Determination method and determination method for seepage starting pressure gradient of low-permeability rock

Abstract

The invention provides a determination method and a determination method for a seepage starting pressure gradient of a low-permeability rock. The determination method comprises the following steps: determining whether a starting pressure gradient and a quasi-starting pressure gradient exist in the movement of a fluid in a seepage channel according to a first corresponding relationship among first parameters of the low-permeability rock; wherein the first parameters comprise the thickness h of weakly bound water on the surface of the low-permeability rock, the distance L from the boundary of the inner-layer weakly bound water and the outer-layer weakly bound water to the surface of the low-permeability rock and the minimum pore throat radius r of single seepage channels communicated with one another of the rock; the first corresponding relation comprises: if rgt; h, the starting pressure gradient and the quasi-starting pressure gradient do not exist; if Llt; rlt; h, the starting pressure gradient does not exist, but the quasi-starting pressure gradient exists; if rlt; l, the starting pressure gradient and the quasi-starting pressure gradient exist at the same time. According to the method, the rock-fluid property and the solid-liquid interface theory are combined, and whether the starting pressure gradient and the quasi-starting pressure gradient exist or not can be judged accurately.

Description

Technical Field

[0001] The present invention belongs to the field of CO2 geological storage and oil and gas field produced water reinjection, and particularly relates to a method for determining and measuring a low-permeability rock seepage starting pressure gradient. Background Art

[0002] Low-permeability rocks are widely distributed in various sedimentary basins, accounting for more than 80% of the total. Due to the low permeability, small pores, and significant fluid-rock interface effects of low-permeability rocks, the seepage pattern of formation water in such rocks usually does not conform to the classical Darcy's law. It is manifested as follows: (1) the flow velocity and pressure gradient have a nonlinear relationship, and the seepage curve consists of nonlinear segments and linear segments; (2) there is a starting pressure gradient. When the applied pressure gradient is greater than the starting pressure gradient, formation water can flow. The intersection of the reverse extension line of the straight segment (i.e., the linear segment) of the seepage curve and the pressure gradient coordinate axis is called the pseudo-starting pressure gradient.

[0003] As a key parameter for the non-Darcy flow characteristics of low-permeability rock fluids, the threshold pressure gradient plays an important role in guiding deep formation space utilization (such as CO2 geological storage and gas field produced water reinjection), oil and gas development, and geothermal water extraction and utilization. The magnitude of the threshold pressure gradient directly affects the seepage characteristics of the original formation water when CO2 or gas field produced water is injected into deep formations. It also determines the pressure dissipation characteristics of the reinjected gas field produced water and the injected and stored CO2, thereby affecting the injection of CO2 or gas field produced water. Furthermore, the threshold pressure gradient is a key parameter in the study of waterflooding in low-permeability reservoirs. It controls the seepage characteristics of fluids in low-permeability reservoirs and influences the recovery rate. To a certain extent, it reflects the difficulty of oilfield development. A clear understanding of the threshold pressure gradient and its influencing factors facilitates the formulation of oilfield development and subsequent adjustment plans. Given the importance of the threshold pressure gradient in disciplines such as hydrogeology and oil and gas geology, the determination methods, formation mechanisms, influencing factors, and empirical formulas related to permeability of the threshold pressure gradient for the seepage of low-permeability rock fluids have attracted considerable attention.

[0004] At present, most of the starting pressure gradient values ​​of low-permeability rocks are determined through indoor research. For example, patent documents CN206945450U and CN203658217U provide measuring devices for measuring starting pressure gradients. For example, documents such as "Unstable Rapid Measurement of Starting Pressure Gradient" (Song Fuquan, Acta Petrolei Sinica), "Experimental Study on Starting Pressure Gradient of Seepage in Low-Permeability Sandstone Reservoirs" (Lv Chengyuan, Petroleum Exploration and Development), and "A New Method for Measuring Starting Pressure of Low-Permeability Reservoirs" (Li Aifen, Journal of China University of Petroleum (Natural Science Edition)) use the steady-state flow-pressure difference method, the non-steady-state pressure reduction method, and the capillary balance method to measure the starting pressure gradient of water in compacted clay or dense rock, but have not established a theoretical determination method for the starting pressure gradient. Their measurement methods are less applicable to measuring the starting pressure gradient of ultra-low permeability rocks (such as shale and mudstone). ; For example, patent documents CN109557010B, CN110984970B, CN109387467B, CN109870396A, CN109580448A, and CN107676083A disclose experimental methods for starting pressure gradient or pseudo-starting pressure gradient; the above methods mostly use the pressure gradient during fluid flow as the starting pressure gradient. In the actual testing process, it is impossible to completely avoid the gas-water two-phase flow caused by the presence of residual gas in the rock pores, and the Jamin effect and breakthrough pressure phenomenon generated during the gas-water two-phase seepage cause large errors in the experimental results; when measuring dense-ultra-low permeability rocks, the error will be further increased when using the above method; secondly, the above method has the shortcomings of poor applicability and limited scope of application in simultaneously obtaining the starting pressure gradient and the pseudo-starting pressure gradient.

[0005] Therefore, it is necessary to provide a method for determining the start-up pressure gradient of seepage in low-permeability rocks to provide a guiding basis for accurately obtaining the parameters of the start-up pressure gradient and the pseudo-start-up pressure gradient. Summary of the Invention

[0006] The present invention provides a method for determining the seepage starting pressure gradient of low-permeability rocks. By combining rock-fluid properties with solid-liquid interface theory, the method can accurately determine whether a starting pressure gradient and a pseudo-starting pressure gradient exist in the rock, thereby providing a guiding basis for accurately obtaining the parameters of the starting pressure gradient and the pseudo-starting pressure gradient.

