Oil-gas well cement sheath interface bonding strength experimental device and determination method

Abstract

The invention discloses an oil and gas well cement sheath interface bonding strength experimental device and a determination method, and relates to the technical field of oil and gas wells. Temperature control is performed on a cement sheath through a numerical control system, so that the temperature of the cement sheath is within a preset temperature range; according to experimental requirements, the cement sheath is subjected to circular treatment, the circular treatment comprises pressurization treatment, heating treatment, cooling treatment and pressure relief treatment, the circular treatment is repeated, the number of times of circulation is consistent with the number of times of demand of field working conditions until the cementing surface in the cement sheath fails in cementing, and the cement sheath is obtained. And determining a load-displacement curve during the cementation failure period of the cementation surface in the cement sheath, and determining the cementation strength of the cement in the cement sheath according to the load-displacement curve. The method can achieve the purposes of being closer to the actual working condition and more accurate in measurement result.

Description

Background Art

[0002] During the drilling and development of oil and gas, the sealing performance of the cementing and sealing system directly affects the overall efficiency of exploration and development. This requires the cement sheath between the formation and the outer wall of the casing to maintain good integrity for a long time. Since large-scale hydraulic fracturing is required to obtain commercial oil and gas production in deep and unconventional oil and gas reservoirs, the sealing failure of the cement sheath is easily caused by temperature and pressure fluctuations, continuous changes, alternating loading and unloading, resulting in interlayer sealing failure, annulus pressure build-up, and casing corrosion, seriously affecting the safe production of oil and gas wells.

[0003] At present, scholars at home and abroad have adopted theoretical and experimental methods, mainly based on the principle of gas channeling, and regarded the internal gas connection of the cement sheath as the failure criterion. The invention patent (CN 103808652B) discloses a method for simulating the cementing failure caused by temperature change. By repeatedly heating and cooling, it detects whether gas channeling occurs between the cement sheath and the casing during the temperature change process, and measures the gas channeling volume. The cement sheath formula that can withstand the most repeated heating and cooling times is the best formula. The invention patent (CN103806865B) discloses a method for simulating the cementing failure caused by pressure change. Under multiple differential pressure levels, it detects whether gas channeling occurs between the cement sheath and the casing during the pressure change process. The cement sheath formula that can withstand the largest differential pressure and the most pressurization and pressure relief cycles is the best formula. In the above patents, by detecting whether gas channeling occurs between the cement sheath and the casing during the pressure and temperature change process, the cementing condition of the cement sheath is judged, and the shear cementing strength value of the cement sheath in deep wells and deep shale gas horizontal wells under high temperature and high pressure cannot be determined, that is, the ultimate load value cannot be given.

[0004] The interfacial cementing strength of the cement sheath is the main factor to measure the sealing performance of the cement sheath. Therefore, it is of great significance to study the evaluation method of the cement sheath cementing strength under high temperature and high pressure in deep wells. At present, aiming at the problem of the complete failure of the cement sheath cementing seal under the actual working conditions such as pressure and temperature changes in the oilfield wellbore, many oilfields and enterprises have studied the evaluation devices and methods. However, the internal pressure and temperature considered by the proposed cement sheath sealing experimental device are limited (internal pressure ≤ 40 MPa, temperature ≤ 200 °C), and the strength test of the first cementing surface of the casing-cement sheath cannot be realized under the internal pressure-temperature cycle condition, which leads to the difference between the measured first interfacial cementing strength and the interfacial cementing strength under the actual working conditions. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an experimental device and a determination method for the interfacial cementing strength of the cement sheath in oil and gas wells in view of the deficiencies of the prior art, as follows:

[0006] 1) In the first aspect, the present invention provides an experimental device for the interfacial cementing strength of the cement sheath in oil and gas wells, and the specific technical solution is as follows:

[0007] Including: an inner casing, an outer formation casing, a heating pad, an annular lower mold, an upper mold of a testing machine, and an internal press;

[0008] The inner casing and the outer formation casing are coaxially arranged, and the outer wall of the inner casing faces the inner wall of the outer formation casing. A cement sheath is formed between the inner casing and the outer formation casing. Plugs are welded to both ends of the inner casing. A pressure injection hole is provided on the first plug at one end of the inner casing. The pressure injection hole is connected to the internal press. The first plug faces the upper mold of the testing machine. The second plug at the other end of the inner casing is placed in the annular lower mold. The heating pad is provided on the outer wall of the outer formation casing.

