Method, system and device for performing geomechanical tests in the rock of hydrocarbon extraction wells during drilling

Abstract

Method for performing geo-mechanical tests comprising a step of drilling a well in a ground of interest (1) by making a hole (2) bounded by a side wall (2a) defined by the ground of interest (1) and a step of performing a geomechanical hydraulic fracturing test using a sensor device, assembled in the drilling string, provided with a working duct (3) extending to an open outlet end and a pressure sensor. The outlet end is placed in abutment against the side wall (2a), the working fluid is pumped within the working duct (3), and a parameter identifying the pressure of the working fluid within the working duct (3) is measured.

Description

[0001] “METHOD, SYSTEM AND DEVICE FOR PERFORMING GEOMECHANICAL TESTS IN THE ROCK OF HYDROCARBON EXTRACTION WELLS DURING DRILLING”

[0002] DESCRIPTION

[0003] The present invention relates to a method for performing geo-mechanical tests in the rock of hydrocarbon wells.

[0004] More specifically, the present invention relates to a method for testing the fracture gradient of the rock of hydrocarbon wells and / or for determining the formation closure pressure (FCP) in the rock of hydrocarbon wells.

[0005] The present invention also provides a drilling system for making a well for the extraction and / or storage of hydrocarbons and a sensor device for performing geo-mechanical tests in the rock of hydrocarbon extraction wells.

[0006] Therefore, the present invention is used in the field of extraction and storage of hydrocarbons such as oil, gas or CO2.

[0007] Generally, the construction of a hydrocarbon extraction field provides a step of exploring the ground where the presence of a deposit is expected or has been detected.

[0008] In detail, in order to ascertain the actual presence of a hydrocarbon deposit, one or more exploratory wells are drilled.

[0009] In order to make an exploratory well, it is known to perform a series of bore holes drilled in the ground in successive steps and at increasing depths. At each step, a hole is drilled with a smaller diameter than in the previous step, until the desired depth is gradually reached.

[0010] Between one step and the next, the drilled hole sections are protected by lining the side walls of the hole with appropriate cemented steel liners.

[0011] Once the exploration is carried out and the actual presence / position of the deposit is verified, the development of the hydrocarbon extraction field starts. In general, a deposit is developed within a porous reservoir rock placed deep in the ground and covered by an impermeable rock layer.

[0012] The hydrocarbons to be extracted are trapped within the reservoir rock and, therefore, the reservoir rock is subjected to hydrostatic pressure.

[0013] Furthermore, in all the formations drilled before reaching the reservoir rock, formation fluids are trapped and could undesirably spill out during drilling.

[0014] In this context, the need to avoid any unwanted spillage of fluids or hydrocarbons in the external environment during the construction of a well is clear.

[0015] Nowadays, it is known to counterbalance the pressure of the fluids trapped in the ground by employing a working mud that is pumped into the hole during drilling to balance the pressure of the formation fluids (drilling in overbalance).

[0016] Drilling is thus performed safely, with the weight of the working mud being greater than the pressure of the fluids contained in the ground.

[0017] However, in order to achieve a correct balance between the pressure exerted by the working mud and that exerted by the reservoir and the formations it passes through, it is necessary to identify the pressure of the fluids trapped in the rock as accurately as possible.

[0018] In this context, a critical issue is that the working mud must have a specific weight such as to balance the pressure of the fluids trapped in the ground but, at the same time, it must not have an excessive specific weight that could fracture the rock.

[0019] In fact, the ground is subjected not only to the hydrostatic pressure generated by the fluids trapped therein, but also to a lithostatic pressure that is independent of the hydrostatic pressure.

[0020] If the mud exerts too much pressure on the walls of the hole, it would lead to the fracturing of the rock. By contrast, if the mud exerts too little pressure on the walls of the hole, fracturing of the rock could occur as a result of the inherent lithostatic pressure in the ground in which the hole is drilled.

[0021] In this case, the risk is that the hydrostatics of the mud column in the hole would no longer be able to balance the pressure of the fluids contained in the formation passed through by the hole. This results in the consequent risk of hydrocarbon entering the hole and the criticalities associated thereto, such as: well loss, uncontrolled eruption of the reservoir.

[0022] In order to derive a value, or range of values, for the optimal specific weight of the working mud to be used for drilling the hole, so as to counterbalance the pressure of the fluids trapped in the ground and at the same time avoid fracturing the rock, it is known to empirically measure the fracture gradient of the rock facing the drilled hole between one step and the next. The fracture gradient is indicative of the rock resistance to pressure.

[0023] In order to determine the fracture gradient, it is common practice to perform one or more geo-mechanical tests, such as the Pressure Integrity Test (PIT), aimed at characterising the mechanical properties of the rock formation.

[0024] Among these, the Formation Integrity Test (FIT) can be carried out in different ways, depending on the type of information to be obtained. In particular, the test can be performed:

[0025] • in a precautionary manner, pushing the pressure up to a value at which there is a reasonably certainty of not fracturing the rock, or

[0026] • extensively, continuing the injection until the fracturing of the formation is caused.

[0027] In the first case, the Fracture Initiation Pressure (FIP), also known as breakdown pressure, is obtained.

[0028] In the second case, the Formation Closure Pressure (FCP), i.e., the pressure at which the fracture closes, is determined.

[0029] The accurate determination of FIP and FCP values is of crucial importance in the oil industry:

[0030] • FIP is essential in the steps of designing and drilling the wells,

[0031] • while FCP plays a crucial role both at a later drilling step and especially during the production step, particularly in stimulation or reinjection processes.

[0032] Tests of this type are designed to measure the strength and integrity of the rock during drilling.

[0033] Nowadays, it is only possible to perform the above-mentioned tests at the beginning of a new drilling step, by closing off the well to the outside environment using appropriate blowout preventers (BOPs) and creating overpressure in the drilled hole by pumping working mud. The pumped working mud exerts a pressure on the wall of the newly drilled hole. During this process, the pressure of the pumped fluid is measured to obtain a pressure curve: in the region where the pressure curve has a linear trend, the rock does not fracture (it has an elastic behaviour) and in the region where the pressure curve has a non-linear trend, the rock fractures (it has a non-elastic behaviour).

[0034] This operation suffers from the drawbacks of being risky and being limited to only the initial step of drilling the ground.

[0035] Moreover, this operation is usually done using standard surface systems, which pump a flow rate of 20 litres per pump stroke into the hole. However, this method does not allow an accurate measurement and may cause unwanted fracturing of the rock. If this happens, the fractures can extend for several metres, causing severe mud loss and potentially damaging, or even compromising, the entire well.

[0036] The risk is particularly high when pressure tests are carried out near the reservoir rock, i.e., in the cap rock of a deposit. If this rock fractures, it loses its ability to effectively seal the deposit, with the consequent danger of spillage of the fluids contained therein.

[0037] For this reason, it is necessary to underestimate the pressure applicable by the drilling mud, keeping it low enough to avoid any risk of fracturing. Generally, only FIT tests (Formation Integrity Test) are carried out, but these often do not provide the desired value.

[0038] Therefore, this test is performed at a significant distance from the reservoir rock (in which the hydrocarbon is contained) to maintain a high safety range.

[0039] In this context, underestimating the maximum injection pressure that the cap rock of the deposit is able to bear inevitably results in lower pressure being applied in the injector wells, thus generating less thrust on the hydrocarbons, reducing and slowing extraction thereof from the deposit. In fact, during the extraction step and while progressively emptying the deposit, the pressure exerted by the injector wells increasingly promotes the production of hydrocarbon.