[0007] The present invention also provides a method for measuring the seepage starting pressure gradient of low-permeability rocks. The method has a wide application range, low test result accuracy, and is simple to operate.

[0008] A first aspect of the present invention provides a method for determining a low-permeability rock seepage starting pressure gradient, comprising the following steps:

[0009] Determine whether there is a starting pressure gradient and a pseudo-starting pressure gradient for the fluid movement in the seepage channel according to the first correspondence relationship between the first parameters of the low-permeability rock;

[0010] Among them, the first parameters include the thickness h of the weakly bound water on the surface of the low-permeability rock, the distance L from the boundary line between the inner-layer weakly bound water and the outer-layer weakly bound water to the surface of the low-permeability rock, and the minimum pore-throat radius r of a single interconnected seepage channel of the rock;

[0011] The first correspondence relationship includes: if r > h, there is no starting pressure gradient and pseudo-starting pressure gradient for the fluid movement in the seepage channel; if L < r < h, there is no starting pressure gradient for the fluid movement in the seepage channel, but there is a pseudo-starting pressure gradient; if r < L, there are both a starting pressure gradient and a pseudo-starting pressure gradient for the fluid movement in the seepage channel.

[0012] For the determination method as described above, among them, the calculation formula for the thickness h of the weakly bound water on the rock surface is as shown in Equation 1:

[0013]

[0014] In the formula: h is the thickness of the weakly bound water, with the unit of nm; ε0 is the vacuum permittivity, with the unit of C 2 J -1 m -1 , ε r is the relative permittivity of water, k B is the Boltzmann constant, with the unit of J / K, e is the elementary charge, with the unit of C, z is the ionic valence state, T is the temperature, with the unit of K, n ∞ is the ionic concentration, with the unit of mol / L.

[0015] For the determination method as described above, among them, the calculation formula for the distance L from the boundary line between the inner-layer weakly bound water and the outer-layer weakly bound water to the rock surface is as shown in Equation 2:

[0016]

[0017] In Equation 2: L is the distance from the boundary line between the inner-layer weakly bound water and the outer-layer weakly bound water to the rock surface, with the unit of nm; A 11 is the Hamaker constant of the rock, with the unit of J, A 22 is the Hamaker constant of the fluid, with the unit of J, K s is the structural force constant, with the unit of N / m 2 , λ is the characteristic attenuation length of water, with the unit of nm.

[0018] In the second aspect of the present invention, a method for measuring the seepage starting pressure gradient of a low-permeability rock is provided, including the following steps:

[0019] Determine whether there is a starting pressure gradient and a pseudo-starting pressure gradient in the flow of fluid in the seepage channel using the method described in the first aspect;

[0020] If there is a starting pressure gradient and a pseudo-starting pressure gradient when the fluid moves in the seepage channel, the starting pressure gradient and the pseudo-starting pressure gradient when the fluid moves in the seepage channel are determined according to the pressure gradient-flow velocity relationship of the rock.

[0021] In the above-mentioned determination method, the method for obtaining the pressure gradient-flow velocity relationship of the rock comprises the following steps:

[0022] The water-saturated low-permeability rock sample is placed in a core holder;

[0023] The ring pressure and temperature of the low permeability rock sample wrapped in the core holder are set according to the pressure and temperature of the reservoir where the low permeability rock sample is located;

[0024] The low permeability rock sample is displaced at different injection pressures to obtain a pressure gradient-flow velocity relationship.

[0025] The measurement method as described above, wherein the pressure gradient-flow velocity relationship of the rock is obtained by using a low-permeability rock seepage start-up pressure gradient measurement device;

[0026] The low permeability rock seepage starting pressure gradient measuring device at least comprises a core holder, a liquid storage tank, a constant speed and constant pressure pump, a vacuum pump, an annular pressure injection pump, a constant temperature box, a pressure gauge, an electronic balance, and a capillary flowmeter;

[0027] The inlet end of the core holder is connected to a liquid storage tank and a vacuum pump respectively, the outlet end of the core holder is connected to an electronic balance and a capillary flowmeter respectively, and the liquid storage tank is connected to a constant speed and constant pressure pump;

[0028] The inlet and outlet ends of the core holder are respectively provided with pressure gauges;

[0029] The annular pressure injection pump is used to simulate the pressure environment of the reservoir where the low permeability rock sample is located;

[0030] The constant temperature box is used to simulate the temperature of the reservoir where the low-permeability rock sample is located.

[0031] The measurement method as described above, before performing displacement treatment at different injection pressures, further includes the following steps: starting a vacuum pump to evacuate the pipeline from the outlet of the liquid storage tank to the inlet end of the core holder, and closing the vacuum pump after the vacuuming is completed.

[0032] The measurement method described above, wherein the displacement treatment is performed at different injection pressures, comprises the following steps: starting a constant speed constant pressure pump, performing a first displacement treatment at a first injection pressure P1, wherein P1 is the highest pressure of the entire displacement, so as to remove residual gas in the device, and after the outlet port continuously discharges liquid without generating bubbles, switching the outlet port to an electronic balance, recording the first liquid outlet flow rate V1 of the fluid, and then gradually reducing the injection pressure at a first interval, and after the outlet port continuously discharges liquid without generating bubbles, recording the injection pressures P1 to P1 respectively. n and the flow rate at the outlet V1~V n ;

[0033] When the flow rate at the outlet is less than 1×10 -6 mL / s, turn off the constant speed and constant pressure pump and switch the outlet to the capillary flowmeter to allow the fluid to continue flowing. When the inlet pressure remains unchanged and the fluid level in the capillary flowmeter does not move significantly, record the inlet pressure Pz and the length of the rock sample.