[0009] The beneficial effects of an experimental device for the interfacial bonding strength of an oil and gas well cement sheath provided by the present invention are as follows:

[0010] The pressure injection hole on the plug is connected to a pressure testing device, which can control the pressure inside the inner casing to a preset value and can also achieve pressure circulation inside the casing. By covering the outer wall of the outer formation casing with a heating pad, the casing is under high-temperature conditions, achieving the purpose of being closer to the actual working conditions and obtaining more accurate measurement results.

[0011] 2) Second, the present invention also provides a method for determining the interfacial bonding strength of an oil and gas well cement sheath. The specific technical solution is as follows:

[0012] Inject the cement of the oil and gas well to be detected into the cement sheath to form a cement sheath. The temperature of the heating pad is controlled by a numerical control system to keep the temperature of the cement sheath within a preset temperature range;

[0013] According to the experimental requirements, perform cyclic processing on the cement sheath. The cyclic processing includes: pressure application processing, heating processing, cooling processing, and pressure relief processing. Repeat the cyclic processing, and the number of cycles is the same as the number of times required for the on-site working conditions until the bonding surface in the cement sheath fails to bond. Determine the load-displacement curve during the failure of the bonding surface in the cement sheath, and determine the bonding strength of the cement sheath according to the load-displacement curve.

[0014] The beneficial effects of an experimental device for the interfacial bonding strength of an oil and gas well cement sheath provided by the present invention are as follows:

[0015] The pressure injection hole on the plug is connected to a pressure testing device, which can control the pressure inside the inner casing to a preset value and can also achieve pressure circulation inside the casing. By covering the outer wall of the outer formation casing with a heating pad, the casing is under high-temperature conditions, achieving the purpose of being closer to the actual working conditions and obtaining more accurate measurement results.

[0016] On the basis of the above solution, the present invention can also be improved as follows.

[0017] Further, the process of determining the bonding strength of the cement sheath according to the load-displacement curve is specifically as follows:

[0018] Determine the interface bonding failure pressure according to the load-displacement curve;

[0019] Based on the interface bonding failure pressure, determine the bonding strength of the cement sheath.

[0020] Further, the process of determining the bonding strength of the cement sheath based on the interface bonding failure pressure is as follows:

[0021] Determine the bonding strength of the cement sheath through the first formula;

[0022] The first formula is:

[0023]

[0024] Wherein, M is the bonding strength, F is the interface bonding failure pressure, d is the inner diameter of the cement sheath, and H is the height of the cement sheath.

[0025] Further, the experimental requirements include:

[0026] At least one of the temperature value, the internal pressure value of the inner casing, and the number of cyclic treatments.

[0027] Further, the temperature value is any value in the range of 20 - 200 °C.

[0028] Further, the internal pressure value is any value in the range of 20 - 200 MPa.

[0029] Further, the number of cyclic treatments is any value in the range of 1 - 100.

[0030] Further, it further includes:

[0031] Determine the influence law according to the bonding strength. Description of the Drawings

[0032] By reading the following detailed description of the non-restrictive embodiments with reference to the accompanying drawings, other features, objects, and advantages of the present invention will become more apparent:

[0033] Figure 1 It is a schematic structural diagram of an experimental device for the interface bonding strength of an oil and gas well cement sheath of the present invention;

[0034] Figure 2 It is a schematic flow diagram of a method for determining the interface bonding strength of an oil and gas well cement sheath 2 of the present invention.

