Downhole flow control system

Abstract

A downhole flow control system, comprising: a downhole flow control device, comprising: a body (1) locatable within a wall of a tubular and defining a primary inlet (2) and a primary outlet (3), wherein a primary flow path is defined between the primary inlet and the primary outlet; a valve member (5) disposed within the body, the valve member being moveable in reverse first and second directions to selectively vary a flow area of the primary flow path; and a pilot pressure chamber (6) for receiving a pilot pressure, the valve member being in pressure communication with the pilot pressure chamber such that pilot pressure may act to bias the valve member in one of the first and second directions; and a pilot pressure system for generating the pilot pressure, comprising: a secondary flow path: a fluid inlet (71) for receiving a fluid into the secondary flow path and a fluid outlet (32) for delivering the fluid from the secondary flow path; and a first flow restrictor (76) and a second flow restrictor (40) arranged in series in the secondary flow path and defining a pilot pressure region therebetween, the pilot pressure region arranged in pressure communication with the pilot pressure chamber of the downhole flow control device, wherein the first and second flow restrictors are each configured to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path, and wherein at least one of the first and second flow restrictors is configured to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the secondary flow path.

Description

[0001] Downhole flow control system

[0002] FIELD

[0003] The present disclosure relates to a downhole flow control system and uses thereof, particularly downhole flow control systems for use in oil and gas operations.

[0004] BACKGROUND

[0005] Wellbore completions often include inflow control devices which assist with controlling the amount and / or type of wellbore fluid that is allowed to pass from a reservoir into a production flow path. A subterranean hydrocarbon reservoir typically produces both hydrocarbons (such as oil and natural gas) and water, with it being only the production of hydrocarbons that is desired. Further, it may also be the case that production of specifically only one kind of hydrocarbon, such as oil, is desired. In this instance, it may be desirable to implement a control system that optimises oil production from the reservoir, while limiting the production of water and natural gas. The concept of selectively producing fluids from a subterranean hydrocarbons reservoir is well known in the art, with the principles being utilised to optimise production of the desired fluid from the reservoir.

[0006] Flow control systems designed to differentiate between fluids based on a property of the fluid flowing through the system are known. In such systems, flow restrictors are often used, which impart a pressure drop across them, dependent on the properties of the fluid flow through them. In many cases, the property of the fluid is one of density and viscosity.

[0007] Systems that differentiate between fluids based on density alone can be inaccurate when differentiating between heavy oil and water, due to their similar densities. Systems that differentiate between fluids based on viscosity alone can struggle to differentiate between light oil and water due to their similar viscosities. It was therefore desirable to provide a system that could selectively control fluid flow based on both density and viscosity. This has previously been done by providing a system including both a flow restrictor that imposes substantially laminar flow characteristics on fluid flowing therethrough and a second flow restrictor that imposes substantially turbulent flow characteristics on fluid flowing therethrough, referred to herein as a laminar flow restrictor and turbulent flow restrictor. In this respect whether a flow restrictor is classified as laminar or turbulent is, according to the common general knowledge in the art, based on classical fluid dynamic principles and is a determination which is made with reference to the Reynolds number. That is, laminar flow is associated with lower Reynolds numbers, and turbulent flow is associated with larger Reynolds number, with a critical Reynolds number defining the transition from laminar to turbulent flow. The reason for using a laminar and turbulent flow restrictor together is outlined below.

[0008] Flow restrictors impose pressure drops on fluid flow. The pressure drop across flow restrictors can be calculated, with different types of restrictors having different governing equations. In this respect both laminar and turbulent flow restrictors are well known, and different pressure drop equations or relationships exists in accordance with whether the restrictors are laminar or turbulent.

[0009] Laminar flow through a porous media is governed by the following equation:

[0010] Pressure drop due to laminar flow through a pipe is calculated as:

[0011] In both cases, the pressure drop for laminar flow (i.e., a laminar flow restrictor) is predominately dependent on viscosity p, without any dependence on density p. Pressure drop is also linearly proportional to the velocity in laminar flow.

[0012] Turbulent flow has different governing equations. Turbulent flow through a restriction is governed by:

[0013] Turbulent flow through a pipe is governed by:

[0014] Where: k: permeability (Darcy or pm2) p: density (kg / m3) p: viscosity (cP - centipoise or Pa-s - Pascal-seconds)

[0015] AP: pressure drop (Pa - Pascal) v: velocity (m / sec)

[0016] L: length (meter)

[0017] D: hydraulic diameter (meter)

[0018] Cd: coefficient of discharge (dimensionless) fo: Darcy friction factor (dimensionless)

[0019] For turbulent flow restrictors, the pressure drop is dependent on density p. Viscosity does not directly appear in the governing equations. Pressure drop is also proportional to the square of velocity in turbulent flow.

[0020] It has therefore been understood within the industry that flow restrictors that primarily operate in the laminar regime are dependent on viscosity. Flow restrictors that primarily operate in the turbulent regime are primarily dependent on density.

[0021] It is for this reason that existing flow control systems often include both a laminar flow restrictor and turbulent flow restrictor where control is desired based on both the viscosity and the density of a fluid. The pressure drop across the laminar flow restrictor is responsive predominantly to viscosity and the pressure drop across the turbulent flow restrictor is responsive predominantly to density. By combining these two restrictor types, movement of the valve can be controlled dependent on variations in viscosity and density to open and close a fluid flow path.

[0022] In such systems, the flow restrictors may be arranged in series, with a pressure located between the two restrictors being used to control movement of the valve (e.g. being applied to one side of the valve). The two flow restrictors may act as a potential divider, essentially dividing the total pressure difference between the annulus and the tubing based on the properties of the two flow restrictors. Such systems suffer from many shortcomings. The velocity of fluid flowing through these systems is typically low. This results in the pressure drop across the turbulent restrictor (which, as highlighted above, is dependent on the square of velocity) being low compared to that across the laminar flow restrictor. The pressure drop across the laminar flow restrictor can dominate the behaviour of the system.

[0023] When the pressure drop across the turbulent flow restrictor is low, the pressure-induced forces acting on the valve may be low. This may make the system incompatible with small valve members, or small pressure actuation surface areas more generally, as the pressure-induced forces acting on the valve member may be insufficient to provide reliable operation. It also provides other challenges. First, it makes the valve’s operation less robust and controllable, as small fluctuations within the system can cause significant changes in valve operation. Second, it inhibits or minimises the ability of the system to throttle production. Production of oil with close to no pressure drop across the system may lead to rapid onset of undesired fluids like water or gas, potentially resulting in bypassed oil in the reservoir. Many, if not most, horizontal wells benefit from some backpressure across the devices at the start of the well’s life. Today, it is common to operate wells with a pressure drop across the devices ranging from 2 to 30 bar (30 - 450 psi).

[0024] The use of two flow restrictors configured to impose different flow characteristics (e.g. laminar and turbulent) on fluid flowing through the secondary flow path results can cause instability within the device. The pressure changes across the two flow restrictors are governed by, and hence responsive to, different properties of the fluid flowing therethrough. The two flow restrictors are configured to react differently to changes in the fluid flow. These factors cause significant changes to the pressure drop across the respective flow restrictors. This, in turn, may cause large pressure swings acting on the valve member and rapid and unstable opening and closing behaviour of the flow control device as a whole.

[0025] There are also disadvantages associated with the use of a laminar flow restrictor per se. The pressure differential across the laminar flow restrictor being predominantly determined by the viscosity means that there are significant swings in pressure differential as the viscosity of the fluid changes. This imposes significant and rapid changes on the forces acting on the valve member, causing rapid movement between an open and closed position and resulting in a binary system in which the control device is either fully open or fully closed. This is undesirable because the undesired fluids (e.g. water) will always gradually increase relative to the desired fluid (e.g. oil), so that it is undesirable to close or highly restrict the flow immediately. In a system using a laminar flow restrictor, the valve will be more predisposed to move rapidly from a fully opened position to a fully closed position at the first sign of water onset, even where only a small volume fraction of water is present. This can lead to premature throttling, locking a lot of oil in the reservoir, with refined and controlled flow control not being achieved. This behaviour can be exacerbated where large valve members are used, that is, where large pressure actuation surfaces are present.