[0034] According to the injection pressure P1~P n and the flow rate at the outlet V1~V n , obtaining a pressure gradient-flow velocity curve; performing linear fitting on the linear segment of the pressure gradient-flow velocity curve to obtain a linear curve; determining the simulated starting pressure gradient of the fluid when seeping through the low permeability rock sample based on the linear curve and the values ​​on the pressure gradient coordinate axis;

[0035] The starting pressure gradient of the fluid during seepage through the low permeability rock sample is determined according to the ratio of the inlet pressure Pz to the length of the low permeability rock sample.

[0036] In the measurement method as described above, the first injection pressure is 5 MPa less than the ring pressure.

[0037] The measurement method as described above, wherein the first interval is 0.1 MPa to 1 MPa.

[0038] The implementation of the present invention has at least the following beneficial effects:

[0039] The method for determining the start-up pressure gradient of seepage in low-permeability rocks provided by the present invention combines rock pore characteristics, rock surface properties, fluid properties, and solid-liquid interface theory to establish a method for determining whether the start-up pressure gradient and pseudo-start-up pressure gradient of the seepage channel in low-permeability rocks exist. This deepens the theoretical understanding of the non-Darcy seepage characteristics of low-permeability rocks and the causes of the start-up pressure gradient, and helps provide a guiding basis for accurately obtaining the parameters of the start-up pressure gradient and pseudo-start-up pressure gradient.

[0040] The invention provides a method for measuring the seepage starting pressure gradient of low-permeability rocks. The method has a wide application range, high test result accuracy, and is easy to operate. Brief Description of the Drawings

[0041] Figure 1 It is a schematic diagram of the distribution of weakly bound water in pore throats in an embodiment of the present invention;

[0042] Figure 2 It is the sandstone pore size distribution curve in Example 1 of the present invention;

[0043] Figure 3 It is the pressure gradient - flow velocity curve (seepage curve) of fluid in sandstone in Example 1 of the present invention. Detailed Embodiment

[0044] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0045] The present invention provides a method for determining the seepage starting pressure gradient of low - permeability rocks, including the following steps: determining whether there is a starting pressure gradient and a quasi - starting pressure gradient for the movement of fluid in the seepage channel according to the first corresponding relationship between the first parameters of the low - permeability rock; wherein, the first parameters include the thickness h of the weakly bound water on the surface of the low - permeability rock, the distance L from the boundary between the inner - layer weakly bound water and the outer - layer weakly bound water to the surface of the low - permeability rock, and the minimum pore - throat radius r of a single interconnected seepage channel in the rock; the first corresponding relationship includes: if r > h, there is no starting pressure gradient and quasi - starting pressure gradient for the movement of fluid in the seepage channel; if L < r < h, there is no starting pressure gradient for the movement of fluid in the seepage channel, but there is a quasi - starting pressure gradient; if r < L, there are both a starting pressure gradient and a quasi - starting pressure gradient for the movement of fluid in the seepage channel.

[0046] Wherein, the fluid refers to the fluid used for subsequent displacement, and the seepage channel refers to the channel formed by the rock itself. The starting pressure gradient is the critical value that must be reached to overcome the maximum binding energy of the bound water in the micro - pores, and the quasi - starting pressure gradient is generated due to the electro - viscous effect caused by the movement of formation water in the micro - pores.

[0047] In the present invention, whether there is a starting pressure gradient and a quasi - starting pressure gradient for the movement of fluid in the seepage channel depends on the corresponding relationship of h, L and r. As Figure 1 shown, it is a schematic diagram of the distribution of weakly bound water in the pore throats of low - permeability rocks.

[0048] The minimum pore radius r refers to the radius of the narrowest point in the interconnected pores of a rock. The size of this minimum pore radius r plays a decisive role in the permeability of low-permeability rocks. Whether oil, gas, and water can flow out of low-permeability rocks under a certain pressure differential depends on the size of this minimum pore radius r. This minimum pore radius r can be determined through a combination of methods, including high-pressure mercury injection, low-pressure nitrogen adsorption, and low-pressure CO2 adsorption.

[0049] In view of the strong heterogeneity of pore size and shape of low-permeability rocks, the minimum pore radius r of the present invention is taken as half of the pore value corresponding to when the cumulative pore volume in the rock pore size distribution curve accounts for 10% of the total pore volume. For example, when the cumulative pore volume in the low-permeability rock pore size distribution curve accounts for 10% of the total pore volume, the corresponding pore value is d10, then r = 0.5d10.

[0050] The interaction between fluids and low-permeability rocks forms strongly bound water and weakly bound water on the surface of low-permeability rocks. Strongly bound water is the first layer of water molecules in the rock, adsorbed to the rock surface by the different polarities of water molecules, and possesses a strong binding affinity with the rock molecules. Weakly bound water is distributed in several layers beyond the strongly bound water on the rock surface. Weakly bound water also adheres to the rock pore surface through the adhesion force generated by the polar electrostatic forces of the molecules. Different rocks have different adhesion forces, and the thickness of weakly bound water varies. For the same rock type, the greater the thickness of weakly bound water, the weaker the rock's control over the outer layer of water molecules. Compared to free water, bound water exhibits shear flow characteristics, requiring a specific external force to move. Strongly bound water is typically less than 2 nm thick and has little impact on fluid flow. However, weakly bound water, with a thickness ranging from a few nanometers to 1 micron, has a significant impact on fluid flow.