[0035] 1, inner casing, 2, cement sheath, 3, outer formation casing, 4, heating pad, 5, circular gasket, 6, circular lower die, 7, plug, 8, upper die of testing machine, 9, internal press. Specific embodiments

[0036] To make the objectives, technical solutions and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0037] As Figure 1 shown, an experimental device for the interfacial bonding strength of the cement sheath in an oil and gas well according to an embodiment of the present invention includes: an inner casing 1, an outer formation casing 3, a heating pad 4, a circular lower die 5, an upper die 8 of a testing machine, and an internal press 9;

[0038] The inner casing 1 and the outer formation casing 3 are coaxially arranged, and the outer wall of the inner casing 1 faces the inner wall of the outer formation casing 3. A cement sheath is formed between the inner casing 1 and the outer formation casing 3. Plugs 7 are welded to both ends of the inner casing 1. A pressure injection hole is provided on the first plug at one end of the inner casing 1. The first plug faces the upper die 8 of the testing machine, and the pressure injection hole is connected to the internal press 9. The second plug at the other end of the inner casing 1 is placed in the circular lower die 6, and the heating pad 4 is arranged on the outer wall of the outer formation casing 3.

[0039] The beneficial effects of an experimental device for the interfacial bonding strength of the cement sheath in an oil and gas well provided by the present invention are as follows:

[0040] The pressure injection hole on the plug is connected to a pressure testing device, which can control the pressure in the inner casing to a preset value and can also achieve the pressure circulation in the casing. By covering the heating pad on the outer wall of the outer formation casing, the casing is under high-temperature conditions, achieving the purpose of being closer to the actual working conditions and more accurate measurement results.

[0041] As Figure 1 shown, it includes an inner casing 1 and an outer formation casing 3 coaxially arranged from the inside to the outside. The annular space between the inner casing 1 and the outer formation casing 3 is filled with cement slurry and cured to form a cement sheath 2. Plugs 7 are welded to both ends of the inner casing 1. The plug 7 includes a first plug (i.e., the upper plug) and a second plug (i.e., the lower plug). The pressure injection hole on the plug 7 is connected to the internal press 9. The outer wall of the outer formation casing 3 is covered with a heating pad 4. The heating pad 4 is connected to a temperature control system. The lower plug of the inner casing 1 is placed in the circular lower die 6, and the top of the upper plug of the inner casing 1 contacts the upper die 8 of the testing machine outside.

[0042] Preferably, the height of the annular lower die 6 is greater than half of the axial length difference between the inner casing 1 and the cement sheath 2, facilitating the placement of the inner casing 1 into the annular lower die 6. The inner diameter of the annular lower die 6 is 20 - 30 mm larger than the outer diameter of the plug 7, and the outer diameter of the annular lower die 6 is more than 5 mm larger than the outer diameter of the outer formation casing 3.

[0043] The pressure injection hole is connected to an internal press, and the internal press applies internal pressure and pressure circulation inside the casing.

[0044] When the length of the inner casing 1 is greater than the length of the cement sheath, the upper die 8 of the testing machine presses the casing 1 downward, and the lower die of the testing machine drives the annular lower die 6 to press the cement sheath 2 and the outer formation casing 3 upward. The testing machine collects the shear load - displacement curve, and when the load suddenly drops, it is determined that the cementing surface of the cement sheath has failed in cementing.

[0045] As Figure 2 shown, the present invention also provides a method for determining the bonding strength of the interface of the oil and gas well cement sheath 2, and the specific technical solution is as follows:

[0046] S1, the temperature of the cement sheath is controlled through a numerical control system to keep the temperature of the cement sheath within a preset temperature range.

[0047] The cement sheath in this solution can be obtained in two forms. One is to generate the cement sheath 2 based on the Figure 1 shown structure, and the other is to directly form an annular gap by coaxially arranging two sleeves with the same length.

[0048] S2, according to the experimental requirements, the cement sheath is subjected to cyclic processing. The cyclic processing includes: pressure application processing, heating processing, cooling processing, and pressure relief processing. The cyclic processing is repeated, and the number of cycles is consistent with the required number of times in the field working conditions until the cementing surface in the cement sheath fails in cementing. The load - displacement curve during the failure of the cementing surface in the cement sheath is determined, and the cementing strength of the cement in the cement sheath is determined according to the load - displacement curve.

[0049] The pressure application processing in the cyclic processing is to directly apply pressure to both ends of the cement sheath, and its processing process is prior art and will not be elaborated here.