[0026] Flow control devices are often installed within existing tubular profiles and, as such, must adhere to very strict geometric constraints. It is therefore also desired to provide such a system that can work within the existing device and tubular footprints.

[0027] SUMMARY

[0028] The present disclosure provides a downhole flow control system that may provide increased control and stability of fluid flow through a flow control device.

[0029] The present disclosure relates to a system in which the disadvantages of the prior art may be overcome. That is, a system is provided for use in the selective production of downhole fluids in which production can be throttled in a controlled and reliable manner.

[0030] According to the disclosure is a downhole flow control system, comprising: a downhole flow control device, comprising: a body locatable within a wall of a tubular and defining a primary inlet and a primary outlet, wherein a primary flow path is defined between the primary inlet and the primary outlet; a valve member disposed within the body, the valve member being moveable in reverse first and second directions to selectively vary a flow area of the primary flow path; and a pilot pressure chamber for receiving a pilot pressure, the valve member being in pressure communication with the pilot pressure chamber such that pilot pressure may act to bias the valve member in one of the first and second directions; and a pilot pressure system for generating the pilot pressure, comprising: a secondary flow path: a fluid inlet for receiving a fluid into the secondary flow path and a fluid outlet for delivering the fluid from the secondary flow path; and a first flow restrictor and a second flow restrictor arranged in series in the secondary flow path and defining a pilot pressure region therebetween, the pilot pressure region arranged in pressure communication with the pilot pressure chamber of the downhole flow control device, wherein the first and second flow restrictors are each configured to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path, and wherein at least one of the first and second flow restrictors is configured to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the secondary flow path.

[0031] The present disclosure relates to a downhole flow control system. The downhole flow control system is for use as part of a downhole completion within a reservoir. It is within this operating context that the present disclosure is provided. Where features of the flow control system are described with reference to fluid flowing through the system, or disclosed as imparting certain characteristics on flow through the system, these statements may be understood as referring to an in use (i.e. under operational conditions) operation of the device within a downhole completion, with such environmental and operation parameters as would be expected in this environment. Normal operation conditions may include normal production or injection within the well. For example, negligible flow during the transitory condition of start-up is generally understood as not considered to fall under normal operational conditions. While it is acknowledged that fluid may flow through the system described herein with a variety of different flow characteristics, it is to be understood that the control functionality described herein is intended to operate when subjected to the specific flow characteristics set out.

[0032] Similarly, where features are referred to as having certain properties during use, or imparting certain characteristics to fluid flowing therethrough, it is to be understood that such statements may be interpreted to mean intentionally imparting such characteristics during the majority of use in normal operation, or to impart substantially those characteristics to the majority of fluid flowing therethrough during normal operation.

[0033] More specifically, and purely for example, where a flow restrictor is described as being “configured to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path”, it is to be understood that the flow restrictor is configured (e.g. designed) to cause fluids (such as oil or water) flowing through it during normal steady state operation within a downhole completion context to have turbulent flow characteristics over the majority of its operational range and timeframe. That is, the flow restrictor is specifically designed with the objective of causing fluids flowing therethrough in normal use to have a Reynolds Number higher than the critical Reynolds number for the specific operational conditions. Furthermore, it is understood that reference to a “turbulent flow restrictor” is to be interpreted identically to “a flow restrictor configured to impose substantially turbulent flow characteristics on fluid”. The same applies to flow restrictors described as being laminar or imposing laminar flow characteristics on fluid flow.

[0034] As noted above, the flow restrictors as defined herein are specifically designed and configured with the objective of causing fluids flowing therethrough to be in the turbulent flow regime, which means that in normal use the Reynolds Number will be higher than the critical Reynolds number for the specific operational conditions. In this respect the Reynolds number is a well-known non-dimensional ratio which is classically and universally used to describe whether a flow is laminar or turbulent. In this respect the

[0035] Reynolds number is a ratio of inertial forces to viscous forces in the fluid, and is represented as:

[0036] Inertial Forces pvD

[0037] Re

[0038] Viscous Forces F

[0039] It is well understood that at large Reynolds numbers the inertial forces are large relative to the viscous forces, meaning that the viscous forces are not strong enough to prevent flow fluctuations and random flow behaviour, such that the fluid flow is classified as turbulent. At small or moderate Reynolds numbers the viscous forces are large enough to suppress flow fluctuations such that the flow remains well ordered and is classified as laminar. The Reynolds number at which the flow transitions from laminar to turbulent is called the critical Reynolds number, which may be, for example around 2300.

[0040] Thus, a fluid flow restrictor may be configured to impose substantially turbulent flow on fluid flowing therethrough when the Reynolds number is greater than the critical Reynolds number.

[0041] A fluid flow restrictor may be configured to impose substantially turbulent flow characteristics on fluid flowing therethrough when it is configured to impose flow with a Reynolds number greater than, for example, 2000, 2300, 2500 or 3000. The first and second flow restrictors may each be configured such that pressure drop across the respective restrictor is proportional to the square of the velocity of fluid flowing therethrough. This is in contrast to laminar flow restrictors where the pressure drop would be linearly proportional to flow velocity.

[0042] A flow restrictor configured to impose substantially turbulent flow characteristics may be such the pressure drop across the flow restrictor during the intended operating conditions is governed by (e.g. can be modelled and estimated using) turbulent flow equations. That is, turbulent flow conditions will be prevailing when the pressure drop is proportional to density and the square of the velocity, in contrast to laminar flow conditions in which the pressure drop will be dependent on viscosity and linearly proportional to flow velocity. This is set out in Figure 12, below.

[0043] The behaviour of a flow restrictor may be considered to be governed by a turbulent flow equation when the pressure drop across the flow restrictor is within + / - 20% of that calculated by the equation, or when it is more closely modelled by a turbulent flow equation than a corresponding laminar flow equation.

[0044] The first and / or second flow restrictor may be configured to operate outside of the complete turbulence regime (e.g. to impose substantially turbulent flow characteristics on fluid flow, but not impose complete turbulence on fluid flowing therethrough).

[0045] In line with the normal approach to flow characterisation, flow characteristics as referred to herein are considered globally - i.e. considering the entire cross-section of fluid flow along the flow path. That is, fringe effects such as laminar boundary layers within otherwise turbulent flow do not affect whether a flow restrictor is considered to impose turbulent flow characteristics.

[0046] A flow restrictor configured to impose substantially turbulent flow characteristics may be configured such that the Reynolds number of heavy oil, light oil, water, and / or a mixture thereof flowing therethrough (e.g. under normal operating conditions) has a Reynolds number higher than the critical Reynolds number, greater than 2000, greater than 2300, greater than 2500 or greater than 3000. The characteristics of downhole fluids and operating conditions are known within the art. Heavy oil may have a density of at least 900 kg / m3 and a viscosity of at least 10cP at 20°C. Light oil may have a density of less than around 850 kg / m3 and a viscosity of around 0.7 - 5 cP at 20°C, for example. Water may have a density of around 1000 kg / m3 and viscosity of around 1cP at 20°C, for example.

[0047] The flow control system is effective in both light and heavy oil reservoirs. In a light oil reservoir, the oil fraction may have a viscosity which is very close to the water fraction. For example, in some light oil reservoirs, the oil fraction might have a viscosity of around 0.8cP (centipoise - unit for viscosity), whereas the water fraction has a viscosity of around 0.5cP. In this case, discriminating based on viscosity alone might not provide the necessary degree of control. Bringing density into the control philosophy therefore increases the sensitivity of discrimination against different fluids. That is, while there might be very little difference in the viscosity of a light oil and water, there will be a difference in density. The flow control system discriminates between different fluids based on viscosity and density in a unique manner, without requiring concessions to be made against other design criteria.

[0048] The system of the present disclosure relates closely to the disclosures provided in WO2023 / 016643 and WO2023 / 016676, the underlying principles and structures of which are relevant to the advances presented in the present disclosure and the features of which are considered to belong to the disclosure of the present application and may be protected as part of the present disclosure.