[0051] Among them, weakly bound water can be divided into inner weakly bound water and outer weakly bound water. The inner weakly bound water is closer to the low-permeability rock, and the outer weakly bound water is located outside the inner weakly bound water. The distance from the dividing line between the two to the surface of the low-permeability rock is L.

[0052] The present invention combines the rock pore size characteristics, rock surface properties and fluid properties to obtain the first corresponding relationship of the first parameter, thereby determining whether a starting pressure gradient and a simulated starting pressure gradient exist, which helps to provide a guiding basis for accurately obtaining the starting pressure gradient and the simulated starting pressure gradient parameters.

[0053] In the first parameter, the following parameters are specifically acquired: (1) acquisition of low permeability rock pore characteristic parameters; (2) acquisition of low permeability rock surface property parameters; (3) acquisition of fluid property parameters.

[0054] The method for obtaining rock pore size characteristic parameters includes the following steps: determining the pore size distribution curve of low-permeability rocks using high-pressure mercury intrusion injection (HIP) and obtaining pore structure parameters such as the average pore size, most probable pore size, and limiting pore size based on the pore size distribution curve. While high-pressure mercury intrusion injection is typically suitable for testing pores with diameters greater than 6 nm, for dense rocks such as shale and limestone, low-pressure nitrogen adsorption and low-pressure carbon dioxide adsorption methods should be combined to systematically characterize the pore structure and pore size distribution of rocks with pore sizes between 0.37 nm and 6 nm. Based on the pore size distribution curve of low-permeability rocks, the minimum pore radius r of a single interconnected seepage channel in the rock is obtained.

[0055] The surface property parameters of low-permeability rocks include the contact angle characteristic parameters and mineral composition of the low-permeability rocks. The hydrophilicity of low-permeability rocks determines the strength of the low-permeability rock-fluid rock interaction and plays a key role in the existence and magnitude of the starting pressure gradient. A contact angle meter is used to measure the contact angles of pure water and salt solutions at room temperature and pressure to characterize the hydrophilicity of the low-permeability rocks. XRD is also used to quantitatively analyze the mineral composition of the low-permeability rocks. Based on the fluid property parameters, contact angle characteristic parameters, and mineral composition, combined with the DLVO theory, the distance L from the boundary between the inner and outer weakly bound water layers to the low-permeability rock surface and the thickness h of the weakly bound water are determined.

[0056] In some embodiments, the thickness h of weakly bound water on the rock surface is calculated as follows:

[0057]

[0058] Where: h is the thickness of weakly bound water (nm); ε0 is the vacuum dielectric constant (C 2 J -1 m -1 ), ε r is the relative dielectric constant of water (dimensionless), k B is the Boltzmann constant (J / K), e is the elementary charge (C), z is the ion valence (dimensionless), T is the temperature (K), n ∞ is the ion concentration (mol / L).

[0059] In some embodiments, the distance L between the boundary line between the inner weakly bound water layer and the outer weakly bound water layer and the rock surface is calculated as shown in Formula 2:

[0060]

[0061] Where: L is the distance between the boundary line between the inner weakly bound water layer and the outer weakly bound water layer and the rock surface (nm); A 11 is the Hamaker constant of rock (J), A 22 is the Hamaker constant of the fluid (J), K sis the structural force constant (N / m 2 ), λ is the characteristic attenuation length of water (nm). Since rocks are a collection of multiple minerals, the Hamaker constant of rocks can be obtained by weighted average of the mineral composition of the rocks; K s is the structural force constant, which is 2×10 8 Pa, for hydrophilic surfaces, the structural force constant can be obtained by multiplying the contact angle test value of the rock by the attenuation coefficient, and λ is the characteristic attenuation length of water, and its value is usually 1.5nm.

[0062] The second aspect of the present invention provides a method for determining the seepage starting pressure gradient of low-permeability rocks, comprising the following steps: using the method provided in the first aspect to determine whether a starting pressure gradient and a pseudo-starting pressure gradient exist when the fluid moves in the seepage channel; if a starting pressure gradient and a pseudo-starting pressure gradient exist when the fluid moves in the seepage channel, then determining the starting pressure gradient and the pseudo-starting pressure gradient of the fluid moving in the seepage channel based on the pressure gradient-flow velocity relationship of the rock.

[0063] In some embodiments, the method for obtaining the pressure gradient-flow velocity relationship of rock includes the following steps: placing a saturated low permeability rock sample on a core holder; setting the annular pressure and temperature of the low permeability rock sample wrapped in the core holder according to the pressure and temperature of the reservoir where the low permeability rock sample is located; and displacing the low permeability rock sample under different injection pressures to obtain the pressure gradient-flow velocity relationship.

[0064] Among them, the low-permeability rock samples are standard cylindrical samples with the following dimensions: diameter of 25 mm and length of 20 to 50 mm.

[0065] The low-permeability rock sample is saturated with water under high pressure to obtain a fully saturated low-permeability rock sample. The fluid used in the saturation treatment is consistent with the fluid used in the subsequent displacement treatment, and the saturation treatment time is not less than 24 hours.

[0066] The water-saturated low-permeability rock sample is placed on a core holder. Specifically, the two sections of the core holder are respectively arranged opposite to the two ends of the low-permeability rock sample to achieve clamping.