[0050] The heating processing, cooling processing, and pressure relief processing are all prior art and will not be elaborated here.

[0051] During the pressure application processing, the shear load - displacement curve is continuously collected, and when the load suddenly drops, it is determined that the cementing surface of the cement sheath has failed in cementing.

[0052] The sudden drop means that at a certain moment, the static friction force suddenly disappears in the load (the sum of the static friction load and the dynamic friction load), and the load approaches the dynamic friction load infinitely.

[0053] The failure period refers to a preset time period before and after the failure of the cementing surface. For example, if the cementing surface fails at time t, the failure period is (t - n, t + n), where n is a fault tolerance value preset according to the actual situation.

[0054] The load-displacement curve refers to the curve in which the load value changes with the loading displacement.

[0055] The cementing strength is determined by the first formula, and the first formula is:

[0056]

[0057] In the formula: M—the mechanical cementing strength of the interface;

[0058] F—the cementing failure pressure of the interface;

[0059] d—the inner diameter of the cement sheath 2;

[0060] H—the height of the cement sheath 2.

[0061] The cementing failure pressure of the interface refers to the applied pressure value corresponding to the failure of the cementing surface of the cement sheath.

[0062] Before performing the interface cementing strength test, it is necessary to first treat the oil and gas well cement to be tested, including but not limited to:

[0063] An inner casing 1 and an outer formation casing 3 are coaxially arranged from the inside to the outside. Cement slurry of the oil and gas well cement to be tested is injected into the annulus volume between the inner casing 1 and the outer formation casing 3, and cured for 7 days until the cement solidifies;

[0064] A circular gasket 5 with a thickness of 10 mm is sleeved at the lower end of the inner casing 1. The inner diameter is 4 - 6 mm larger than the outer diameter of the inner casing 1, and the outer diameter is more than 5 mm larger than the outer diameter of the outer formation casing 3, which is used to hold the cement sheath 2 and the outer formation casing 3 together for loading;

[0065] The outer diameter of the inner casing 1 is ≤ 95 / 8″, and the length is 900 - 1500 mm. The two ends of the outer formation casing 3 are respectively 50 - 70 mm shorter than the inner casing 1. Cylindrical plugs 7 are respectively welded at the two ends of the inner casing 1. The outer diameter of the plug 7 is 40 - 50 mm larger than the outer diameter of the inner casing 1. The pressure injection holes on the plug 7 are connected with a pressure test device, which can control the pressure in the inner casing 1 to 20 - 200 MPa, and can also realize the pressure cycle in the casing, that is, the pressure is increased to the maximum value, held for a certain time and then depressurized, and then increased to the maximum value again and held and depressurized, so as to cyclically load the pressure in the casing;

[0066] Place the overall structure of the inner casing 1 - cement sheath 2 - outer formation casing 3 on the testing machine. Place the lower plug 7 of the inner casing 1 on the circular lower die 6 of the testing machine, and the top of the upper plug 7 of the inner casing 1 contacts the upper die of the testing machine.

[0067] S1. Inject the cement for the oil and gas well to be detected into the annulus between the inner casing 1 and the outer formation casing 3, and cure to form the cement sheath 2. During the formation of the cement sheath, control the temperature of the heating pad 4 through the numerical control system so that the temperature of the cement sheath is within the preset temperature range. Among them:

[0068] The numerical control system refers to: heating elements, temperature sensors, controllers, display panels, etc. First, set the required control temperature, sample the real-time temperature through the temperature sensor until the set control temperature is reached, then stop heating, and the temperature control accuracy is ±0.3°C.

[0069] The preset temperature range is set according to the actual situation and the type of cement for the oil and gas well to be detected, etc., and is not limited here.

[0070] The outer wall of the outer formation casing 3 is coated with a heating pad 4. The heating pad 4 is connected to the temperature numerical control system with an accuracy of ±0.3°C. The heating pad 4 can heat the outer casing to a temperature of 20 - 200°C, and it can cool down to room temperature when the heating pad 4 is turned off. In this way, cycling can achieve the temperature cycling of the entire wellbore structure of the outer formation casing 3 - cement sheath 2 - casing.