[0049] The secondary flow path may comprise the first and second flow restrictors. The first and second flow restrictors may collectively define the pilot pressure which acts on one side of the valve member to bias the valve member, for example towards a closed position.

[0050] The flow control system, for example the valve member, primary flow path and pilot pressure system, may be configured such that the valve member never closes during use.

[0051] It is beneficial in flow control systems to establish a high pilot pressure. This may allow the flow control system to operate with flow control devices of restricted geometry (i.e. where the overall area over which the pilot pressure can act is limited). It can also improve the reliability and accuracy of the control of fluid flowing through the flow control device. Setting an appropriate pilot pressure can also facilitate throttling or choking of the well, which provides benefits discussed below.

[0052] In order achieve a suitably high pilot pressure, a pressure drop across the second flow restrictor as a percentage of the combined pressure drop across the first and second flow restrictors may be higher than in existing systems. The percentage that the pressure drop across the second flow restrictor provides of the combined pressure drop across the first and second restrictor is referred to herein as the “Pressure Boost (%)” or “Boost value (%)”.

[0053] Pressure Boost (%) = 100

[0054] Pressure Boost may be defined as the pressure drop across the second flow restrictor as a percentage of the combined pressure drop across the first and second restrictor, as outlined above.

[0055] The geometry of the flow control system (e.g. valve member, body inlet and outlet, pilot pressure chamber, ...) may define a Pressure Boost at which fluid pressure forces acting across the valve member are balanced (so the valve member would remain still in steady state without any biasing member). This Pressure Boost may be seen as a “sweet spot”, because operating at, or around, this Pressure Boost may allow more ready choking or throttling of production, which can improve recovery performance. An estimate of the Pressure Boost “sweet spot” can be calculated from a force balance equation for the valve member.

[0056] A sweet spot for a valve member having a diameter of 12mm and an inlet to the downhole flow control device having a diameter of 9.7mm may be around 65%. The first and second flow restrictors may be configured to provide a Pressure Boost of at least 50%, 60% or 65% during use.

[0057] The use of such Pressure Boost levels provide a system with higher stability and control, since the use of larger actuating forces can assist in minimising and damping instabilities within the system. To achieve a suitable pressure drop across the second flow restrictor for providing a higher Pressure Boost, the secondary flow path may be configured for high fluid velocity. The first flow restrictor and second flow restrictor may be configured to provide such a high fluid velocity through the secondary flow path.

[0058] Previous systems have been designed on the assumption that high fluid velocities tend towards turbulent fluid flow through restrictors and that, without a laminar flow restrictor, it would not be possible to distinguish between fluids with similar viscosities. Contrary to this accepted understanding, the present disclosure is based on the discovery that suitable control can be achieved with a system using two turbulent restrictors, and that significant benefits are achieved in such a system.

[0059] The first and second flow restrictors may each be configured to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path. That is, the first flow restrictor and second flow restrictor may be configured such that the flow through them during normal use has a high velocity and is operating within the turbulent regime.

[0060] At least one of the first and second flow restrictors may be configured to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the secondary flow path.

[0061] The first and second flow restrictors may be configured to impose substantially the same flow characteristics on fluid flowing through the secondary flow path, e.g. turbulent flow. Having both of the first and second flow restrictors imposing similar, or the same, flow characteristics can provide a more stable control, since significant and rapid pressure changes within the pilot pressure chamber are avoided as the properties of the fluid change.

[0062] Providing a system with two turbulent flow restrictors (and without any laminar flow restrictors) controlling the operation of the valve member may provide a number of benefits. The Pressure Boost can be increased. This increases the pressures acting on the valve member, minimising and damping instabilities within the system. The availability of an increased Pressure Boost also allows the flow control system to throttle production - that is, to provide a pressure drop across the flow control system even when the desired fluid (e.g. oil) is flowing into the tubing through the system.

[0063] As outlined above, it was traditionally understood that the pressure drop across turbulent flow restrictors was dependent on density, but not viscosity. It was therefore previously understood that a flow control system with two turbulent flow restrictors and no laminar flow restrictors would be insensitive to changes in viscosity and thus the system as a whole would be unable to accurately differentiate between reservoir products with similar densities. The present disclosure provides an arrangement in which a first and second turbulent flow restrictor can accurately differentiate between the majority of reservoir products during normal use.

[0064] At least one of the first and second flow restrictors may be configured to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the secondary flow path. A property of the fluid may include density and / or viscosity.

[0065] In particular, at least one of the first and second flow restrictors may be configured to influence the pressure within the pilot pressure region as a function of both density and viscosity of the fluid flowing through the secondary flow path.

[0066] That is, the pressure within the pilot pressure region changes when either, or both, of the density or viscosity of fluid flowing through the flow restrictor change.

[0067] Where reference is made to pressure within the pilot pressure region changing, it is to be understood that this is caused by the pressure drop across the first (and / or second) flow restrictor changing in response to changing fluid properties. Reference to changes in the pilot pressure region can be interchanged with references to changes in the pressure drop across the flow restrictor.

[0068] The other of the first and second flow restrictors may be configured to influence the pressure within the pilot pressure region as a function of density of the fluid flowing through the secondary flow path. The first flow restrictor may be configured to influence the pressure within the pilot pressure region as a function of both density and viscosity of the fluid flowing through the secondary flow path. Influencing the pressure within the pilot pressure region as a function of density and viscosity of the fluid flowing through the secondary flow path can result in a system that has increased accuracy in differentiating between heavy oil and water, and light oil and water.

[0069] The fact that the first flow restrictor and second flow restrictor are both configured to impose substantially turbulent flow characteristics means that neither flow restrictor is responsive predominantly to viscosity alone. This avoids excessive rapid force changes within the system, improving operating control and reliability.

[0070] One of the first and second flow restrictors may be a conduit, e.g. a tube. The first flow restrictor may be a conduit.

[0071] The pressure drop across the first flow restrictor may be governed by the turbulent flow equation:

[0072] 2 t^Pn = f CD* - P * V — * L 1.

[0073] J U2 D

[0074] The conduit may be configured to operate (e.g. during normal operation) within a region on the Moody diagram (see Figure 5, for example) bounded on one side by the transition region (i.e. from laminar to turbulent flow) and on the other side by the boundary with complete turbulence.

[0075] The conduit may be configured to operate outside of the complete turbulence regime (e.g. to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path, but not impose complete turbulence on fluid flowing therethrough). Operation outside of the complete turbulence regime is to the left of the “complete turbulence” line in the Moody chart. Fluids flowing through the secondary flow path may be water, light oil, heavy oil and / or mixtures thereof. The flow characteristics may be taken during normal operation in a downhole environment.

[0076] The conduit may be configured to impose turbulent, but not completely turbulent, flow characteristics on fluid flowing through the secondary flow path. The property of the fluid flowing through the secondary flow path that may influence the pressure in the pilot pressure region may be density and viscosity. The first flow restrictor, e.g. the conduit, may be configured to influence the pressure within the pilot pressure region as a function of both viscosity and density of the fluid flowing through the secondary flow path.

[0077] Traditionally, it was thought that changes in pressure imposed by turbulent flow (e.g. through a pipe or across a valve) would be independent of, or insensitive to, changes in viscosity. It was previously understood that the pressure drop across a turbulent flow restrictor would be responsive predominantly to changes in density. However, the pilot pressure of the flow control system described herein may be sensitive to changes in viscosity (as well as density).

[0078] As set out above, the pressure drop across a flow restrictor in the form of a conduit under turbulent flow will be governed by the equation:

[0079] As noted above, the pressure drop is directly proportional to density, but not directly proportional to viscosity, p, which does not appear in the above equation. The present disclosure provides a method by which the pressure drop across a turbulent flow restrictor can be made to vary with viscosity. This is achieved through manipulation of the Darcy friction factor fo.