[0067] In some embodiments, a low-permeability rock seepage starting pressure gradient measuring device is used to obtain the pressure gradient-flow rate relationship of the rock; the low-permeability rock seepage starting pressure gradient measuring device includes at least a core holder, a liquid storage tank, a constant speed and constant pressure pump, a vacuum pump, an annular pressure injection pump, a constant temperature box, a pressure gauge, an electronic balance, and a capillary flowmeter; the inlet end of the core holder is respectively connected to the liquid storage tank and the vacuum pump, and the outlet end of the core holder is respectively connected to the electronic balance and the capillary flowmeter, and the liquid storage tank is connected to the constant speed and constant pressure pump; pressure gauges are respectively provided at the inlet and outlet ends of the core holder; the annular pressure injection pump is used to simulate the pressure environment of the reservoir where the low-permeability rock sample is located; and the constant temperature box is used to simulate the temperature of the reservoir where the low-permeability rock sample is located.

[0068] Among them, the core holder is the core component of the measuring device, which is used to place the low-permeability rock sample; the annular pressure injection pump is used to simulate the stress environment of the actual formation where the low-permeability rock sample is located; the constant temperature and constant pressure pump is used to inject the fluid into the low-permeability rock sample in the core holder in a constant pressure or constant flow mode, so as to realize the study of fluid seepage law under different injection pressures; the vacuum pump is used to evacuate the test device before injecting the fluid to avoid the air in the pipeline from affecting the seepage experiment; the pressure gauge is used to monitor the pressure at both ends of the core holder in real time, and the pressure gauge needs to be accurate to four decimal places (0.0001MPa); the electronic balance and capillary flowmeter are used to measure the flow rate of the fluid after it flows out of the outlet of the core holder under different injection pressures, and the movement of bubbles in the capillary flowmeter is used to determine whether there is fluid flow in the low-permeability rock sample in the core holder, so as to determine the starting pressure and starting pressure gradient. The electronic balance is used to test the high flow rate (>10 -4 mL / s), capillary flowmeter is used to test low-ultra-low flow (10 -5 ~10 -8 mL / s); the thermostat is used to simulate the actual formation environment. By setting the thermostat temperature, the start-up pressure gradient measurement experiment under different formation temperatures can be carried out. The fluid storage tank contains the fluid used for displacement.

[0069] The inlet of the fluid reservoir is connected to a constant-speed, constant-pressure pump, the outlet to the inlet of the core holder, the vacuum pump to the inlet of the core holder, and the outlet of the core holder to an electronic balance or capillary flowmeter. Valves are installed at the inlet and outlet of the fluid reservoir, the outlet of the vacuum pump, and the inlet and outlet of the core holder to control the opening and closing of the pipelines. An annular pressure injection pump is connected to the center of the core holder. The fluid reservoir, core holder, electronic balance, and capillary flowmeter are placed in a constant temperature chamber to simulate the annular pressure and temperature of the actual formation.

[0070] The low-permeability rock seepage starting pressure gradient measuring device of the present invention has a simple composition and is easy to operate. The fluid metering system at the outlet end combines two metering methods, an electronic balance and a capillary flowmeter, to solve the problem of narrow measurement range of a single method. The pressure gradient test of the device can meet the testing requirements from ultra-low permeability cap rock (shale) to low permeability reservoir rock (sandstone), further improving the scope of application of the method and device.

[0071] Before performing displacement treatment at different injection pressures, the process also includes: setting the annular pressure and thermostat temperature of the low-permeability rock sample wrapped in the core holder according to the pressure and temperature of the reservoir where the low-permeability rock is located; then starting the annular pressure injection pump to apply the annular pressure; and starting the thermostat to stabilize for 24 hours so that the stress state and temperature of the low-permeability rock sample reach an approximate equilibrium state.

[0072] In some embodiments, before performing the displacement treatment at different injection pressures, the following steps are further included: starting a vacuum pump to evacuate the pipeline from the outlet of the liquid storage tank to the inlet of the core holder, and then turning off the vacuum pump after the evacuation is completed. This evacuation prevents the formation of two-phase flow between air and solution in the pipeline, which could cause testing errors.

[0073] In some embodiments, performing displacement treatment at different injection pressures includes the following steps: starting a constant speed constant pressure pump, performing a first displacement treatment at a first injection pressure P1, after the flow rate at the outlet is stabilized, switching the outlet to an electronic balance, recording the first outlet flow rate V1 of the fluid, and then gradually reducing the injection pressure at a first interval, after the flow rate at the outlet is stabilized, recording the injection pressures P1 to P2 respectively. n and the flow rate at the outlet V1~V n ; When the flow rate at the outlet is less than 1×10 -6 mL / s, turn off the constant speed and pressure pump, switch the outlet to the capillary flowmeter, and allow the fluid to continue to flow. When the pressure at the inlet remains unchanged and the fluid level in the capillary flowmeter does not move significantly, record the inlet pressure Pz and the length of the rock sample. n and the flow rate at the outlet V1~V n , obtain the pressure gradient-flow velocity curve; perform linear fitting on the linear segment of the pressure gradient-flow velocity curve to obtain a linear curve; determine the pseudo-starting pressure gradient of the fluid when it seeps through the low-permeability rock sample based on the linear curve and the values ​​on the injection pressure coordinate axis; determine the starting pressure gradient of the fluid when it seeps through the low-permeability rock sample based on the ratio of the inlet pressure Pz to the length of the low-permeability rock sample.