[0071] S2. According to the experimental requirements, perform cyclic processing on the cement sheath. The cyclic processing includes: pressure application processing, heating processing, cooling processing, and pressure relief processing. Repeat the cyclic processing, and the number of cycles is the same as the on-site working conditions. After the cyclic processing, the upper die of the testing machine loads the first plug until the cementing surface in the cement sheath fails to bond. Determine the load-displacement curve during the failure of the cementing surface in the cement sheath, and determine the cementing strength of the cement sheath according to the load-displacement curve. Among them:

[0072] The experimental requirements include at least one of the following: temperature value, internal pressure value of the inner casing 1, and number of cyclic processing times.

[0073] The failure of the cementing surface to bond means that when the length of the inner casing 1 is greater than the length of the cement sheath, the upper die 8 of the testing machine presses the casing 1 downward, and the lower die of the testing machine drives the circular lower die 6 to press the cement sheath 2 and the outer formation casing 3 upward. The testing machine collects the shear load-displacement curve, and when the load suddenly drops, it is determined that the cementing surface of the cement sheath fails to bond.

[0074] After the pressure inside the inner casing 1 and the temperature of the outer formation casing 3 reach the preset values, in accordance with the requirements of the test procedure, turn off the heating pad 4 to cool down, and maintain the preset pressure value for the duration of one cycle. Then relieve the pressure to complete the loading process of one entire cycle. Continue to repeat the above processes of pressurization, heating, cooling, and pressure relief to complete the experiments with the specified number of multiple cycles.

[0075] The requirements of the test procedure mean that the application of the internal pressure load and the heating of the casing are carried out simultaneously. After the internal pressure load reaches the preset value, it is maintained for a certain period of time, which can be equal to the pressure-holding time during on-site fracturing construction. Within one cycle, after heating the casing to the preset temperature, turn off the heating system and let it cool naturally.

[0076] The cycle time refers to the pressure-holding time during on-site fracturing construction.

[0077] Start the testing machine. The upper cylinder and the lower cylinder of the testing machine load towards each other simultaneously, and the loading rate of both is 0.1 mm / min. The upper cylinder loads downward at a constant speed. After contacting the upper plug 7 of the casing, apply pressure to the inner casing 1. The lower cylinder loads upward at a constant speed and simultaneously supports the cement ring 2 and the outer formation casing 3 through the circular gasket 5 to apply pressure. The upper and lower cylinders load until the bonding at the first bonding surface of the casing-cement ring 2 fails.

[0078] Through the load-displacement curve recorded by the testing machine, obtain the bonding failure pressure of the cement ring 2 interface under the cyclic load condition. Based on the bonding failure pressure, calculate the bonding strength of the first bonding surface of the casing-cement ring 2 under this cyclic load condition, which is calculated by the following formula:

[0079]

[0080] In the formula: M - the mechanical bonding strength of the interface;

[0081] F - the bonding failure pressure of the interface;

[0082] d - the inner diameter of the cement ring 2;

[0083] H - the height of the cement ring 2.

[0084] The test of the bonding strength of the first interface of the casing-cement ring 2 is carried out as described in the above steps to determine the bonding strength of the cement ring 2 under the target parameters;

[0085] The target parameters include the preset temperature (20 - 200 °C), the preset internal pressure value (20 - 200 MPa), the number of internal pressure cycles (1 - 100), etc.

[0086] The load-displacement curve refers to the curve of the load values of the upper die and the lower die collected by the testing machine changing with the loading displacement.

[0087] Example 1

[0088] In a first aspect, the present technology proposes a full-size bonding strength experimental device for the interface of an oil and gas well cement sheath 2, including an inner casing 1 and an outer formation casing 3 coaxially arranged from the inside to the outside. The annulus volume between the inner casing 1 and the outer formation casing 3 is filled with cement slurry until it cures and solidifies into a cement sheath 2. Plugs 7 are welded to both ends of the inner casing 1, and a pressure injection hole on the plug 7 is connected to an internal press 9. The outer wall of the outer formation casing 3 is covered with a heating pad 4, and the heating pad 4 is connected to a temperature control system. The lower plug 7 of the inner casing 1 is placed on a circular lower mold 6, and the top of the upper plug 7 of the inner casing 1 contacts the upper mold 8 of an external testing machine.