[0080] The Darcy friction factor fo can be derived from the Moody chart, included as Figure 5. The Moody chart sets out friction factors against Reynolds Number for fully developed flow in a circular pipe for a range of different pipe roughnesses. In accordance with the principles of the Moody chart, the relationship between Reynolds number and friction factor is presented for multiple values of relative surface roughness in the turbulent flow regime. Relative surface roughness is the ratio of surface roughness to pipe diameter.

[0081] As shown in the chart of Figure 5, curves for pipes with a higher relative pipe roughness (i.e., higher surface roughness for equivalent diameter pipe) are flatter in nature, indicating less variation of friction factor for increasing Reynolds number. In other words, the friction factor becomes substantially constant at lower Reynolds numbers for pipes with a higher relative pipe roughness. Conversely, pipes with a lower relative pipe roughness (i.e. , lower surface roughness for equivalent diameter pipe) will see a larger variation of friction factor for increasing Reynolds number. Further, the friction factor will continue to drop at a reasonable gradient as the Reynolds number increases.

[0082] The Reynolds Number is dependent on both density and viscosity, thus if the friction factor varies with Reynolds Number, it inherently varies with viscosity. This means that a conduit or pipe, operated in the turbulent regime, can be configured to influence the pressure within the pilot pressure region as a function of both density and viscosity by using a conduit with a very low surface roughness.

[0083] The Reynolds Number varies in proportion to density divided by viscosity (p / p). The friction therefore varies in line with the ratio of these two characteristics of the fluid.

[0084] When differentiating between oil and water, it has been found that the present disclosure is most sensitive to changes in fluid properties, and thus operates most effectively, when Poll Pwater Poll Pwater

[0085] Although this may be a particularly sensitive operational range, the flow control system provides increased productivity across the operational range.

[0086] By using a turbulent flow restrictor in the form of a conduit (i.e. pipe) with low surface roughness, the following conditions may be established with respect to the turbulent flow restrictor across the majority of the useful range of the operating regime:

[0087] 1 . If viscosity increases: Reynolds number will decrease, friction factor will increase and as a result, the pressure drop in the conduit flow restrictor will increase. This results in lower boost (pilot) pressure, because more of the total pressure drop is in the conduit flow restrictor.

[0088] 2. If viscosity decreases: Reynolds number will increase, friction factor will decrease and as a result, the pressure drop in the conduit flow restrictor will decrease. This results in higher boot (pilot) pressure, because less of the total pressure drop is in the conduit flow restrictor.

[0089] For this reason, the conduit may comprise a substantially smooth surface. The relative pipe roughness of the conduit may be less than 0.001 , 5x1 O'4, 2x1 O'4, 10'4, 5x1 O'5, 10'5, 5x1 O'6or 10'6.

[0090] The relative pipe roughness may be between 8 and 9 x10'5. The relative roughness may be 8.5x1 O'5.

[0091] The relative pipe roughness is defined as the absolute roughness of a pipe divided by the inside diameter of a pipe, (e / d). By using a conduit configured to impose substantially turbulent flow characteristics on the fluid flowing through the secondary flow path with a low relative pipe roughness, the pilot pressure may be dependent on a function of the viscosity and density.

[0092] In order to provide a conduit with the required relative pipe roughness, the internal surface of the conduit may be treated to increase smoothness.

[0093] For example, the inside surface of the conduit may comprise an electropolished surface. That is, the inside surface may be electrochemically polished.

[0094] By using a first and second flow restrictor that both impose substantially turbulent flow characteristics on the fluid flowing through the secondary flow path, the pressure drop (dependent on the friction factor of the Moody chart) is less sensitive than laminar flow to small changes in the viscosity of the fluid. This provides the advantage of more stable, controllable and reliable flow control through the control device.

[0095] This is evidenced by reference to Figures 6 through 11 , discussed below.

[0096] The conduit may be provided in at least one of a coiled or serpentine form.

[0097] In some examples, the conduit may be configured to be coiled around the tubular

[0098] The conduit may be coiled and have a curvature ratio of less than 0.02. The curvature ratio is defined as the inner diameter of the conduit divided by the outer diameter of the helical coil. The conduit may be a helical coil with an outside coil diameter of 140mm and an inner conduit diameter of 2mm.

[0099] Managing the curvature ratio of the conduit may ensure that the critical Reynolds number (i.e. the point at which flow transitions from laminar to turbulent flow) is appropriate to maintain the flow in the turbulent regime for the purposes of establishing the desired functionality to control flow through the system based on fluid property changes.

[0100] The conduit may have a length of 3m.

[0101] The second flow restrictor may comprise a nozzle, e.g. an orifice or valve.

[0102] For the majority of flow regimes, a nozzle will operate in the turbulent regime, however for particularly high viscosity and low boost values (e.g. low fluid velocities) a nozzle may instead operate in the laminar regime. Another advantage of the present system is that the higher velocity and boost pressures ensure that the nozzle imparts a substantially turbulent flow on fluid flowing therethrough during use. That is, by the first fluid flow restrictor operating in the turbulent flow regime, with correspondingly higher flow velocities, the fluid will be delivered to the second flow restrictor with such a higher velocity providing greater assurance that this will also continue to operate in the turbulent flow regime and provide the desired pressure boost for appropriate operation. The pressure drop across the nozzle is therefore predominantly determined by the density of the fluid.

[0103] The second flow restrictor may have a diameter of less than 1 mm. The second flow restrictor may have a diameter of less than 0.8mm, for example 0.685mm.

[0104] The pilot pressure system may comprise a manifold associated with the fluid inlet, configured to circumscribe the tubular and comprising a plurality of circumferentially arranged inlet ports for receiving a downhole fluid into the manifold.

[0105] The downhole flow control device may comprise a manifold associated with the primary inlet, configured to circumscribe the tubular and comprising a plurality of circumferentially arranged inlet ports for receiving a downhole fluid into the manifold. The pilot pressure chamber may be sealed.

[0106] The downhole flow control system may be configured such that there is substantially no flow between the pilot pressure region and the pilot pressure chamber.

[0107] The pilot pressure chamber is configured to tap a static pressure value from the pilot pressure region. During use, there is substantially no flow through the pilot pressure chamber from the pilot pressure region. This provides significant advantages, since there will be significantly reduced risk of debris build up and it will avoid the risk of kinematic effects influencing the valve member movement - this greatly increases the reliability and accuracy of the flow control device.

[0108] The pilot pressure chamber may be physically separated from the pilot pressure region by means of a pressure conveying separator. For example, a pressure-conveying piston or diaphragm may be located between the pilot pressure chamber and pilot pressure region.

[0109] The valve member may be configured to be exposed to the pilot pressure on one side and a second pressure - for example annulus pressure or reservoir pressure - on the opposing side. The pressure differential across the valve member may urge the valve member in one of the first and second directions.

[0110] The downhole flow control system may be configured such that the valve member does not close the primary flow path during use. That is, the flow area of the primary flow path may always greater than zero during use.

[0111] The flow control system may be designed such that, during normal, steady state operation, the primary flow path is never closed. The downhole flow control device may be configured to always permit flow from the annulus into the tubular, but to provide various degrees of choking or throttling dependent on the nature of the fluid encountered.

[0112] The valve member may be unable to fully close during operation due to its relatively small diameter (e.g. less than 20mm or 15mm, for example 12mm). The small diameter of the valve member may mean that there are insufficient pressures available to exert sufficient force to close the primary flow path entirely during use. Avoiding a binary system that moves from fully open to fully closed and instead providing a system that provides a more accurate and graduated response to throttle fluid production can greatly improve reliability and recovery performance.

[0113] The system may further comprise a biasing member arranged to bias the valve member towards a closed arrangement for restricting flow through the primary flow path.

[0114] The provision of a biasing member may improve the stability of the system and allow the valve member to throttle fluid flow across its entire operation range. Such throttling is desirable to improve operational efficiency.

[0115] The biasing member may be a diaphragm. The biasing member may be a diaphragm that surrounds the valve member (or a part thereof). Arranging the diaphragm to surround the valve member (rather than being arranged on one side of the valve member, for example) may provide a more compact design.

[0116] Part of the diaphragm may be sandwiched between parts of the valve member. This may ensure a particularly secure connection between the diaphragm and valve member.