[0040] Consequently, the known solutions, while avoiding rock fracture during strength tests, cause a considerable economic loss for companies involved in the extraction and marketing of hydrocarbons.

[0041] In other words, the known methods for carrying out geo-mechanical tests during drilling do not allow for the optimal exploitation of a hydrocarbon deposit.

[0042] In addition, known methods do not allow to safely measure the fracture closure pressure (FCP), an extremely important parameter for designing and exploiting wells. In fact, in order to measure this parameter, it is necessary to pump working mud into the hole until a radial stress is generated on its walls such as to generate a fracture and, after stopping the mud pumping, it is possible to identify the fracture closure pressure by measuring the mud pressure within the hole.

[0043] It is clear that this process, that requires fracturing the rock - which can extend for many metres in an uncontrolled manner - is extremely risky. Therefore, FCP is a difficult parameter to measure directly, due to the risks associated with performing the test, including: the possible loss of cap rock integrity or hole stability, the undesired extension of the fracture from the cap rock to the hydrocarbon deposit, as well as the time required to perform the test, which negatively affects its economic sustainability. For these reasons, the FCP value is often estimated rather than directly measured, especially in the cap rock of the deposit, where this parameter should be measured. This estimate almost always leads to the use of a conservative approach, with significant economic impacts both on the well design - especially in the exploration step - and, even more so, on the production and storage step of the oil field.

[0044] In addition, as is well known, in order to maximise the productivity of oil fields, injector wells are made. The latter are subjected to an operational constraint represented by the maximum pressure for injecting the working mud into the well: this pressure must not exceed the fracture closure pressure (FCP), i.e. the threshold beyond which there is a risk of reopening pre-existing fractures in the rock.

[0045] The ability to support the deposit production is directly related to the flow rate of the injectors, which is in turn limited by the maximum pressure that can be exerted at the bottom of the well, determined by the FCP of the cap rock.

[0046] As mentioned, since determining FCP directly involves significant operational risks, indirect estimates are frequently used, adopting conservative values to safeguard the integrity of the system: this conservative approach, although necessary for safety reasons, leads to a reduction in operating pressure and, consequently, a limitation of the deposit overall production potential.

[0047] A technical task of the present invention is therefore to provide a method for performing geo-mechanical tests capable of overcoming the drawbacks resulting from the prior art.

[0048] A technical task of the present invention is to provide a method for testing the fracture gradient of the rock of a production or injection well that allows to calculate more precisely, safely and accurately, the actual fracture pressure of the rock layer, in order to apply an injection pressure closer to this value to the working mud, so as to optimise the subsequent process of drilling the well and subsequent production or injection.

[0049] A technical task of the present invention is to provide a method for determining the fracture initiation pressure (FIP) and, in particular, the fracture closure pressure (FCP) of a well intended for the production or injection that allows to calculate accurately and safely the actual fracture closure pressure (FCP), without compromising the integrity of the hole, and thereby optimising the subsequent process of drilling the well and subsequent production or injection.

[0050] A further technical task of the present invention is to provide a drilling system for drilling a hydrocarbon well that can overcome the drawbacks resulting from the prior art.

[0051] A further technical task of the present invention is to provide a sensor device for performing geo-mechanical tests (e.g. for testing the fracture gradient (FIP - Fracture Initiation Pressure) and / or the fracture closure pressure - FCP) of the rock of a hydrocarbon extraction well at the desired depth of the well, which, in the state of the art, is only limited to the start depth of the new section of the drilled hole, so as to better plan further oil field developments.

[0052] A further object of the present invention is to provide an operating device configured to perform a plurality of geo-mechanical tests during drilling at different depths along the drilled hole section, overcoming the limitations of known solutions, which typically allow a single test to be performed at the initial drilling steps.

[0053] An object of the present invention is to provide a drilling system and an associated method for performing geo-mechanical tests, such as determining the fracture gradient (FIP - Fracture Initiation Pressure) and / or the fracture closure pressure (FCP), selectively along the horizontal axis of the stress state, in either the minor or major horizontal stress direction.

[0054] A further object of the present invention is to provide a drilling system and a method to perform geo-mechanical tests (e.g. to test the fracture gradient FIP - Fracture Initiation Pressure) and / or the fracture closure pressure - FCP) with a more precise methodology that allows operations to be performed with greater control, safety and in less time.

[0055] The specified technical task and objects are substantially achieved by a method and a sensor device for performing geo-mechanical tests, (e.g. for testing the fracture gradient (FIP - Fracture Initiation Pressure) and / or the fracture closure pressure - FCP - of the rock) comprising the technical features set forth in one or more of the preceding claims. The dependent claims correspond to possible embodiments of the invention.

[0056] Further features and advantages of the present invention will become clearer from the indicative, and therefore non-limiting, description of an embodiment of a method and a sensor device for performing geomechanical tests (e.g. for testing the rock fracture gradient and / or fracture closure pressure - FCP) on the rock of a hydrocarbon extraction well and a drilling system for making a hydrocarbon extraction well.

[0057] This description will be set forth herein below with reference to the accompanying drawings, provided for merely indicative and therefore nonlimiting purposes, wherein:

[0058] - Figures 1 to 3 show a schematic view of respective steps of the method object of the present invention and the drilling system object of the present invention;

[0059] - Figure 4 shows a detail of Figure 3;

[0060] - Figure 5 shows a schematic view of a sensor device object of the present invention.

[0061] The present invention provides a method for performing localised and oriented geo-mechanical tests of the rock of a ground of interest 1 .

[0062] The method can be used to test the fracture gradient of the rock of the ground of interest 1.

[0063] “Fracture gradient” refers to a hydraulic pressure expressed per unit depth that, when exerted on a rock, measures its strength until its fracture value.

[0064] In other words, the present invention provides a method for testing and determining the reference margin for the pressure strength of the rock of a hydrocarbon deposit or storage.

[0065] The method can be used to test both the fracture initiation pressure (FIP) and fracture closure pressure (FCP) of the rock of a ground of interest 1 .

[0066] “Initiation pressure” refers to the pressure that, exerted on a rock, is required to initiate a fracture.

[0067] "Closure pressure” refers to the pressure that, exerted on a rock, is required to keep a fracture open. In other words, the closure pressure corresponds to a pressure below which the fractures in the rock do not remain open and, as will become clear from the following description, corresponds to the minimum main stress of the ground.

[0068] Structurally, the ground of interest 1 has a walkable surface 10 facing the external environment.

[0069] The ground of interest 1 comprises a plurality of rock layers. Specifically, as schematically shown in Figure 1 , the ground of interest 1 comprises an impermeable rock layer 1 a, e.g. clayey, and a permeable rock layer 1 b, e.g. sandy.

[0070] The impermeable rock layer 1 a is placed between the walkable surface 10 and the permeable rock layer 1 b.

[0071] The hydrocarbon to be extracted is therefore trapped in the permeable rock layer 1 b.

[0072] In order to reach the hydrocarbon trapped within the permeable rock layer 1 b, the method comprises a step of drilling the ground of interest 1 .

[0073] The step of drilling the ground of interest 1 is carried out by drilling a blind hole 2 extending along at least one extension direction X transverse to the walkable surface 10.

[0074] For example, the hole 2 extends along a plurality of mutually transverse extension directions X comprising at least one extension direction X transverse to the walkable surface 10.

[0075] The hole 2 is bounded by a side wall 2a defined by the ground of interest 1 itself. The method comprises positioning, within the hole 2, a working duct 3 extending to an outlet end U that is open and adapted to contain a working fluid.

[0076] Therefore, the working duct 3 is a probe that extends to the outlet end U.