[0089] Preferably, the height of the circular lower mold 6 is greater than half of the axial length difference between the casing 1 and the cement sheath 2. The inner casing 1 is of Q125 steel grade with a length of 900 mm; the outer formation casing 3 is of Q125 steel grade with a length of 800 mm; the circular lower mold 6 is made of H13 die steel, with an inner diameter of 200 mm, an outer diameter of 306 mm, and a height of 200 mm; the plug 7 is made of H13 die steel, with an inner diameter of 10 mm, an outer diameter of 180 mm, and a height of 100 mm.

[0090] In a second aspect, the present technology proposes a method for evaluating the full-size bonding strength of the interface of an oil and gas well cement sheath 2, applying the full-size bonding strength experimental device for the interface of the cement sheath 2 as described in the first aspect, including:

[0091] S1: An inner casing 1 and an outer formation casing 3 are coaxially arranged from the inside to the outside. Cement slurry is injected into the annulus volume between the inner casing 1 and the outer formation casing 3, and it is cured for 7 days until it solidifies into a cement sheath 2. The casing and the cement sheath 2 are loaded in opposite directions;

[0092] S2: A circular gasket 5 with a thickness of 10 mm, an inner diameter of 14 mm, and an outer diameter of 310 mm is sleeved at the lower end of the inner casing 1, which is used to hold the cement sheath 2 and the outer formation casing 3 together for loading;

[0093] S3: The inner casing 1 is 50 mm longer than the outer formation casing 3 at both ends respectively. Cylindrical plugs 7 are welded to both ends of the inner casing 1. The outer diameter of the plug 7 is 40.3 mm larger than the outer diameter of the inner casing 1. A pressure injection hole on the plug 7 is connected to an internal press 9, and the pressure inside the inner casing 1 can be controlled to 14 MPa (90% of the yield strength), and the pressure is maintained for 30 minutes;

[0094] S4: Place the overall assembly of the inner casing 1 - cement sheath 2 - outer formation casing 3 on an unconventional testing machine. Place the lower plug 7 of the inner casing 1 on the circular lower die 6 of the testing machine, and the top of the upper plug 7 of the inner casing 1 contacts the upper die 8 of the testing machine.

[0095] S5: Wrap a heating pad 4 around the outer wall of the outer formation casing 3. The heating pad 4 is connected to a temperature numerical control system. In this embodiment, the temperature is 20°C at room temperature.

[0096] S6: After the pressure inside the inner casing 1 and the temperature of the outer formation casing 3 reach the preset values of 14 MPa and 20°C, according to the requirements of the test procedure, maintain the preset pressure value for 30 min, and then relieve the pressure to complete a whole cycle of loading experiment.

[0097] S7: Start the testing machine. The upper cylinder and the lower cylinder of the testing machine load towards each other simultaneously, and the loading rate is 0.1 mm / min for both. The upper cylinder loads downward at a constant speed. After contacting the upper plug 7 of the casing, apply pressure to the inner casing 1. The lower cylinder loads upward at a constant speed, and simultaneously hold the cement sheath 2 and the outer formation casing 3 by the circular gasket 5 to apply pressure. The upper and lower cylinders load until the first bonding surface between the casing 1 - cement sheath 2 fails to bond.

[0098] S8: Through the load - displacement curve recorded by the testing machine, as Figure 2 , obtain the interface failure pressure of the cement sheath 2 as 186.3 MPa (612.2 + 574.1 = 186.3 MPa). Calculate the bonding strength of the first bonding surface between the casing - cement sheath 2 from the failure pressure, which is calculated by the following formula:

[0099]

[0100] Where: M - Interface mechanical bonding strength;

[0101] F - Interface bonding failure pressure;

[0102] d - Inner diameter of the cement sheath 2;

[0103] H - Height of the cement sheath 2.