[0117] In one particular arrangement, the valve member may comprise a flow surface configured to move in reverse first and second directions to vary the flow area of the primary flow path and a stem extending away from the flow surface. The stem may extend through the diaphragm such that the diaphragm surrounds the stem of the valve member.

[0118] The diaphragm and valve member may be a single part. They may be an integral part. That is, the valve member may comprise the diaphragm. The valve member may comprise a central sealing region configured to provide the throttling functionality with respect to the primary flow path, for example with high corrosive resistance, and a circumferential diaphragm region, for example configured to provide increased elasticity for providing a biasing force as described herein.

[0119] The diaphragm may be configured to exert a closing force on the valve member when the valve member is in a closed position. It is noted that the valve member may be configured to never close during use, however it may be advantageous for the valve member to be located in a closed position when not in use - e.g. during transportation or storage, when the flow control device is not installed in a completion. That is, the valve member may be configured to be capable of closing, but only in the absence of operational fluid pressure forces acting on it.

[0120] The diaphragm may be configured to exert a biasing force on the valve member, regardless of the position of the valve member within the body. That is, the diaphragm may be arranged such that is strained (e.g. bowed) throughout the operational range of movement of the valve member and, even when the valve member is in the closed position, the diaphragm is still strained to exert a biasing force holding the valve member in the closed position. Again, it is reiterated that the valve member is configured to only reach a fully “closed” position when not in use.

[0121] The diameter of the valve member may be less than 20mm, for example 12mm, and the diameter of an inlet to the body of the downhole flow control device may be less than 15mm, for example 9.7mm.

[0122] As noted above, the valve member may be smaller than those used in existing systems. This advantageously allows the valve member, and hence the flow control system, to be used within the footprint of existing inflow control devices. Furthermore, the use of a small valve member with high Pressure Boost (and hence operating forces) can greatly improve the control accuracy provided by the device.

[0123] Further according to the disclosure is a pilot pressure system for generating a pilot pressure. The pilot pressure system may comprise a flow path. It may further comprise a fluid inlet, for example for receiving a fluid into the flow path. The pilot pressure system may further comprise a fluid outlet for delivering the fluid from the flow path. The pilot pressure system may further comprise a first flow restrictor and a second flow restrictor, which may be arranged in series in the flow path and may define a pilot pressure region therebetween. The first and second flow restrictors may each be configured to impose substantially turbulent flow characteristics on fluid flowing through the flow path. At least one of the first and second flow restrictors may be configured to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the flow path.

[0124] The pilot pressure system may be as described anywhere herein.

[0125] Further according to the disclosure is a further downhole flow control system. The further downhole flow control system may comprise a downhole flow control device, for example as described herein. The downhole flow control device may comprise a body. The body may be locatable within a wall of a tubular and may define a primary inlet and a primary outlet. The primary flow path may be defined between the primary inlet and the primary outlet.

[0126] The further downhole flow control device may further comprise a valve member, for example as described herein. The valve member may be disposed within the body. The valve member may be moveable in reverse first and second directions to selectively vary a flow area of the primary flow path.

[0127] The further downhole flow control device may further comprise a pilot pressure chamber, for example as described herein. The pilot pressure chamber may be for receiving a pilot pressure. The valve member may be in pressure communication with the pilot pressure chamber such that pilot pressure may act to bias the valve member in one of the first and second directions. The valve member may be configured to be exposed to the pilot pressure on one side and a second pressure - for example annulus pressure or reservoir pressure - on the opposing side. The pressure differential across the valve member may urge the valve member in one of the first and second directions.

[0128] The further downhole flow control system may comprise a pilot pressure system, for example as described herein, for generating the pilot pressure. The pilot pressure system may comprise a secondary flow path. It may further comprise a fluid inlet for receiving a fluid into the secondary flow path. It may further comprise a fluid outlet for delivering the fluid from the secondary flow path. The pilot pressure system fluid inlet may be spaced from the primary inlet.

[0129] The pilot pressure system may comprise a first flow restrictor and a second flow restrictor, for example as described herein. The flow restrictors may be arranged in series in the secondary flow path and may define a pilot pressure region therebetween. The pilot pressure region may be arranged in pressure communication with the pilot pressure chamber of the downhole flow control device. The secondary flow path may comprise the first and second flow restrictors. The first and second flow restrictors may collectively define the pilot pressure which acts on one side of the valve member to bias the valve member, for example towards a closed position.

[0130] Further according to the disclosure is a method of controlling fluid flow. The method may comprise using the downhole flow control system described herein.

[0131] The method may comprise providing a downhole flow control system. The system may have a downhole flow control device comprising: a body locatable within a wall of a tubular and defining a primary inlet and a primary outlet, wherein a primary flow path is defined between the primary inlet and the primary outlet; a valve member disposed within the body, the valve member being moveable in reverse first and second directions to selectively vary a flow area of the primary flow path; and a pilot pressure chamber for receiving a pilot pressure, the valve member being in pressure communication with the pilot pressure chamber such that pilot pressure may act to bias the valve member in one of the first and second directions; and a pilot pressure system for generating the pilot pressure, comprising: a secondary flow path: a fluid inlet for receiving a fluid into the secondary flow path and a fluid outlet for delivering the fluid from the secondary flow path; and a first flow restrictor and a second flow restrictor arranged in series in the secondary flow path and defining a pilot pressure region therebetween, the pilot pressure region arranged in pressure communication with the pilot pressure chamber of the downhole flow control device. The method may further comprise operating the first and second flow restrictors in a turbulent fluid flow regime to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the secondary flow path.

[0132] Further according to the disclosure is a flow restrictor configured to influence the pressure drop across the flow restrictor during use as a function of the viscosity and density of the fluid flowing through the secondary flow path.

[0133] The flow restrictor may be a downhole flow restrictor for use with oil and gas production. The flow restrictor may be as described anywhere herein.

[0134] Further according to the disclosure is a method for providing or manufacturing a flow restrictor as described herein.

[0135] BRIEF DESCRIPTION OF THE FIGURES

[0136] Figure 1 is a schematic side view of a downhole flow control system according to the disclosure;

[0137] Figure 2 is a cross-section view of a downhole flow control device;

[0138] Figure 3 is a cross-section view of part of a pilot pressure system;

[0139] Figure 4 is a cross-section view of the downhole flow control system of Figure 1 ;

[0140] Figure 5 is a Moody chart with a representative curve for the first flow restrictor of the downhole flow control system added;

[0141] Figure 6 is a plot showing numerical friction factor calculations for a laminar flow restrictor according to the prior art;

[0142] Figure 7 is a plot showing Boost Percentage calculations for the flow restrictor of Figure 6;

[0143] Figure 8 is a plot showing numerical friction factor calculations for a turbulent flow restrictor according to the disclosure;

[0144] Figure 9 is a zoomed in view of Figure 8;

[0145] Figure 10 is a plot showing Boost Percentage calculations for the flow restrictor of Figure 8;

[0146] Figure 11 is a plot showing pilot flow rate calculations for the flow restrictor of Figure 8; and Figure 12 is a plot showing the dependency of pressure drop across a flow restrictor with velocity in both the laminar and turbulent regimes.

[0147] DETAILED DESCRIPTION

[0148] Figures 1 and 4 illustrate a downhole flow control system, generally identified by reference numeral 100, mounted on a tubular 50. The flow control system 100 includes a downhole flow control device 102 mounted in a wall of the tubular 50, a manifold 10 serving as an inlet of a primary flow path of the flow control device 102, and a pilot pressure system 70. The manifold 10 is connected to the flow control device 102 to collect and deliver wellbore fluid from annulus region 16 to the flow control device 102, which is then then received within the tubular 50.

[0149] The pilot pressure system comprises a fluid inlet (in the form of a second manifold 71) and a fluid outlet 32 (see Figure 3) and defines a secondary flow path therebetween.