[0077] The outlet end U is placed in abutment against the side wall 2a of the hole 2, defining a tight fluid-dynamic connection between the working duct 3 and the side wall 2a of the hole 2.

[0078] In detail, the outlet end U is pressed against the side wall 2a of the hole 2, thus defining a tight fluid-dynamic connection between the working duct 3 and the side wall 2a of the hole 2.

[0079] The outlet end U is pressed against the side wall 2a by applying on the outlet end U a pressure above an expected pressure value which, when exerted on the side wall 2a, causes the fracture thereof.

[0080] It is thereby possible to ensure hydraulic isolation and put in communication the working duct 3 with the side wall 2a, creating a closed system and isolating the working duct 3 from the rest of the hole 2.

[0081] In detail, the outlet end U is provided with a rubber sealing ring, preferably either made of rubber or made of a rubber which is expandable with the compression exerted by the working duct 3 against the side wall 2a.

[0082] The sealing ring is a body placed to cover an edge of the outlet end U, adapted to contact the side wall 2a of the hole 2 and to define the hydraulic sealing connection between the working duct 3 and the side wall 2a. Preferably, the outlet opening U has a support surface, adapted to adhere in abutment with the side wall 2a, having a convex portion having a radius defined based on the diameter of the hole 2. Preferably, the sealing ring defines the support surface.

[0083] The working duct 3 is comprised within a sensor device 200.

[0084] Therefore, the method comprises arranging the sensor device 200.

[0085] The sensor device 200 is positioned within the hole 2.

[0086] This positioning can advantageously be performed while drilling the ground 1 . The sensor device 200 is provided with a pressure sensor 201 configured to measure a parameter identifying a pressure of the working fluid within the working duct 3. The pressure sensor 201 is capable of transmitting real-time data to a user interface device, preferably in a wireless mode (e.g. mud pulse telemetry)

[0087] The pressure sensor 201 is therefore fluid-dynamically connected to the working duct 3.

[0088] In other words, the pressure sensor 201 is associated with the working duct 3 so that an accurate measurement of the pressure of the working fluid can be performed.

[0089] With reference to Figure 5, the sensor device 200 can be provided with hydraulic ducts C fluid-dynamically connected to the working duct 3 and configured to bring working fluid to the working duct 3.

[0090] The sensor device 200 can be provided with pumps P adapted to pump working fluid within the working duct 3.

[0091] Therefore, the hydraulic ducts C are fluid-dynamically connected to the pumps P.

[0092] The pressure sensor 201 is at least partially arranged within a hydraulic duct C. In other words, the pressure sensor 201 is fluid-dynamically connected to the working duct 3 through the hydraulic duct C within which it is at least partially arranged.

[0093] Therefore, the pressure sensor 201 is adapted to measure the pressure of the working fluid in the hydraulic duct C: the value of this pressure corresponds to the parameter identifying the pressure of the working fluid within the working duct 3.

[0094] Preferably, the pressure sensor 201 is arranged within a hydraulic duct C downstream of the pumps P.

[0095] “Downstream” means subsequent to a direction of a flow F of the working fluid.

[0096] Specifically, the drilling of the ground of interest 1 is performed by means of a drilling string 101 , and the sensor device 200 is integral with the drilling string 101 .

[0097] The pumps P are in fluid-dynamic contact with a working fluid supply line of the drilling string 101 (and its drilling rods in detail).

[0098] The sensor device 200 is provided with an inlet port commanding an inlet valve that allows the working fluid to be drawn (e.g. by means of pumps P) from the supply line of the drilling string 101 to perform the method.

[0099] The hydraulic ducts C are at least partially or completely arranged within the inner volume of the drilling string 101 .

[0100] The pumps P are at least partially or completely arranged within the inner volume of the drilling string 101

[0101] Preferably, the pressure sensor 201 is arranged within a duct C arranged in the inner volume of the drilling string 101 .

[0102] Advantageously, the pressure sensor 201 and the working duct 3 are protected within the drilling string 101 , and not subject to damage during the well drilling operations.

[0103] Once the desired depth where the test is to be performed has been reached, drilling is stopped, and the outlet end U of the working duct 3 abuts against the side wall 2a.

[0104] Therefore, during drilling, the working duct 3 is housed within the drilling string 101 and, when the desired depth is reached, it is extracted from the drilling string 101 and abuts against the side wall 2a by means of a hydraulic piston thrust.

[0105] Preferably, the pressure sensor 201 is housed within the drilling string 101 during all the steps of the method object of the present invention.

[0106] It is therefore possible to avoid any damage to the sensor device 201 , which is “protected” inside the drilling string 101 .

[0107] More in detail, as schematically shown in Figure 2, during drilling, the positioning of the sensor device 200 within the hole 2 is performed by keeping the outlet end U of the working duct 3 spaced from the side wall 2a of the hole 2, protected within the drilling string 101 .

[0108] Once drilling is stopped, the sensor device 200 is placed at the required depth where the test is to be performed and the working duct 3 is moved closer to the side wall 2a of the hole 2.

[0109] As schematically shown in Figure 3, the working duct 3 is moved closer to the side wall 2a of the hole 2 by placing the outlet end U in abutment against the side wall 2a of the hole 2.

[0110] The working duct 3 is therefore moved closer to the side wall 2a of the hole 2, defining a tight fluid-dynamic connection between the working duct 3 and the side wall 2a of the hole 2.

[0111] The step of moving the working duct 3 towards the side wall 2a of the hole 2 thus comprises a step of bringing the outlet end U into contact with the side wall 2a and, subsequently, a step of pushing the outlet end U against the side wall 2a by applying a pressure greater than an expected value of the hydraulic pressure which, when exerted on the side wall 2a, causes it to fracture.

[0112] In order to carry out these operations, as will become clearer in the following description, the sensor device 200 is provided with a positioning device 202 connected to the working duct 3 and configured to move the working duct 3 along an axis Z around which the outlet opening U extends. The method comprises pumping a working fluid within the working duct 3.

[0113] In other words, the step of pumping the working fluid is performed by generating a flow F of working fluid within the working duct 3, the flow F being directed towards the outlet opening U in abutment with the side wall 2a.

[0114] Preferably, the working fluid is a mud, but in one of its variants it could be a working fluid stored within a tank housed within the sensor device 200. Preferably, the working fluid is pumped within the working duct 3 until the working fluid flows through the side wall 2a.

[0115] Preferably, the pumping step is carried out by means of the pumps P of the sensor device 200, which draw the working fluid from the supply line of the drilling string 101 or from the tank within the sensor device 200 itself.

[0116] During the pumping step and in successive first time instants, the parameter identifying a pressure of the working fluid within the working duct 3 is measured.

[0117] An first time set of identification parameters is thus obtained.

[0118] Preferably, this measuring step is carried out by means of the pressure sensor 201 of the sensor device 200.

[0119] In greater detail, the first time set is processed at each of the first subsequent time instants.

[0120] In other words, the processing of the first time set is continuous and is performed each time the pressure of the working fluid is detected.

[0121] Therefore, each time an identification parameter is detected, the first time set is updated with the detected parameter and the updated time set is processed.

[0122] This processing allows to generate a first pressure curve.

[0123] The first pressure curve identifies a pressure of the working fluid inside the working duct 3 at which the working fluid flows into the side wall 2a of the hole 2, i.e. at which a fracture in the side wall 2a begins to occur.

[0124] Advantageously, the method allows to assess the fracture gradient of the side wall 2a by testing a limited region thereof, i.e., in a region subtended by the outlet opening U.

[0125] In detail, the outlet opening U has a substantially circular shape with a radius of 0.5 to 5 cm.