[0074] Start a constant-speed, constant-pressure pump and inject the fluid from the reservoir into the core holder at constant pressure. Connect the outlet to a capillary flowmeter. Set the initial displacement pressure (i.e., the first injection pressure) as the maximum displacement pressure. Continue displacement in high-pressure mode until no bubbles escape from the capillary flowmeter. At this point, the flow rate at the core holder outlet stabilizes. Switch the outlet to an electronic balance and use it to measure the steady-state flow rate at this injection pressure. Then, gradually reduce the injection pressure at the first interval and measure the flow rate (i.e., the core seepage velocity) at each corresponding injection pressure.

[0075] Specifically, the outlet end of the core holder is connected to the capillary flowmeter, the constant speed and constant pressure pump is started, and the first displacement treatment is performed at the first injection pressure P1. After no bubbles flow out of the capillary flowmeter and the flow rate at the outlet end is stable, the outlet end is switched to the electronic balance, and the first outlet flow rate V1 of the fluid is recorded. Then, the second displacement treatment is performed at the second injection pressure P2 (P2 < P1). After no bubbles flow out of the capillary flowmeter and the flow rate at the outlet end is stable, the outlet end is switched to the electronic balance, and the second outlet flow rate V2 of the fluid is recorded. And so on, each injection pressure P1 to P n Flow velocity at the lower outlet V1~V n .

[0076] When the flow rate at the outlet is less than 1×10 -6 mL / s, turn off the constant speed and constant pressure pump, switch the outlet to the capillary flowmeter, and let the measured fluid continue to flow under the injection pressure. When the pressure at the inlet remains basically unchanged and the liquid level in the capillary flowmeter does not move significantly, the ratio of the inlet pressure Pz to the length of the low permeability rock sample is the starting pressure gradient.

[0077] Arrange each injection pressure P1~P n Steady-state flow velocity V1~V at the lower outlet n , draw the pressure gradient-flow velocity curve (seepage curve), fit the fluid seepage equation of the low permeability rock sample, perform linear fitting on the linear segment of the seepage curve, and the value of the pressure gradient axis corresponding to the obtained linear equation is the simulated starting pressure gradient of the fluid in the low permeability rock sample.

[0078] Specifically, each injection pressure P1~P n Pressure gradient under the test = (injection pressure - outlet pressure) / rock sample length. The outlet pressure is standard atmospheric pressure, and the pressure gradient is the ratio of injection pressure to the length of the low-permeability rock sample.

[0079] The measurement method provided by the present invention can establish a mathematical model of the starting pressure gradient / pseudo-starting pressure gradient and the rock permeability, porosity and other physical parameters by changing the rock type and combining the rock permeability, porosity and other physical parameters.

[0080] In some embodiments, the first injection pressure is 5 MPa less than the confining pressure. That is, the first injection pressure P1 is 5 MPa less than the confining pressure applied to the low-permeability rock sample. Further, the first injection pressure P1 is 5 MPa less than the pressure of the reservoir where the low-permeability rock sample is located.

[0081] In some embodiments, the first interval is 0.1 MPa to 1 MPa. Specifically, the single-step pressure reduction value is large in the high-pressure stage and small in the low-pressure stage.

[0082] The present invention couples the high-pressure displacement method, the steady-state pressure gradient - flow rate method, and the unsteady pressure reduction method, and proposes a method for simultaneously obtaining the seepage starting pressure gradient and the pseudo-starting pressure gradient of low-permeability rock fluids. Moreover, the influence of residual air in the pipeline and the core is completely excluded in the high-pressure displacement stage, avoiding gas-liquid two-phase flow during the test, and significantly improving the accuracy and reliability of the test results.

[0083] The present invention will be further described below through specific examples and comparative examples.

[0084] Example 1

[0085] In this example, taking the low-permeability sandstone in the Qaidam Basin as an example, the starting pressure gradient and the pseudo-starting pressure gradient of deionized water are tested to verify the determination method and the measurement method of the above pressure gradient.

[0086] I. The determination method of this example includes the following steps:

[0087] (1) Obtain the pore size characteristic parameters, surface property parameters, and fluid property parameters of the low-permeability sandstone; the pore size distribution curve of the sandstone is as Figure 2 shown, the pore size structure parameters are shown in Table 1, and the mineral composition of the sandstone is obtained by XRD as shown in Table 2 (the Hamaker constants of common minerals are shown in Table 3); the contact angle of the low-permeability sandstone is about 28°, which is not completely wetting, and the structural force constant is taken as 6×10 7 Pa. Substituting into Equation 1 and Equation 2 for calculation, it can be known that the distance L from the boundary between the inner-layer weakly bound water and the outer-layer weakly bound water of the sandstone to the rock surface is 17.50 nm, the thickness h of the weakly bound water is 960 nm, and the minimum pore throat radius r of a single interconnected seepage channel of the rock is taken as 11.2 nm;

[0088] (2) According to the corresponding relationship of r < L, determine that there are starting pressure gradient and pseudo-starting pressure gradient for the seepage of deionized water in this sandstone. [[ID=~32]]

[0089] Second, the low-permeability rock seepage starting pressure gradient measurement device used in this embodiment includes at least a core holder, a liquid reservoir, a constant-speed and constant-pressure pump, a vacuum pump, an annular pressure injection pump, a thermostat, a pressure gauge, an electronic balance, and a capillary flowmeter. The inlet of the liquid reservoir is connected to the constant-speed and constant-pressure pump, the outlet of the liquid reservoir is connected to the inlet of the core holder, the vacuum pump is connected to the inlet of the core holder, and the outlet of the core holder is connected to an electronic balance or a capillary flowmeter. Valves are provided at the inlet and outlet of the liquid reservoir, the outlet of the vacuum pump, and the inlet and outlet of the core holder to control the opening and closing of the pipelines. The annular pressure injection pump is connected to the middle of the core holder. The liquid reservoir, core holder, electronic balance, and capillary flowmeter are placed in the thermostat to simulate the annular pressure and temperature of the actual formation. Pressure gauges are provided at the inlet and outlet of the core holder.