[0104] The beneficial effect of this embodiment is: Obtain the bonding load - displacement curve of the Q125 steel grade casing after maintaining pressure for 30 min under an internal pressure of 14 MPa. Intuitively evaluate the bonding strength of the first interface between the casing - cement sheath 2 through a full - scale test, realize the bonding strength test of the casing after maintaining pressure for a period of time under high - pressure conditions, and solve the problem in related technologies that it is difficult to conduct a full - scale test on the bonding strength of the cement sheath 2 under complex working conditions.

[0105] Example 2

[0106] The cementing strength testing device of this embodiment is as follows Figure 1 shown. The sizes of the inner casing 1, the cement sheath 2, the outer formation casing 3, the circular gasket 5, the circular lower mold 6, and the plug 7 are the same as those in Embodiment 1. When no pressure is applied inside the casing 1 and no heating is applied outside the outer formation casing 3, the cementing strength of the first interface between the casing and the cement sheath 2 is directly tested. Through the load-displacement curve recorded by the testing machine, the interface failure pressure of the cement sheath 2 is obtained as 1547.4 MPa (791.1 + 756.3 = 1547.4 MPa). Based on the failure pressure, the cementing strength of the first cementing surface between the casing and the cement sheath 2 is calculated, and is obtained by the following formula:

[0107]

[0108] Furthermore, by comparing the cementing strength in the case where internal pressure is applied to the inner casing 1 and maintained for 30 minutes and the case where no internal pressure is applied to the inner casing 1, the influence law of different internal pressure values on the cementing strength can be obtained.

[0109] Similarly, by controlling the heating temperature of the outer formation casing 3, the influence law of different temperature values on the cementing strength can be tested.

[0110] Embodiment 3

[0111] The cementing strength testing device of this embodiment is as follows Figure 1 shown. The sizes of the inner casing 1, the cement sheath 2, the outer formation casing 3, the circular gasket 5, the circular lower mold 6, and the plug 7 are the same as those in Embodiment 1. Pressure starts to be applied inside the casing 1, rising from 0 to 100 MPa. At the same time, the outer wall of the outer formation casing 3 is covered with a heating pad 4 and starts to be heated to 90 °C. After keeping the temperature constant for 1 hour, the power supply of the heating pad 4 is turned off, and natural heat dissipation and cooling start. The internal pressure of the casing 1 remains at 100 MPa for 4 hours and then the pressure is relieved, completing a whole cycle. The above loading process is repeated. After 20 cycles, the cementing strength of the first cementing surface between the casing and the cement sheath 2 is calculated through the load-displacement curve recorded by the testing machine.

[0112] Based on the above solution, the present invention can also be improved as follows.

[0113] Furthermore, the process of determining the cementing strength of the cement sheath according to the load-displacement curve is specifically as follows:

[0114] Determine the interface cementing failure pressure according to the load-displacement curve;

[0115] Based on the interface cementing failure pressure, determine the cementing strength of the cement sheath.

[0116] Furthermore, the process of determining the cementing strength of the cement sheath based on the interface cementing failure pressure is as follows:

[0117] Determine the bonding strength of the cement sheath through the first formula;

[0118] The first formula is:

[0119]

[0120] Wherein, M is the bonding strength, F is the interfacial bonding failure pressure, d is the inner diameter of the cement sheath 2, and H is the height of the cement sheath 2.

[0121] Furthermore, the experimental requirements include:

[0122] At least one of the temperature value, the internal pressure value of the inner casing 1, and the number of circulation treatments.

[0123] Furthermore, the temperature value is any value in the range of 20 - 200 °C.

[0124] Furthermore, the internal pressure value is any value in the range of 20 - 200 Mpa.

[0125] Furthermore, the number of circulation treatments is any value in the range of 1 - 100.

[0126] Furthermore, it also includes:

[0127] Determine the influence law according to the bonding strength.

[0128] In the above embodiments, although the steps are numbered S1, S2, etc., these are only specific embodiments given by the present invention. Those skilled in the art can adjust the execution order of S1, S2, etc. according to the actual situation, and this is also within the protection scope of the present invention. It can be understood that in some embodiments, it may include some or all of the above embodiments.