[0126] Preferably, the outlet opening U has a substantially circular shape with a radius of 0.5 up to 20cm or elliptical with a smaller radius of 0.5 up to 10cm and a larger radius of 5 to 30cm.

[0127] Therefore, should the side wall 2a be damaged, this damage would be advantageously limited to the region occupied by the outlet end U of the working duct 3.

[0128] Advantageously, it is thus possible to safely assess the fracture gradient of the side wall 2a: by pumping fluid into the working duct 3 (and not directly from the drilling rods of the drilling string 101 by closing blowout devices known as BOP to isolate a certain section of hole 2), it is possible to precisely regulate the flow rate of pumped working fluid.

[0129] Advantageously, it is also possible to assess the fracture gradient of the side wall 2a with greater precision: by pumping fluid into the working fluid line 3 (and not directly from the drilling rods of the drilling string 101 by closing blowout devices known as BOPs to isolate a certain section in the hole 2), it is possible to precisely measure the pressure at which the working fluid flows within the side wall 2a.

[0130] The method therefore allows to perform an integrity test (FIT - Formation Integrity Test) or other strength and integrity geo-mechanical tests of the rock, with increased safety and accuracy.

[0131] The method allows to determine the fracture initiation pressure (FIP), and the fracture closure pressure (FCP) in the side wall 2a of the hole 2.

[0132] In order to determine the fracture initiation pressure (FIP), the first time set is recorded up to a rupture instant, when a fracture is generated in the side wall 2a. Specifically, the working fluid is pumped into the working duct 3 until a fracture is generated in the side wall 2a, which occurs at the rupture instant. The pressure value of the working fluid in the working duct 3 recorded at the rupture instant corresponds to the fracture initiation fracture (FIP).

[0133] In order to determine the fracture closure pressure (FCP), the pumping of the working fluid into the working duct 3 continues for a predetermined time interval after the rupture instant (identifying the attainment of the FIP). At this step, the pressure in the working duct 3 tends to decrease as the opening of the fracture creates an additional volume that absorbs some of the injected fluid. Pumping is stopped when the fracture has a certain extent.

[0134] Therefore, following the rupture instant, the method comprises measuring, at second time instants subsequent to each other and subsequent to the rupture instant, a parameter identifying a pressure of the working fluid within the working duct 3 obtaining a second time set of identification parameters. This measurement is carried out both in a time interval between the rupture instant and the interruption of the pumping of working fluid into the working duct 3 and in a time interval after the interruption of pumping.

[0135] The method comprises processing, at each of the second successive time instants, the second time set generating a second pressure curve, identifying a pressure of the working fluid within the working duct 3 at which the fracture generated in the side wall 2a closes.

[0136] The analysis of the second pressure curve advantageously allows to determine the fracture closure pressure (FCP) with a considerable margin of certainty.

[0137] The interruption of the pumping of the working fluid can be based on a numerical pressure value and / or the analysis of the trend of the second pressure curve, possibly combined with the operator’s experience.

[0138] Preferably, the interruption of the pumping of the working fluid is not only based on a numerical pressure value but also on the analysis of the pressure trend over time, combined with the operator’s experience. In fact, it is essential to identify the moment at which the fracture is sufficiently extended, but without overcoming the sealing zone defined by the outlet end U (possibly provided with the sealing rubber ring) so as to avoid interference in the measurement and guarantee the quality of the acquired data. In other words, pumping is stopped after a fracture has been generated in the side wall 2a and before this fracture extends beyond the area of the side wall 2a subtended by the outlet end U.

[0139] To this end, the method comprises a step of:

[0140] • comparing the parameter identifying the pressure of the working fluid within the working duct 3, measured after the rupture instant, with a threshold value representative of a fracture extent, in the side wall 2a, positioned in a region thereof facing the outlet end U, and / or

[0141] • processing the second time set to recognise a reference trend, identifying a certain extent of a fracture in the side wall 2a positioned in a region thereof facing the outlet end U;

[0142] Specifically, the step of pumping fluid into the working duct is stopped when:

[0143] • the parameter identifying the pressure of the working fluid within the working duct 3, measured after the rupture instant, reaches the threshold value, and / or

[0144] • the reference trend is recognised.

[0145] In accordance with an aspect of the present invention in the step of pumping working fluid within the working duct 3 (performed both before and after the rupture instant), all the portions of the hole 2 outside the working duct 3 are, entirely, fluid-dynamically connected to each other. In other words, no section of the hole 2 is isolated from the rest of the hole 2, not even the section containing the sensor device 200 and the working duct 3.

[0146] Advantageously, it is therefore not necessary to use sealing packers, (which tend to wear out quickly during drilling and are only adopted with wire-line methodology) upstream and downstream of the portion where the sensor device 200 is positioned in order to isolate it from the rest of the hole 2.

[0147] In accordance with an aspect of the present invention, the steps of the method can be safely repeated multiple times in order to more accurately determine the value of the fracture initiation / closure pressure (FIT / FCP). With reference to the detection of the first and second time sets, the first and second successive time instants, in which the parameter identifying the pressure of the working fluid is measured, can be more or less spaced apart from each other. The spacing can also be selected as desired by the operator also depending on the objectives to be achieved in the geomechanical analysis of the formation.

[0148] By way of example, it is also possible to measure the parameter identifying the pressure of the working fluid at intervals of a few seconds or minutes depending on the pumping speed of the working fluid. With reference to the positioning of the working duct 3, preferably, the method comprises performing a detection of the impermeable rock layer 1a, e.g., by applying the known techniques below.

[0149] The step of drilling the ground of interest 1 is performed by drilling the impermeable rock layer 1 a so that the side wall 2a of the hole 2 is at least partially defined by the impermeable rock layer 1 a. Therefore, preferably, the step of moving the working duct 3 closer to the side wall 2a is performed by abutting the outlet end U against the side wall 2a of the hole

[0150] 2 defined by the impermeable rock layer 1a.

[0151] A further application of the invention is the possibility of performing injection tests (i.e. of the type described in the herein description) in a section of the side wall 2a defined by the permeable layer 1 b using a fluid suitable for performing the test.

[0152] With reference to the pumping of fluid in the working duct, this is preferably done by pumping a constant volume of working fluid into the working duct

[0153] 3 at successive time intervals.

[0154] In other words, this step is performed by imposing a constant volume flow F on the working fluid. Alternatively, the flow rate can also be variable.

[0155] The flow F is directed towards the side wall 2a of the hole 2.

[0156] Preferably, the volume of the working fluid pumped in the unit of time must have a controlled flow ranging from 5 cc / s to 30 cc / s. The value of the flow is determined on a case-by-case basis as necessary.

[0157] This value is selected within the above range depending on the strength of the rock, the information already known about the rock petrophysical properties and the type of geo-mechanical test to be performed.

[0158] Advantageously, the limited volume of fluid allows to precisely and safely measure the pressure resistance of the side wall 2a.

[0159] With reference to the processing steps of the first and second time sets, these are performed by defining a time trend of the detected identification parameters (first and second pressure curves).

[0160] In other words, they are generated: - a pressure-time graph (first pressure curve) of the parameters identifying the pressure of the working fluid within the working duct 3 detected at the first time instants;

[0161] - a pressure-time graph (second pressure curve) of the parameters identifying the pressure of the working fluid within the working duct 3 detected at the second time instants.

[0162] With reference to the first pressure curve, obtained in the first time instants during the pumping of working fluid within the working duct 3, the time trend comprises a linear stretch, which is representative of the time trend of the identification parameters detected at initial time instants, where the time trend is linear.