[0090] 3. The determination method of this embodiment includes the following steps:

[0091] (1) Process the sandstone into a standard cylindrical sample with a diameter of 25 mm and a length of 20 mm (i.e., sandstone sample);

[0092] (2) Pre-saturate the sandstone sample with water under high pressure. The fluid used for saturation should be consistent with the fluid used in subsequent displacement experiments. The saturation time should be no less than 24 hours.

[0093] (3) The saturated sandstone sample is placed in a core holder. The inlet of the core holder is connected to a liquid storage tank (containing deionized water) and a constant speed and constant pressure pump, and the outlet is connected to an electronic balance and a capillary flowmeter respectively;

[0094] (4) According to the pressure and temperature of the reservoir where the sandstone is located, the ring pressure of the sandstone to be tested, which is wrapped in the core holder, is set to 9 MPa and the constant temperature box is set to 30°C. After the ring pressure is applied and the constant temperature box is opened, it is stabilized for 24 hours to allow the stress state and temperature of the sandstone to reach an approximate equilibrium state;

[0095] (5) Turn on the vacuum pump and evacuate the pipeline from the outlet of the liquid storage tank to the inlet of the clamp to avoid the formation of two-phase flow between the air and the solution in the pipeline, which may cause test errors;

[0096] (6) Turn off the vacuum pump and turn on the constant speed and constant pressure pump to inject the solution in the liquid storage tank into the core holder in constant pressure mode. Set the initial displacement pressure (first injection pressure) as the highest displacement pressure of the entire test process, which is 4 MPa. Continue displacement in this high pressure mode until no bubbles flow out of the capillary flowmeter. At this time, switch the outlet end to the electronic balance to measure the steady-state flow rate under this injection pressure.

[0097] (7) The injection pressure is gradually reduced to 4 MPa, 3.3 MPa, 2.6 MPa, 2 MPa, 5 MPa, 1 MPa, and 0.6 MPa, and the fluid velocity (in cm / s) is measured at each injection pressure. The pressure gradient corresponding to each injection pressure = injection pressure / rock sample length, and the unit of pressure gradient is MPa / cm.

[0098] (8) After the displacement experiment with an injection pressure of 0.6 MPa was completed, the water injection pump was turned off and the outlet was switched to a capillary flowmeter. Deionized water continued to flow under the residual pressure. When no flow occurred, the corresponding inlet pressure Pz was 0.027 MPa. The ratio of the inlet pressure Pz to the length of the rock sample was used as the starting pressure gradient, which was 0.0135 MPa / cm.

[0099] (9) Draw the pressure gradient-flow rate curve as shown in Figure 3 As shown, the simulated starting pressure gradient G can be obtained by fitting the linear segment. P It is 0.252MPa / cm.

[0100] Table 1 Pore structure parameters of sandstone

[0101]

[0102] Table 2 Mineral composition of sandstone

[0103]

[0104] In Table 2, the main components of clay are illite, kaolinite and montmorillonite.

[0105] Table 3 Hamaker constants of common minerals

[0106] name Hamaker constant value quartz <![CDATA[6.8×10 -20 J]]> calcite <![CDATA[1.2×10 -19 J]]> dolomite <![CDATA[1.35×10 -19 J]]> Illite <![CDATA[1.8×10 -19 J]]> Kaolinite <![CDATA[1.67×10 -19 J]]> Montmorillonite <![CDATA[1.98×10 -19 J]]> water <![CDATA[3.7×10 -20 J]]>

[0107] It can be seen from this that (1) the present invention combines the rock pore characteristics, rock surface properties, fluid properties and the classic solid-liquid interface theory DLVO theory to establish a method for determining whether the start-up pressure gradient and pseudo-start-up pressure gradient of the seepage channel in low-permeability rocks exist, deepening the theoretical understanding of the non-Darcy seepage characteristics of low-permeability rocks and the causes of the start-up pressure gradient.

[0108] (2) The present invention couples the high-pressure displacement method, the steady-state pressure gradient-flow method and the non-steady-state pressure reduction method, and proposes a method for simultaneously obtaining the seepage starting pressure gradient and the pseudo-starting pressure gradient of low-permeability rock fluid. In addition, the high-pressure displacement stage completely eliminates the influence of residual air in the pipeline and the core, avoids gas-liquid two-phase flow during the test process, and significantly improves the accuracy and credibility of the test results.

[0109] (3) The experimental device provided by the present invention has a simple composition and is easy to operate. The fluid metering system at the outlet combines two metering methods, an electronic balance and a capillary flowmeter, which solves the problem of narrow measurement range of a single method. The pressure gradient test of the device can meet the testing requirements from ultra-low permeability cap rock (shale) to low permeability reservoir rock (sandstone), further improving the scope of application of the method and device.

[0110] The above describes in detail the preferred embodiments of the present invention and their experimental verification. It should be understood that a person skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solution that can be derived by a person skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art shall be within the scope of protection of the present invention.