[0150] The outlet 74 of a conduit 76, serving as a first flow restrictor, of the pilot pressure system 70 is connected to a pressure controller 30 (described in further detail below with Figure 3), also mounted in the wall of the tubular 50, with a control or pilot conduit 79 extending between a pilot pressure region arranged within the pressure controller 30 and a pilot pressure chamber 6 of the flow control device 102. The control or pilot conduit 79 functions to deliver a control pressure to the flow control device 102, wherein the control pressure is used, at least partially, in the control or operation of the flow control device 102 to vary or control inflow of wellbore fluid into the tubular 50 via a primary flow path of the downhole flow control device 102. In this respect the control pressure may be established by virtue of both the first flow restrictor (conduit 76) and a second flow restrictor located within the pressure controller 30 and described below.

[0151] Fluid provided from the manifold 10 is provided at the inlet of the flow control device 102, to flow into the tubular 50. The control pressure provided from the pilot pressure region of the pilot pressure system 70 is provided to the flow control device 102 and used to control the movement of the valve member 5 to control flow through the flow control device 102. In some examples the flow control device 102 may be provided in accordance with that described in the applicant’s co-pending application PCT / EP2021 / 072427, the disclosure of which is incorporated herein by reference.

[0152] The flow control device 102 is primarily responsible for conveying fluid from the annulus into the tubular via the primary flow path 4.

[0153] A diagrammatic illustration of an example form of the flow control device 102 is provided in Figure 2. The device 102 includes a threaded body 1 to be secured in a threaded port of the tubular 50. The device 102 further includes an inlet 2, outlet 3 (or multiple outlets) and a primary flow path 4 extending between the inlet 2 and outlet 3. A control member in the form of a valve member 5 is provided within the flow path 4, and is illustrated in a closed arrangement in Figure 2. It is noted that, during use, the valve member 5 is never in a fully closed position as illustrated. The valve member 5 may only be in a closed configuration as illustrated when not in use, e.g. during storage or transportation).

[0154] The device 102 further includes a pilot pressure chamber 6 behind the valve member 5. The pilot pressure chamber 6 is in pressure communication with the pilot pressure region (located between the first and second flow restrictors in pressure controller 30) via the pilot conduit 79, such that control pressure may be delivered to the pilot pressure chamber 6 and act against the valve member 5. There is substantially no flow through pilot conduit 79 into pilot pressure chamber 6. The control pressure delivered from the pilot pressure system 70, in combination with any other operational pressures, may be used to control movement of the valve member 5 and thus the flow rate through the flow path 4 and flow control device 102.

[0155] In the present example the pilot pressure chamber 6 is sealed, such that flow therethrough is not possible. In this example the control pressure delivered to the pilot pressure chamber 6 from the second pilot pressure system may be defined as a static pressure.

[0156] Fluid is delivered into the primary flow path 4 via the primary inlet, which comprises a manifold 11. The fluid is then conveyed via a tube 12 to the inlet 2. The inlet 2 is defined by a nozzle disc 13, inserted into the body 1 of the flow control device 102. The nozzle disc 13 is removable from the body 1. It is also corrosion resistance treated. This allowed the nozzle disc 13 to be replaced either when damaged, or when a different flow profile is desired.

[0157] A valve member 5 is located within the body 1 and arranged to move towards and away from the nozzle disc 13 to provide a throttling effect on fluid flowing through the primary flow path 4. During use, the valve member 5 is configured to provide a choke effect on fluid flowing through the primary flow path 4, but it is never able to fully close.

[0158] Outlets 3 are provided through which fluid flowing through the flow control device 102 can enter the tubular 50. The outlets 3 are arranged in an axial direction (with respect to the flow control 102) around the circumference of the body 1.

[0159] A biasing member in the form of a diaphragm 14 is provided to urge the valve member 5 towards a closed configuration. In the present example, the diaphragm 14 surrounds the valve member 5. The valve member 5 comprises a flow surface 5a configured to move in reverse first and second directions to vary the flow area of the primary flow path 4 and a stem 5b extending away from the flow surface 5a. The stem 5b extends through the diaphragm 14. The valve member 5 comprises a locking plate 5c, provided on the rear surface of the diaphragm 14, to sandwich the diaphragm 14 between parts of the valve member 5. This provide a particularly compact and secure attachment between the diaphragm 14 and valve member 5, improving reliability of the device. The body 1 comprises a closure part 1a configured to sandwich the outer perimeter of the diaphragm 14 between parts of the body and thus lock the diaphragm 14 in place.

[0160] Although not clearly visible in Figure 2, the diaphragm 14 is slightly bowed even with the valve member 5 in a closed configuration. This ensures that the diaphragm 14 exerts a force in the closing direction of the valve member 5 throughout operation of the device.

[0161] Circumferential ports 15 are provided through the body 1 (specifically the closure part 1a) to provide pressure communication between the pilot pressure chamber 6 and the pilot pressure region.

[0162] In the present example, the pressure controller 30 forms part of the pilot pressure system 70. The functionality of the pressure controller 30, and its role within the flow control system 100 will be explained in greater detail below with reference to Figures 1 , 3 and 4.

[0163] The pilot pressure system 70 includes a second manifold 71 configured to circumscribe the tubular 50 and including an array of circumferentially distributed inlet ports. The pilot pressure system 70 includes an outlet 32 for delivering fluid from the pilot pressure system 70 (for example into the tubular via the pressure controller 30).

[0164] The pilot pressure system 70 comprises a first flow restrictor in the form of an extended flow path conduit 76, which will be explained below in greater detail. The pilot pressure system 70 also comprises a second flow restrictor 40 in the form of an orifice (see Figure 3).

[0165] In between the first flow restrictor and a second flow restrictor a pilot pressure region is defined, from which fluid pressure is communicated to the pilot pressure chamber of the downhole flow control device 102. In the present example, the pilot pressure region comprises a chamber 34.

[0166] In the present example the first flow restrictor is a conduit 76 that extends along a serpentine path which is arranged to partially surround the tubular 50 upon which the pilot pressure system 70 is disposed. Providing an extended flow path conduit 76 as in the example presented is such that a relatively long length of conduit may be packaged within a short axial footprint.

[0167] The first flow restrictor is configured to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path. In this respect the first flow restrictor may be defined as a first turbulent flow restrictor. This is achieved due to the geometry and surface finish of the conduit 76, as described elsewhere herein.

[0168] During operation, the pressure drop across the first flow restrictor, by virtue of this being configured to impose substantially turbulent flow characteristics, is proportional to the square of the velocity of fluid flowing therethrough. The fluid flowing through the first flow restrictor has a Reynolds number greater than the critical Reynolds number, which would be well understood to be the point at which fluid flow transitions from laminar flow to turbulent flow. The length of the conduit 76 may be selected to provide a desired pressure at the outlet 74. For example, the required pressure at the outlet 74 may be determined by the Darcy- Weisbach equation, shown below: wherein Ap is the pressure loss along the length of the conduit 76, fo is the friction factor of the conduit 76, L is the length of the conduit 76, v is the velocity of the fluid within the conduit 76 and DH is the internal diameter of the conduit 76. The length (Li) of the conduit 76 is therefore a factor in providing the desired pressure drop behaviour.

[0169] An example of a conduit for use with a pilot pressure system may be a coiled conduit with a length of 3m, inner diameter of 1 ,75mm, curvature ratio of less than 0.02 (defined as the inner diameter of the conduit divided by the outer diameter of the helical coil) and roughness of 0.15 micron. Such a conduit may be configured to impose substantially turbulent flow characteristics on each of oil, water and gas flowing through the secondary flow path in a desired operating regime.

[0170] Figure 3 is a cross-sectional view of the pressure controller 30 of Figure 1 . The pressure controller 30 forms part of the pilot pressure system. It comprises a threaded body 42 which is configured to be secured to a threaded port on the wall of the tubular 50. This threaded body may be of substantially the same form as threaded body 1 of the flow control device 102 of Figure 7, thus providing a degree of standardisation.

[0171] The pressure controller 30 includes a pressure controller inlet 31 (different to the inlet of the pilot pressure system as a whole) connected to the conduit 76, an outlet 32 which opens into the interior of the tubular 50, and a pressure tap opening located at a pilot pressure region between the first restrictor 76 and second restrictor 40. The pressure tap 38 may communicate pressure to the pilot conduit 79 (which connects to the pilot pressure chamber 6).