[0163] A linear time trend Id otifiess an elastic behaviour of the side wall 2a of the hole 2.

[0164] In greater detail, each point of the linear stretch can be referred to the FIT (Formation Integrity Test) value, i.e., the actual pressure applied to the formation in the absence of fracturing phenomena.

[0165] The time trend comprises a non-linear stretch, subsequent to the linear stretch, representing the time trend of the identification parameters detected at time instants subsequent to the initial time instants, where the time trend is non-linear.

[0166] A non-linear time trend identifies a non-elastic behaviour of the side wall 2a of the hole. In other words, the non-linear stretch indicates that the rock elastic limit of the side wall 2a has been exceeded and that it has consequently entered a regime of local plastic deformation.

[0167] The point at which the linear stretch stops and the non-linear stretch begins is defined as LOP (Leak-Off Point) and represents the beginning of local failure of the rock in the side wall 2a due to the micro-cracks induced in it.

[0168] The first pressure curve identifies values such as Leak Off Pressure (LOP) or Fracture Initiation Pressure (FIP).

[0169] The processing of the first time set is performed by recognising / identifying a transition stretch comprising a first portion, of the linear stretch, and a second portion, of the non-linear stretch.

[0170] The transition stretch is therefore a stretch in between the linear stretch and the non-linear stretch.

[0171] The transition stretch is representative of the information identifying the pressure of the working fluid within the working duct 3 at which the working fluid flows within the side wall 2a of the hole 2.

[0172] In order to determine the fracture gradient of the side wall 2a, the step of measuring the parameter identifying the pressure of the working fluid within the working duct 3 and the step of processing the time set are substantially repeated until the transition stretch is recognised.

[0173] To determine the fracture initiation pressure (FIP), the step of measuring the parameter identifying the pressure of the working fluid within the working duct 3 and the step of processing the first time set are repeated until the non-linear stretch is reached and extended at least until a rupture instant, in which a pressure drop occurs, is identified. As mentioned, the value of the pressure of the working fluid in the working duct 3 at the rupture instant corresponds to the fracture initiation pressure (FIP).

[0174] The working fluid is therefore pumped without interruption into the working duct 3, causing a continuous increase in the pressure within the duct 3. In this way, it is possible to reach a critical value at which a sudden drop in pressure in the working duct 3 can be observed. This drop occurs at the aforementioned rupture instant, when the pressure value in the working duct 3 corresponds to the fracture initiation pressure (FIP), i.e., the pressure at which the propagation of a macroscopic fracture in the side wall 2a begins.

[0175] Therefore, the first time set is processed to identify, among the first time instants, the rupture time instant at which the side wall 2a is macroscopically fractured, i.e., at which the identification parameter in at least one first time instant directly preceding the rupture time instant has a value greater than the value of the identification parameter in the rupture time instant.

[0176] As said, the step of pumping the working fluid into the working duct 3 is maintained after the rupture time instant and then stopped when the fracture in the side wall has a certain extent. As said, the second time set of parameters identifying the pressure of the working fluid in the working fluid duct 3 is detected from the rupture instant.

[0177] To this end, the second pressure curve (second time set) is processed by defining the time trend of the identification parameters detected at the second time instants. This trend has three distinct features:

[0178] • a decreasing section, indicating the initiation and propagation of a fracture in the side wall 2a of the hole 2. In particular, when the side wall 2a fractures (reaching the FIP), the pressure in the working duct 3 tends to decrease, as part of the injected fluid is absorbed within the fracture;

[0179] • a constant stretch, characterised by a substantially zero first derivative, representative of the fracture propagation step in the side wall 2a of the hole - the pumping of fluid is stopped at one of the second time instants of the constant stretch;

[0180] • a pressure drop stretch, which has a decreasing trend determined by the fluid pumping shut-in. This stretch is indicative of the successful natural closure of the fracture in the side wall 2a of the hole and can be used to determine the fracture closure pressure (FCP) as it represents the moment at which the pressure stabilises following the fracture closure.

[0181] Therefore, after the onset (rupture instant, FIP) and propagation (decreasing stretch) of the fracture, there is a constant stretch in the second pressure curve representing a step in which the fracture stabilises and continues to propagate. Subsequently, the fluid pumping is shut-in and the analysis of the second pressure curve is started.

[0182] The fracture closure pressure (FCP) is determined by analysing the first derivative of the time trend of the second pressure curve (during shut-in, i.e. after stopping the pumping of the working fluid in the working duct 3), and corresponds to the point at which the fracture in the side wall 2a of the hole closes and the pressure stabilises. In other words, the method provides to:

[0183] • derive the first derivative of the second pressure curve;

[0184] • process the first derivative of the pressure drop stretch of the second pressure curve and determine the value of the fracture closure pressure (FCP).

[0185] More specifically, processing the second time set provides to:

[0186] • define a time trend of the identification parameters detected in the second time instants;

[0187] • process the time trend to recognise its constant and pressure-drop stretches (following the constant stretch) of the time trend;

[0188] • define a first derivative of the pressure drop stretch and identify a time instant at which the first derivative tends to zero; the parameter identifying the pressure in the working duct 3 in the time instant at which the first derivative tends to zero is representative of the fracture closure pressure (FCP).

[0189] The phrase “tends to zero”, means that the pressure drop stretch has a substantially constant i.e. asymptotic trend towards a constant function.

[0190] The step of pumping working fluid into the working duct 3 is repeated until the constant stretch is recognised and when pumping stops.

[0191] The steps of measuring the pressure in the working duct 3 and processing the second time set are repeated until the fracture closure pressure (FCP) is identified.

[0192] The method of the present invention can advantageously be performed together with additional petrophysical information of the ground of interest 1.

[0193] More specifically, petrophysical information may comprise data recorded by known logging while drilling (LWD) techniques, such as: Gamma Ray, Resistivity and Density-Neutron and / or, preferably, Magnetic Resonance and sonic.

[0194] For example, it is possible to employ Gamma Ray, Resistivity and Density- Neutron and / or Magnetic Resonance to determine the range of impermeable rock, and then employ sonic to identify the maximum (o'2 or SHmax) and minimum (o'3 or Shmin) stress directions in the horizontal plane of the rock of the side wall 2a.

[0195] It is thus advantageously possible to orient the working duct 3 in the direction useful for the geo-mechanical information to be measured. Preferably, the method comprises a step of detecting, at the side wall 2a of the hole 2, a first, a second and a third lithostatic pressure of the ground of interest, in detail, they are detected:

[0196] - a first parameter o1 (lithostatic pressure), identifying a first lithostatic pressure of the ground of interest 1 along a first vertical direction, i.e., oriented parallel to the weight force;

[0197] - a second parameter o2 (SHmax - maximum horizontal stress), identifying a second lithostatic pressure of the ground of interest 1 , along a second direction transverse and preferably orthogonal to the first direction;

[0198] - a third parameter o3 (Shmin - minimum horizontal stress), identifying a third lithostatic pressure of the ground of interest 1 , along a third direction transverse and preferably orthogonal to the first and second directions.

[0199] The terms first, second and third are not intended to define any kind of hierarchy, but merely to give the different components a clear and unambiguous designation.

[0200] Generally, for most sites, the first parameter o1 identifies a stress due to the weight of the rock itself and is greater than both the second and third parameters (Shmax or o2; Shmin or o3), which instead result from confinement and tectonic effects.

[0201] More specifically, the third parameter o3 is lower than the second parameter o2 which is, in turn, lower than the first parameter o1 (o1 > o2 > o3).