Claims

1. A method for determining the starting pressure gradient of seepage in low-permeability rocks, characterized in that: The steps include: Determine whether there is an initial pressure gradient and a pseudo-initial pressure gradient for the fluid movement in the seepage channels according to the first correspondence relationship between the first parameters of the low-permeability rock; Among them, the first parameters include the thickness h of the weakly-bound water on the surface of the low-permeability rock, the distance L from the boundary line between the inner-layer weakly-bound water and the outer-layer weakly-bound water to the surface of the low-permeability rock, and the minimum pore-throat radius r of a single interconnected seepage channel in the rock; The first correspondence relationship includes: if r > h, there is no initial pressure gradient and pseudo-initial pressure gradient for the fluid movement in the seepage channels; if L < r < h, there is no initial pressure gradient for the fluid movement in the seepage channels, but there is a pseudo-initial pressure gradient; if r < L, there are both an initial pressure gradient and a pseudo-initial pressure gradient for the fluid movement in the seepage channels.

2. The determination method according to claim 1, wherein: The calculation formula for the thickness h of the weakly-bound water on the rock surface is as shown in Equation 1: In formula 1, h is the thickness of weakly bound water, in nm; ε0 is the vacuum dielectric constant, in C 2 J -1 m -1 , ε r is the relative dielectric constant of water, k B is the Boltzmann constant in J / K, e is the elementary charge in C, z is the ion valence, T is the temperature in K, n ∞ is the ion concentration, in mol / L.

3. The determination method according to claim 1 or 2, characterized in that: The calculation formula for the distance L between the boundary line of the inner-layer weakly-bound water and the outer-layer weakly-bound water and the rock surface is as shown in Equation 2: In formula 2, L is the distance between the boundary line between the inner weakly bound water layer and the outer weakly bound water layer and the rock surface, in nm; A 11 is the Hamaker constant of the rock, in J, A 22 is the Hamaker constant of the fluid, in J, K s is the structural force constant, in N / m 2 , λ is the characteristic attenuation length of water, in nm.

4. A method for measuring the seepage starting pressure gradient of low permeability rock, characterized in that: The steps include: Use the method described in any one of claims 1-3 to determine whether there is an initial pressure gradient and a pseudo-initial pressure gradient for the fluid movement in the seepage channels; If there are an initial pressure gradient and a pseudo-initial pressure gradient for the fluid movement in the seepage channels, determine the initial pressure gradient and the pseudo-initial pressure gradient for the fluid movement in the seepage channels according to the pressure gradient - flow rate relationship of the rock.

5. The measuring method according to claim 4, wherein The method for obtaining the pressure gradient - flow rate relationship of the rock includes the following steps: Place the water-saturated low-permeability rock sample in a core holder; Set the confining pressure and temperature of the low-permeability rock sample wrapped in the core holder according to the pressure and temperature of the reservoir where the low-permeability rock sample is located; Displace the low-permeability rock sample at different injection pressures to obtain the pressure gradient - flow rate relationship.

6. The measuring method according to claim 4 or 5, characterized in that Use a device for measuring the initial pressure gradient of low-permeability rock seepage to obtain the pressure gradient - flow rate relationship of the rock; The device for measuring the initial pressure gradient of low-permeability rock seepage at least includes a core holder, a liquid storage tank, a constant-speed and constant-pressure pump, a vacuum pump, a confining pressure injection pump, a constant-temperature box, a pressure gauge, an electronic balance, and a capillary flowmeter; The inlet end of the core holder is respectively connected to the liquid storage tank and the vacuum pump, the outlet end of the core holder is respectively connected to the electronic balance and the capillary flowmeter, and the liquid storage tank is connected to the constant-speed and constant-pressure pump; Pressure gauges are respectively arranged at the inlet end and the outlet end of the core holder; The confining pressure injection pump is used to simulate the pressure environment of the reservoir where the low-permeability rock sample is located; The constant-temperature box is used to simulate the temperature of the reservoir where the low-permeability rock sample is located.

7. The measuring method according to claim 6, wherein Before displacement at different injection pressures, the following steps are further included: Start the vacuum pump, evacuate the pipeline from the outlet of the liquid storage tank to the inlet end of the core holder, and close the vacuum pump after the evacuation ends.

8. The measuring method according to claim 6 or 7, characterized in that The displacement under different injection pressures includes the following steps: starting a constant speed constant pressure pump, performing the first displacement process at a first injection pressure P1, where P1 is the highest pressure of the entire displacement to remove residual gas in the device; after the outlet continuously discharges liquid without generating bubbles, switching the outlet to an electronic balance, recording the first outlet flow rate V1 of the fluid, and then gradually reducing the injection pressure at a first interval, and after the outlet continuously discharges liquid without generating bubbles, recording the injection pressures P1 to P1 respectively. n and the flow rate at the outlet V1~V n ; When the flow rate at the outlet is less than 1×10 -6 mL / s, turn off the constant speed and constant pressure pump and switch the outlet to the capillary flowmeter to allow the fluid to continue flowing. When the inlet pressure remains unchanged and the fluid level in the capillary flowmeter does not move significantly, record the inlet pressure Pz and the length of the rock sample. According to the injection pressure P1~P n and the flow rate at the outlet V1~V n , obtaining a pressure gradient-flow velocity curve; performing linear fitting on the linear segment of the pressure gradient-flow velocity curve to obtain a linear curve; determining the simulated starting pressure gradient of the fluid when seeping through the low permeability rock sample based on the linear curve and the values ​​on the pressure gradient coordinate axis; Determine the initial pressure gradient for the fluid seepage in the low-permeability rock sample according to the ratio of the inlet-end pressure Pz to the length of the low-permeability rock sample.

9. The measuring method according to claim 8, characterized in that The first injection pressure is 5 MPa less than the confining pressure.

10. The measuring method according to claim 8, characterized in that The first interval is 0.1 MPa to 1 MPa.

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