[0172] The pressure tap 38 is configured to convey fluid pressure, but not fluid flow. The pressure controller 30 includes a chamber 34 located between the first and second flow restrictors. The chamber 34 forms the pilot pressure region of the described example. The full communication path between the chamber 34 and the pilot conduit 79 is not visible in Figure 3 (i.e., out of the plane of cross-section), and is instead represented by dashed line 38.

[0173] The pressure controller 30 includes the second flow restrictor 40 between the chamber 34 and the first outlet 32. The second flow restrictor 40 of the present example is an orifice. The second flow restrictor 40 is configured to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path.

[0174] In use, fluid delivered to the pressure controller 30 via conduit 76 (first restrictor) will flow into the chamber 34, through the flow restrictor 40 and into the tubular 50 via the outlet 32. Accordingly, a back pressure will be created within the chamber 34 as described above, which will be dictated by the first and second fluid restrictors. The pressure within the chamber 34 is communicated to the flow control device 102 via the pilot line 79.

[0175] In the present example the flow restrictor 40 is provided as a removable insert 44 securable with the body 42. The insert 44 may be readily changed in order to provide a different flow restrictor and thus a different level of pressure control.

[0176] The pressure controller 30 may comprise a non-return valve, comprising a ball 46 and a seat 48. In use, the non-return valve may prevent or restrict reverse flow from the tubular 50. In an embodiment, the non-return valve, e.g. ball 46 and / or seat 48, may be degradable and / or dissolvable, for example over a predetermined period of time. This may avoid or mitigate risks that the valve may become stuck or blocked over time.

[0177] Figure 4 is a cross-section of the flow control system as a whole, including coloured arrows to demonstrate fluid flow. Flow of fluid through the pilot pressure system is shown in blue. Fluid flow through the flow control device 102 is shown in red. Figure 4 also shows a sand screen 16.

[0178] Figure 5 is a Moody diagram with a curve 101 superimposed that shows the relative pipe roughness of a conduit according to the downhole flow control system. The curve 101 is for a conduit with a 1.75mm inner diameter with a 0.150um surface finish (relative roughness = 0.150 / 1750 = 8.6*1 O'5). As can be seen, such a conduit behaves as a substantially smooth pipe. This allows the first flow restrictor to operate in the turbulent regime, but still provide a pressure drop dependent on both viscosity and density of the fluid flowing therethrough.

[0179] Typically, for rough conduits, pressure drop across a conduit in which the flow is turbulent is dependent on the density of the fluid flowing through the conduit and is insensitive to viscosity. This is shown in the upper region of Figure 5, where the relative roughness is high - the friction factor is largely constant regardless of the Reynolds number (which is dependent on viscosity).

[0180] However, as the roughness reduces and the conduit becomes substantially smooth, the friction factor varies as the Reynolds number varies across the entire operational range. This is important, as it means that turbulent flow within substantially smooth conduits provides a pressure drop which is sensitive to both density and viscosity (via the viscosity term in the Reynolds number).

[0181] As can be seen, the curve added to Figure 5 has a clear negative gradient across their entire operational extents - i.e. for the entire range of Reynolds numbers shown. This means that the pressure drop (determined by fo) across the first flow restrictor is sensitive to Reynolds number and, in turn, is sensitive to viscosity as well as density. In other words, in the flow control system, the pressure drop across the first flow restrictor, operating in the turbulent regime, is sensitive to both of density and viscosity.

[0182] It is also important to note from both of Figures 5 and 6 that the gradient is far steeper in the laminar flow regime than the turbulent flow regime. This means that changes in pressure across the turbulent restrictor are less sensitive to changes in fluid viscosity than they would be across a laminar flow restrictor. This is beneficial, as it provides a system that is less susceptible to rapid pressure fluctuations or variations, which can cause kinetic effects and instability. In turn, the disclosed flow control system provides a more measured, predictable and consistent pressure change, thus resulting in a more robust, stable and easily controlled system.

[0183] Turning now to Figure 6, numerical simulations are plotted on a graph showing the Darcy Friction Factor (fo) versus Reynolds number (Re) for a flow restrictor in the form of a conduit (e.g. tube) having the characteristics shown. This conduit is configured to impose substantially laminar flow characteristics on oil and water flowing therethrough.

[0184] Figure 6 depicts calculated values of various examples of oil (5.6cP), water (0.66cP) and gas (0.02cP) flowing through a 6-meter-long, 1mm ID tube (first flow restrictor) and 0.7mm nozzle (second flow restrictor). As illustrated, the flow is predominantly in the laminar range for oil and water under operating conditions. That is, the overwhelming majority of data points for oil and water are located within the laminar region of the graph. It should be noted that gas is always in turbulent flow. Figure 6 demonstrates that there is a significant difference in the friction factor for oil compared to water. This will result in a significant change in pressure drop across the flow restrictor and a correspondingly large change in Pressure Boost (see Figure 7). In Figure 7, the Pressure Boost is set out for the same system. The Pressure Boost achieved using a laminar first flow restrictor is much lower for oil than water / gas due to the relatively low flow rate through the turbulent restrictor resulting in a much larger pressure drop across the laminar than the turbulent restrictor. This may have the effect of allowing high oil flow rates through the flow control system, because the low boost (pilot) pressure would not be able to provide any choking to restrict the flow. It is known to be beneficial to provide choking to the flow of oil into the tubular. The low Boost Percentage also results in a more unstable and difficult to control system.

[0185] In this example, since the viscosity of water and gas is significantly lower than oil, then upon water or gas breakthrough the pilot pressure would rapidly and significantly increase. This will cause the valve to rapidly move towards its closed position for water and gas while remaining almost open for oil. This approach will thus not achieve more refined and controlled flow control. In table 1 below, Reynolds numbers (Re), friction factor (fo), pressure drop and boost are calculated for the 5.6 cP oil.

[0186] In another theoretical example, in a heavier oil reservoir, if viscosity drops from a 5cP oil to 0.5cP water, the control pressure acting against a valve may increase tenfold when transitioning from oil to water in a system using a laminar flow restrictor. As noted, one of the main consequences of this effect could be that the device rapidly moves from open towards the closed position when water becomes the flowing medium. This is undesirable. This can be contrasted with corresponding calculated results for a turbulent first flow restrictor in the form of a smooth conduit.

[0187] This is presented in Figures 8 and 9 which provides corresponding calculations for oil (0.82cP), water (0.46cP) and gas (0.02cP) flowing through a 3-meter-long, 1.77mm ID tube (first flow restrictor) with a very low surface roughness and a 0.7mm nozzle (second flow restrictor).

[0188] The first thing to note is that nearly all of the data points for oil and water are located within the turbulent region of the graph. For light oil (which is very common in the North Sea and Middle East), the example of Figures 8 and 9 shows the reduction in friction factor (fD) in the turbulent regime for all three phases (oil, water and gas), with an associated continual reduction in friction factor. Importantly, the change in friction factor (fD) is less extreme than for flow restrictor operating in the laminar regime.

[0189] The friction factor in the pilot line is 15% lower for water compared to oil due to the increase in Reynolds number (Re) following the smooth pipe friction line. This reduction in the pressure drop across the first flow restrictor causes a relative increase in the pressure drop across the second flow restrictor which increases the Pressure Boost (i.e. pressure in the pilot pressure region and pilot pressure chamber). The reduction in pressure drop for the turbulent flow restrictor is far less severe than that for the laminar flow restrictor previously described. This results in a more accurate and sensitive control of fluid flow. It allows production to be throttled and controlled, rather than being subjected to a binary (i.e. fully open or fully closed) and rapidly fluctuating valve. This greatly improves reliability, control and overall recovery performance.

[0190] Furthermore, the combination of increased density and lower viscosity increases the Reynolds number for water compared to vs oil. By moving into the turbulent regime for all phases, this method allows for effective control of water in light oil mainly due to its higher density compared to oil.