[0202] When drilling the ground of interest, working mud is introduced into the hole 2:

[0203] • if the working mud pressure is insufficient, the rock surrounding the side wall 2a tends to push inwards into the hole 2 - the greatest force is exerted by the maximum horizontal stress (second parameter - o2, Shmax) which deforms the hole 2 until it assumes an oval shape; this condition leads the wall to be subjected to compressive stresses in the direction of the maximum horizontal stress (second parameter - o2, SHmax), with possible (and undesirable) fracture initiation of the side wall 2a;

[0204] • if the working mud pressure is excessive, it exerts a radial stress on the side wall 2a which can lead to the formation of a fracture in the rock. In detail, the fracture is generated along a plane perpendicular to the direction of maximum horizontal stress (second parameter, o2 or SHmax), i.e. along a plane parallel to the direction of maximum horizontal stress (o3, Shmin).

[0205] When a fracture has been generated according to what described in the second point of the previous list and the working mud pressure is reduced, the generated fracture tends to close as soon as the pressure in the hole 2 falls below the minimum horizontal stress (o3 or Shmin). In fact, it is precisely the minimum horizontal stress (o3 or Shmin) that determines the closure of the generated fracture.

[0206] To summarise, the maximum horizontal stress (o2 or SHmax) is oriented along the preferential direction of fracture propagation, while the minimum horizontal stress (o3 or Shmin) is a critical parameter to be determined, as it indicates the pressure value below which the hydrostatic pressure of the mud, once fracturing has been triggered, does not cause further spillage into the well.

[0207] During drilling and production operations, fractures are formed when the pressure of the drilling fluid exceeds the fracture initiation pressure (FIP), resulting in mud losses.

[0208] These losses only stop when the hydrostatic pressure of the fluid reduces until reaching the value corresponding to the minimum horizontal stress (o3 or SHmin), thus causing the fracture to close.

[0209] Advantageously, the step of positioning the sensor device 200 is performed by positioning it in such a way that the main axis Z around which the outlet opening U of the working duct 3 extends is substantially parallel to the second direction.

[0210] The term substantially means that the main axis Z may have an inclination of no more than 5°, preferably no more than 3°, with respect to the second or third direction - this value is determined as a function of the area of the outlet end U, which is in contact with the formation.

[0211] Preferably, the step of positioning the sensor device 200 is performed by positioning it so that the main axis Z around which the outlet opening U of the working duct 3 extends is parallel to the second direction.

[0212] An orientation of the main axis Z substantially parallel, or parallel, to the second direction allows to obtain a more accurate value of the fracture initiation pressure (FIP) as well as fracture closure pressure (FCP).

[0213] The orientation of the main axis Z substantially parallel, or parallel, to the second direction allows to obtain a value of fracture closure pressure (FCP) which identifies the minimum horizontal stress o3 or SHmin. In other words, the method allows to obtain an identification value of the minimum horizontal stress o3 or Shmin.

[0214] In one version, it is possible to orient the main axis Z parallel, or substantially parallel, to the third direction and carry out the method object of the present invention to obtain the pressure of the working fluid within the working duct 3 at which the fracture generated in the side wall 2a closes, which identifies the maximum horizontal stress o2 or SHmax.

[0215] Advantageously, the present invention makes it possible to estimate maximum (o2 or SHmax) and minimum (o3 or Shmin) horizontal stresses, significantly improving the characterisation of the stress field and the management of the well operating conditions.

[0216] In one version, the method comprises a step of pumping a working fluid directly into the hole 2 before performing the steps of moving the working duct 3 closer to the side wall 2a of the hole 2 and pumping a working fluid within the working duct 3.

[0217] Advantageously, thanks to this step, it is possible to introduce, before performing geo-mechanical tests using the sensor device 200, a stress on the side wall 2a similar to that to which it is subjected during hydrocarbon extraction, thus making the geo-mechanical tests performed more reliable and inducing only a less extensive fracture in the hole.

[0218] Preferably, in order to avoid spillage of fluids and / or hydrocarbons from the hole 2 drilled into the ground of interest 1 during the drilling step, the method comprises the steps of selecting a working mud to be pumped within the hole 2 to counterbalance a pressure of the fluids and / or hydrocarbons trapped in the ground 1 during the drilling step.

[0219] By way of example, the selection of the working mud can be performed by selecting a working mud having a specific weight calculated as a function of the information identifying the pressure of the working fluid within the working duct 3 at which the working fluid flows within the side wall 2a.

[0220] In particular, the mud may have such a specific weight that it exerts a hydrostatic pressure on the side walls 2a of the hole 2 that is lower than the pressure of the working fluid within the working duct 3 at which the working fluid flows within the side wall 2a.

[0221] Advantageously, an undesired rupture of the side wall 2a can thus be avoided.

[0222] Preferably, the working mud is obtained by mixing an amount of an earthy solid substance and an amount of a liquid substance.

[0223] Preferably, the liquid substance is inert with respect to clay materials.

[0224] The amounts of solid earthy substance and liquid substance are calculated based on the volume of the hole 2 and the calculated specific weight of the working mud.

[0225] Preferably, as schematically shown in Figure 1 , the step of measuring the fracture initiation pressure (FIP) and / or fracture closure pressure (FCP) is repeated in successive regions of the side wall 2a, placed at different depths along at least one extension direction X.

[0226] Thus, a spatial profile of the fracture gradient and / or closure pressure (FCP) can be obtained, which is useful for geo-mechanical characterisation of the subsurface.

[0227] The present invention also provides a drilling system 100 for making a hydrocarbon well, schematically shown in Figures 2 and 3.

[0228] For simplicity in the remainder of this description, the drilling system will be referred to as “system 100”.

[0229] The system 100 is configured to perform one or more steps of the method.

[0230] The system 100 comprises a drilling string 101 extending along an extension axis Y.

[0231] The drilling string 101 extends between a first end 101 a, carrying a chisel 102 adapted to dig the hole 2, and a second end 101 b.

[0232] Preferably, the drilling string 101 comprises a plurality of drilling rods and LWD instrumentation for log recording (not shown in the accompanying figures). The rods comprise a supply line for the working fluid.

[0233] The system 100 comprises a handling device 103 connected to the second end 101 b of the drilling string 101 .

[0234] The handling device 103 is configured to determine a rotation of the drilling string 101 around the extension axis Y.

[0235] Preferably, the handling device comprises a motor.

[0236] The step of drilling the ground of interest 1 is preferably carried out by actuating the handling device 103 to determine a rotation of the drilling string 101 and the chisel 102.

[0237] Preferably, the chisel 102 can rotate independently of the system 100 with the aid of a dedicated motor. The system comprises the sensor device 200.

[0238] The working duct 3 of the sensor device 200 is movable transversely to the extension axis Y between a moved-closer position and a moved-away position. In the moved-closer position (schematically shown in Figure 2), the outlet end U is moved closer in the drilling string 101 .

[0239] Preferably, in the moved-closer position, the outlet end U is housed in the drilling string 101 .

[0240] Preferably, the drilling string 101 is provided with a housing (not shown in the accompanying figures) configured to receive the insertion of at least part of the working duct 3 when it is in the moved-closer position.

[0241] Preferably, as schematically shown in Figure 2, the working duct 3, when in the moved-closer position, is completely contained and protected within the housing, in order to prevent it from being damaged during drilling.

[0242] In other words, the drilling string 101 comprises on one of its outer surfaces a concavity within which the working duct 3 is inserted in a moved-closer position.

[0243] In the moved-away position (schematically shown in Figure 3), the outlet end U is moved away from the drilling string 101 and abuts with the side wall 2a of the hole 2, defining a tight fluid-dynamic connection between the working duct 3 and the side wall 2a of the hole 2.