[0129] It should be noted that the beneficial effects of the method for determining the interfacial bonding strength of the oil and gas well cement sheath 2 provided in the above embodiments are the same as those of the experimental device for the interfacial bonding strength of the oil and gas well cement sheath 2, and will not be elaborated here. In addition, when the system provided in the above embodiments realizes its functions, only the above division of each functional module is used for illustration. In actual applications, the above functions can be allocated to different functional modules according to needs, that is, the system can be divided into different functional modules according to the actual situation to complete all or part of the functions described above. In addition, the system and method embodiments provided above belong to the same concept, and the specific implementation process can be seen in the method embodiments, and will not be elaborated here.

[0130] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects, rather than to limit a specific order or sequence. In appropriate cases, the order of use of similar objects can be interchanged so that the embodiments of the present application described herein can be implemented in an order other than the illustrated or described order.

[0131] Those skilled in the art know that the present invention can be implemented as a system, method, or computer program product. Therefore, the present disclosure can be specifically implemented in the following forms, namely: it can be completely hardware, can also be completely software (including firmware, resident software, microcode, etc.), and can also be in the form of a combination of hardware and software, which is generally referred to as "circuit", "module", or "system" in this article. In addition, in some embodiments, the present invention can also be implemented in the form of a computer program product in one or more computer-readable media, which contains computer-readable program code.

[0132] Any combination of one or more computer-readable media can be adopted. The computer-readable media can be a computer-readable signal medium or a computer-readable storage medium. The computer-readable storage medium can be, for example, but not limited to, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, device, or component, or any combination of the above. More specific examples (non-exhaustive list) of the computer-readable storage medium include: an electrical connection with one or more wires, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the above. In this document, the computer-readable storage medium can be any tangible medium that contains or stores a program that can be used by or in conjunction with an instruction execution system, device, or component.

[0133] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for determining the bonding strength of the cement sheath interface in an oil and gas well, characterized in that, Including: Controlling the temperature of the cement sheath through a numerical control system to keep the temperature of the cement sheath within a preset temperature range; Performing cyclic treatment on the cement sheath according to experimental requirements. The cyclic treatment includes: pressure application treatment, heating treatment, cooling treatment, and pressure relief treatment. Repeat the cyclic treatment, and the number of cycles is the same as the required number of cycles in the field working conditions until the cementing surface in the cement sheath fails to cement. Determine the load-displacement curve during the failure of the cementing surface in the cement sheath, and determine the cementing strength of the cement in the cement sheath according to the load-displacement curve.

2. The method for determining the bonding strength of the cement sheath interface in an oil and gas well according to claim 1, wherein The process of determining the cementing strength of the cement in the cement sheath according to the load-displacement curve is specifically as follows: Determine the interface cementing failure pressure according to the load-displacement curve; Based on the interface cementing failure pressure, determine the cementing strength of the cement in the cement sheath.

3. A method for determining the bonding strength of the cement sheath interface in an oil and gas well according to claim 1, characterized in that, The process of determining the cementing strength of the cement in the cement sheath based on the interface cementing failure pressure is: Determine the cementing strength of the cement in the cement sheath through the first formula; The first formula is: Wherein, M is the cementing strength, F is the interface cementing failure pressure, d is the inner diameter of the cement sheath, and H is the height of the cement sheath.

4. A method for determining the bonding strength of the cement sheath interface in an oil and gas well according to claim 1, characterized in that, The experimental requirements include: At least one of the temperature value, the internal pressure value of the inner casing, and the number of cyclic treatments.

5. A method for determining the bonding strength of the cement sheath interface in an oil and gas well according to claim 4, characterized in that, The temperature value is any value in the range of 20 - 200 °C.

6. A method for determining the bonding strength of the cement sheath interface in an oil and gas well according to claim 4, characterized in that, The internal pressure value is any value in the range of 20 - 200 MPa.

7. A method for determining the bonding strength of the cement sheath interface in an oil and gas well according to claim 4, characterized in that, The number of cyclic treatments is any value in the range of 1 - 100.

8. A method for determining the bonding strength of the cement sheath interface in an oil and gas well according to claim 1, characterized in that, It also includes: Determine the influence law according to the cementing strength.

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