[0191] Figure 10 shows corresponding example Pressure Boost values that can be attained using an inlet diameter of 9.7mm and a valve member of 12mm. It can be seen that the relative change in Boost values is much lower than with the laminar flow restrictor, increasing control and facilitating production throttling. Figure 11 illustrates pilot flow rates versus pressure differential across the flow control system for oil (0.82cP), water (0.46cP) and gas (0.02cP). Figure 12 illustrates how the pressure drop across a restrictor varies with respect to Reynolds number. As can be seen, at Reynolds numbers below the critical Reynolds number (Rec), flow is laminar. In the laminar regime, the pressure drop AP varies linearly with viscosity p and velocity v. Above the critical Reynolds number, the flow becomes turbulent. In the turbulent regime, the pressure drop AP varies linearly with density p and with the square of velocity v. Accordingly, restrictors configured to impose substantially turbulent flow characteristics on fluid flowing therethrough have a pressure drop across them that varies with the square of velocity during operation.

[0192] It should be understood that the examples described herein are merely exemplary and that various modifications may be made thereto within the understanding of the skilled person, without departing from the scope of the present disclosure.

Claims

CLAIMS:

1. A downhole flow control system, comprising: a downhole flow control device, comprising: a body locatable within a wall of a tubular and defining a primary inlet and a primary outlet, wherein a primary flow path is defined between the primary inlet and the primary outlet; a valve member disposed within the body, the valve member being moveable in reverse first and second directions to selectively vary a flow area of the primary flow path; and a pilot pressure chamber for receiving a pilot pressure, the valve member being in pressure communication with the pilot pressure chamber such that pilot pressure may act to bias the valve member in one of the first and second directions; and a pilot pressure system for generating the pilot pressure, comprising: a secondary flow path: a fluid inlet for receiving a fluid into the secondary flow path and a fluid outlet for delivering the fluid from the secondary flow path; and a first flow restrictor and a second flow restrictor arranged in series in the secondary flow path and defining a pilot pressure region therebetween, the pilot pressure region arranged in pressure communication with the pilot pressure chamber of the downhole flow control device, wherein the first and second flow restrictors are each configured to impose substantially turbulent flow characteristics on fluid flowing through the secondary flow path, and wherein at least one of the first and second flow restrictors is configured to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the secondary flow path.

2. The downhole flow control device of claim 1 , wherein the first and second flow restrictors are each configured to impose flow with a Reynolds number greater than 2300.

3. The downhole flow control system of claim 1 or 2, wherein the first flow restrictor is configured to influence the pressure within the pilot pressure region as a function of the viscosity and density of the fluid flowing through the secondary flow path.

4. The downhole flow control device of any preceding claim, wherein the first and second flow restrictors are each configured such that pressure drop across the respective restrictor is proportional to the square of the velocity of fluid flowing therethrough.

5. The downhole flow control system of any preceding claim, wherein the first flow restrictor is a conduit.

6. The downhole flow control system of claim 5, wherein the first flow restrictor is configured such that pressure drop across the first flow restrictor is governed by the turbulent flow equation:2 t^Pn = f CD* - P * V — * L 1.J U2 D7. The downhole flow control system of claim 5 or claim 6, wherein the first flow restrictor is configured to operate outside of the complete turbulence regime.

8. The downhole flow control system of any of claims 5 to 7, wherein the conduit is provided in at least one of a coiled or serpentine form.

9. The downhole flow control system of any of claims 5 to 8, wherein the conduit comprises a substantially smooth surface.

10. The downhole flow control system of any of claims 5 to 9, wherein the relative pipe roughness of the conduit is less than 0.002, 0.001 , 5x1 O'4, 2x1 O'4, 10'4, 5x1 O'5, 10'5, 5x1 O'6or 10'6.11 . The downhole flow control system of any of claims 5 to 10, wherein the inside surface of the conduit comprises an electropolished surface.

12. The downhole flow control system of any one of claims 5 to 11 , wherein the conduit is coiled and has a curvature ratio of less than 0.02, wherein the curvature ratio is defined as the inner diameter of the conduit divided by the outer diameter of the coil.

13. The downhole flow control system of claim 12, wherein the conduit is a helical coil with an outside coil diameter of 140mm and an inner conduit diameter of 2mm; optionally the conduit has a length of 3m.

14. The downhole flow control system of any of the preceding claims, wherein the second flow restrictor is an orifice or valve.

15. The downhole flow control system of any of the preceding claims, wherein the first and second flow restrictors are configured to provide a Pressure Boost of at least 50% during use; wherein Pressure Boost is defined as the pressure drop across the second flow restrictor as a percentage of the combined pressure drop across the first and second restrictor; optionally the first and second flow restrictors are configured to provide a Pressure Boost of at least 50% for all properties of the fluid flowing through the secondary flow path during use.

16. The downhole flow control system of any of the preceding claims, wherein the pilot pressure system comprises a manifold associated with the fluid inlet, configured to circumscribe the tubular and comprising a plurality of circumferentially arranged inlet ports for receiving a downhole fluid into the manifold.

17. The downhole flow control system of any of the preceding claims, wherein the downhole flow control device comprises a manifold associated with the primary inlet, configured to circumscribe the tubular and comprising a plurality of circumferentially arranged inlet ports for receiving a downhole fluid into the manifold.

18. The downhole flow control system of any of the preceding claims, wherein the pilot pressure chamber is sealed.

19. The downhole flow control system of any of the preceding claims, wherein the downhole flow control system is configured such that there is substantially no flow between the pilot pressure region and the pilot pressure chamber.

20. The downhole flow control system of any preceding claim, configured such that the valve member does not close the primary flow path during use, such that the flow area of the primary flow path is always greater than zero during use.

21. The downhole flow control system of any preceding claim, further comprising a biasing member arranged to bias the valve member towards a closed arrangement for restricting flow through the primary flow path.

22. The downhole flow control system of claim 21 , wherein the biasing member is a diaphragm that surrounds the valve member.

23. The downhole flow control system of claim 22, wherein part of the diaphragm is sandwiched between parts of the valve member.

24. The downhole flow control system of any of claims 21 to 23, wherein the diaphragm is configured to exert a closing force on the valve member when the valve member is in a closed position.

25. The downhole flow control system of any preceding claim, wherein the diameter of the valve member is less than 20mm, for example 12mm, and the diameter of the primary inlet is less than 15mm, for example 9.7mm.

26. A pilot pressure system for generating a pilot pressure, comprising: a flow path: a fluid inlet for receiving a fluid into the flow path and a fluid outlet for delivering the fluid from the flow path; and a first flow restrictor and a second flow restrictor arranged in series in the flow path and defining a pilot pressure region therebetween, wherein the first and second flow restrictors are each configured to impose substantially turbulent flow characteristics on fluid flowing through the flow path, and wherein at least one of the first and second flow restrictors is configured to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the flow path.

27. A method of controlling fluid flow, the method comprising: providing: a downhole flow control device, comprising:a body locatable within a wall of a tubular and defining a primary inlet and a primary outlet, wherein a primary flow path is defined between the primary inlet and the primary outlet; a valve member disposed within the body, the valve member being moveable in reverse first and second directions to selectively vary a flow area of the primary flow path; and a pilot pressure chamber for receiving a pilot pressure, the valve member being in pressure communication with the pilot pressure chamber such that pilot pressure may act to bias the valve member in one of the first and second directions; and a pilot pressure system for generating the pilot pressure, comprising: a secondary flow path: a fluid inlet for receiving a fluid into the secondary flow path and a fluid outlet for delivering the fluid from the secondary flow path; and a first flow restrictor and a second flow restrictor arranged in series in the secondary flow path and defining a pilot pressure region therebetween, the pilot pressure region arranged in pressure communication with the pilot pressure chamber of the downhole flow control device, the method further comprising operating the first and second flow restrictors in a turbulent fluid flow regime to influence the pressure within the pilot pressure region as a function of a property of the fluid flowing through the secondary flow path.

28. A flow restrictor configured to influence the pressure drop across the flow restrictor during use as a function of the viscosity and density of the fluid flowing through the secondary flow path.

29. A method for providing a flow restrictor according to claim 28.