[0244] In order to define the movement between the moved-closer and moved- away positions, the sensor device 200 comprises a positioning device 202. The positioning device 202 is connected to the working duct 3 and the drilling string 101 .

[0245] The positioning device 202 is configured to move, preferably by means of a hydraulic piston, the working duct 3 axially along the main axis Z around which the outlet opening U extends.

[0246] In other words, the positioning device 202 is configured to move the working duct 3 transversely to the extension axis Y of the drilling string 101.

[0247] Preferably the positioning device 202 comprises at least one piston operatively connected to the working duct 3.

[0248] Specifically, the positioning device 202 is configured to bring the outlet end U in contact with the side wall 2a and, subsequently, to push the outlet end U against the side wall 2a by applying a pressure higher than an expected value of the hydraulic pressure which, when exerted on the side wall 2a, causes it to fracture.

[0249] During the step of moving the working duct 3 closer to the side wall 2a, the handling device 103 is inactive.

[0250] Preferably, the sensor device 200 comprises an orientation device 202' configured to move the working duct 3 in rotation around the extension axis Y to orient its main axis Z in space. Thanks to the orientation device 202', it is possible, for example, to orient the main axis Z parallel, or substantially parallel, to the second or third direction.

[0251] In accordance with an aspect of the present invention, the drilling string 101 defines its own inner volume.

[0252] In other words, the drilling string 101 is internally hollow.

[0253] The hydraulic lines C and pumps P of the sensor device 200 are at least partially, preferably completely, arranged within the inner volume of the drilling string 101 .

[0254] The pressure sensor 201 of the sensor device 200 is also arranged within the inner volume of the drilling string 101. For example, the sensor device is arranged within a hydraulic duct C arranged within the inner volume of the drilling string 101 .

[0255] The pressure sensor 201 is thereby protected from any damage.

[0256] Therefore, the sensor device 200 is integrated in the system 100. As a result, the system 100 makes it possible to measure geo-mechanical data at any depth of the drilled hole stretch, and is not limited to the initial drilling stretch as is instead in the state of the art.

[0257] According to a preferred embodiment, the drilling system is designed in the absence of sealing packers, traditionally used to isolate and pressurise selected stretches of the hole 2.

[0258] As is well known, sealing packers are not applicable in drilling string configurations - such as the system 100 - as they are easily subject to damage or destruction during the operating steps. The sealing packers are not suitable for drilling string configurations, as they are particularly exposed to mechanical damage in these operating contexts, due to both the high advancement dynamics and the complexity of the multi-section geometry. The removal of sealing packers therefore allows for greater reliability and continuity of geo-mechanical testing under critical operating conditions.

[0259] The system 100 thus has a limited complexity and, as it is free of sealing packers subject to considerable wear, has small maintenance costs.

[0260] The present invention also makes available a sensor device 200 for testing the fracture gradient of the rock in a hydrocarbon deposit.

[0261] The sensor device 200 can be installed on a drilling system for making a hydrocarbon well.

[0262] The sensor device 200 comprises a working duct 3 extending to an outlet end U that extends around a main axis Z.

[0263] The sensor device 200 comprises a pressure sensor 201 , configured to measure the parameter identifying the pressure of the working fluid within the working duct 3.

[0264] The pressure sensor 201 is fluid-dynamically connected with the working duct 3.

[0265] The sensor device 200 comprises a positioning device 202 connected to the working duct 3 and configured to move the working duct 3 axially along the main Z axis.

[0266] Preferably, the positioning device 202 comprises at least one piston.

[0267] Preferably, the sensor device 200 comprises an orientation device 202' configured to move the working duct 3 in rotation around the extension axis Y to orient its main axis Z in space. Thanks to the orientation device 202', it is possible, for example, to orient the main axis Z parallel, or substantially parallel, to the second or third direction. The working duct 3 is adapted to contain a working fluid.

[0268] Preferably, the sensor device 200 is provided with pumps P adapted to pump working fluid into the working channel 3. Preferably, the sensor device 200 comprises a pair of pumps P arranged in series. Advantageously, the pair of pumps P in series allows to regulate the pressure of the fluid injected into the working duct 3 in an improved manner.

[0269] The working duct 3 has an inlet end opposite to the outlet end U fluid- dynamically connected to the hydraulic ducts C, and the positioning device 202 is adapted to move the working duct 3 while maintaining the hydraulic sealing connection between the hydraulic ducts C and the inlet end.

[0270] The hydraulic ducts C and the pumps P can be inserted within the inner volume of a drilling string 101 .

[0271] Preferably, the pressure sensor 201 is arranged within the containment volume of the drilling string 101 and is at least partially arranged within a hydraulic duct C to measure the pressure of the working fluid within the latter.

[0272] The ducts C define an inlet port commanding an inlet valve that allows to use the working fluid in the supply line of the drilling string 101 to perform the method.

[0273] Advantageously, the present invention is capable of overcoming the drawbacks emerged from the prior art.

[0274] Advantageously, the present invention makes it possible to precisely and safely measure the fracture initiation pressure (FIP) and fracture closure pressure (FCP) in the rock of the side wall 2a during well drilling.

[0275] Advantageously, the present invention makes it possible to measure the fracture initiation pressure (FIP) and fracture closure pressure (FCP) in the rock of the side wall 2a near the reservoir rock during well drilling.

[0276] Advantageously, the present invention makes it possible to perform any geo-mechanical test by preventing unwanted spillage of fluids and / or hydrocarbons trapped in the ground of interest.

[0277] Advantageously, the present invention makes it possible to measure the fracture initiation pressure (FIP) and fracture closure pressure (FCP) of the rock during drilling and at the desired depth. Advantageously, the present invention makes it possible to determine parameters identifying the maximum horizontal stress o2 or SHmax and the minimum horizontal stress o3 or Shmin.

[0278] Advantageously, the present invention makes it possible to measure with high accuracy, in passive mode (i.e. without further fluid injections), the pressure drop over time after the injection operation has stopped.

[0279] The invention makes it possible, in one of its potential multiple applications, to perform mini-injection tests in permeable rocks using a dedicated fluid. This fluid is drawn from a specially prepared reservoir upstream of the pumping system (not shown in the drawings), integrated within the instrument itself and specifically designed for this purpose.

Claims

CLAIMS1. Method for performing geo-mechanical tests in the rock of a hydrocarbon extraction well located in a ground of interest (1), said method comprising the steps of:- drilling the ground of interest (1 ) by making a dead-end hole (2) extending along at least an extension direction (X) transverse to a walkable surface (10) of said ground of interest (1 ), said hole (2) being bounded by a side wall (2a) defined by the ground of interest (1 );- preparing a sensor device (200) comprising:- a working duct (3) extending to an outlet end (U) that is open and adapted to contain a working fluid;- a pressure sensor (201 ), fluid-dynamically connected to the working duct (3) and configured to measure a parameter identifying a pressure of the working fluid within the working duct (3);- positioning the sensor device (200) within the hole (2) while keeping the outlet end (U) spaced from the side wall (2a) of the hole (2), said step of positioning the sensor device (201 ) being carried out during the step of drilling the ground of interest;- moving the working duct (3) closer to the side wall (2a) of the hole (2) by placing the outlet end (U) in abutment with the side wall (2a) of the hole (2) and establishing a tight fluid-dynamic connection between the working duct (3) and the side wall (2a) of the hole (2);- pumping a working fluid into the working duct (3) until the working fluid flows through the side wall (2a);- during the previous step, measuring, in first successive time instants, a parameter identifying a pressure of the working fluid within the working duct (3) obtaining a first time set of identification parameters;- processing, in each of said first successive time instants, the first time set generating a first pressure curve identifying a pressure of the working fluid within the working duct (3) at which the working fluid flows within the side wall (2a) of the hole (2).

2. Method according to the preceding claim, wherein the step of pumping working fluid into the working duct (3) is carried out by keeping all the portions of the hole (2) outside the working duct (3), entirely, fluid- dynamically connected with each other.

3. Method according to claim 1 or 2, wherein:- the step of processing the first time set comprises identifying, among the first time instants, a rupture time instant when the side wall (2a) is fractured; the identification parameter in at least one first time instant directly preceding the rupture time instant having a value greater than the value of the identification parameter in the rupture time instant;- the step of pumping the working fluid into the working duct (3) is stopped after the rupture time instant;- the method comprises, following the rupture time instant, a step of measuring the fracture closure pressure of the side wall (2a) of the hole (2) comprising the steps of:- measuring, in second time instants subsequent to each other and subsequent to the rupture time instant, a parameter identifying a pressure of the working fluid within the working duct (3) obtaining a second time set of identification parameters;- processing, in each of said second time instants, the second time set generating a second pressure curve, identifying a pressure of the working fluid within the working duct (3) at which the fracture generated in the side wall (2a) closes.

4. Method according to the preceding claim, comprising a step of:- comparing the parameter identifying the pressure of the working fluid within the working duct (3), measured after the rupture instant, with a threshold value, wherein said threshold value is representative of a fracture extension in the side wall (2a), positioned in a region of the sidewall (2a) facing the outlet end (U) and / or- processing the second time set to recognise a reference trend, identifying a certain extent of a fracture in the side wall (2a) positioned in a region of the side wall (2a) facing the outlet end (U); wherein the step of pumping fluid into the working duct is stopped when:- said parameter identifying the pressure of the working fluid within the working duct (3), measured after the rupture instant, reaches said threshold value and / or- said reference trend is recognised.

5. Method according to claim 3 or 4, wherein the step of processing the second time set comprises the steps of:- defining a time trend of the identification parameters detected in the second time instants;- processing the time trend to recognise:- a constant stretch of said time trend along which the identification parameters measured at the respective second time instants all have substantially equal values- a pressure drop stretch following said constant stretch along which said time trend shows a decreasing trend;- defining a first derivative of said pressure drop stretch and identifying a time instant at which said first derivative tends to zero; wherein the parameter identifying the pressure in the working duct (3) at said time instant in which the first derivative tends to zero is representative of the fracture closure pressure.

6. Method according to the preceding claim, wherein the step of measuring a parameter identifying a pressure of a working fluid within the working duct (3) and the step of processing the second time set are repeated until the constant stretch and the pressure drop stretch are recognised.

7. Method according to one or more of the preceding claims, comprising a step of detecting an impermeable rock layer (1 a) and wherein:- the drilling step is performed by drilling said impermeable rock layer (1 a) so that the side wall (2a) of the hole (2) is at least partially defined by said impermeable rock layer (1 a);- the step of moving the working duct (3) closer to the side wall (2a) is performed by placing the outlet end (U) in abutment with the side wall (2a) of the hole (2) defined by the impermeable rock layer (1 a).

8. Method according to any one or more of the preceding claims, wherein the step of pumping a working fluid is performed by pumping at successive time intervals a constant volume of working fluid within the working duct (3) and imposing on the working fluid a flow (F) at a constant volume flow rate wherein the volume of working fluid is preferably less than 1 litre, more preferably less than 0.5 litre; more preferably the volume of working fluid being between 50 millilitres and 2 millilitres, even more preferably between 30 millilitres and 2 millilitres.

9. Method according to one or more of the preceding claims, wherein the step of processing the first time set comprises the steps of:- defining a time trend of the detected identification parameters;- processing the time trend to recognise a transition stretch comprising:- a first portion, representative of the time trend of the identification parameters measured at initial time instants and wherein the time trend is linear;- a second portion, representative of the time trend of the identification parameters measured at time instants subsequent to the initial time instants and wherein the time trend is non-linear; wherein the transition stretch is representative of the information identifying the pressure of the working fluid within the working duct (3) at which the working fluid flows within the side wall (2a) of the hole (2).

10. Method according to the preceding claim, wherein the step of measuring a parameter identifying a pressure of the working fluid within the working duct (3) and the step of processing the first time set are repeated until the transition stretch is recognised.11 . Method according to one or more of the preceding claims, comprising a step of measuring, at the side wall (2a) of the hole (2):- a first parameter identifying a first lithostatic pressure of the ground of interest (1 ) along a first vertical direction;- a second parameter identifying a second lithostatic pressure of the ground of interest (1 ) along a second direction which is transverse and preferably orthogonal to the first direction;- a third parameter identifying a third lithostatic pressure of the ground of interest (1 ) along a third direction which is transverse and preferably orthogonal to the first and second directions; wherein the third parameter identifies a third lithostatic pressure lower than said first and second lithostatic pressures; and wherein the step of positioning the sensor device (200) is performed by positioning the working duct (3) so that a main axis (Z) around which the outlet opening (U) of the working duct (3) extends is substantially parallel to the second direction.

12. Method according to any one of the preceding claims wherein the step of positioning the sensor device (200) and the steps thereafter are repeated in regions of the side wall (2a) of the hole (2) successive to each other along the at least one extension direction (X) to obtain a spatial trend, along the at least one extension direction (X), of the information identifying the pressure of the working fluid within the working duct (3).

13. Method according to any one or more of the preceding claimscomprising a step of pumping a working fluid within the hole (2) performed prior to the steps of moving the working duct (3) closer to the side wall (2a) of the hole (2) and pumping a working fluid within the working duct (3).

14. Drilling system (100) for making a hydrocarbon extraction well or an injector well comprising:- a drilling string (101 ) extending along an extension axis (Y) between a first end (101a), carrying a chisel (102) adapted to dig a hole (2) in a ground of interest (1 ), and a second end (101 b);- a handling device (103) connected to the second end (101 b) of the drilling string (101 ) and configured to rotate the drilling string (101 ) around the extension axis (Y); characterised in that it comprises a sensor device (200) provided with:- a working duct (3), extending to an outlet end (U) and adapted to contain a working fluid;- a pressure sensor (201 ), fluid-dynamically connected to the working duct (3) and configured to measure a parameter identifying a pressure of the working fluid within the working duct (3), the working duct (3) being movable transversely to the extension axis (Y) between a moved-closer position, wherein the outlet end (U) is moved closer to the drilling string (101 ), and a moved-away position wherein the outlet end (U) is moved away from the drilling string (101 ) and in abutment with a side wall (2a) of the hole (2) by defining a tight fluid-dynamic connection between the working duct (3) and the side wall (2a) of the hole (2), the drilling system (100) being configured to perform one or more steps of the method according to any one of the preceding claims.

15. Drilling system (100) according to the preceding claim, characterised in that the drilling system (10) is free of sealing packers.

16. Sensor device (200) for performing geo-mechanical tests in the rock ofa hydrocarbon extraction well that can be installed on a drilling system for making a hydrocarbon well comprising:- a working duct (3), extending to an outlet end (U) around a main axis (Z), said working duct (3) being adapted to contain a working fluid; - a pressure sensor (201 ), fluid-dynamically connected to the working duct (3) and configured to measure a parameter identifying a pressure of the working fluid within the working duct (3);- a positioning device (202) connected to the working duct (3) and configured to move said working duct axially along the main axis (Z).

Images