Method for evaluating shale wettability based on alternate spontaneous imbibition

Abstract

The invention discloses a method for evaluating the wettability of shale based on alternate spontaneous imbibition, which comprises the following steps: step (1), obtaining the apparent volume of a to-be-tested sample, then smearing sealant on the periphery of the to-be-tested sample, standing, and entering step (2) after the to-be-tested sample is air-dried; step (2), based on a low-field nuclear magnetic resonance method, calibrating a relation between the mass of the imbibition fluid and a nuclear magnetic semaphore; (3) alternately imbibing dodecane and a 8wt% KCl aqueous solution into the to-be-detected sample treated in the step (1), and monitoring the weight change of imbibition fluid imbibed into the to-be-detected sample by using a two-dimensional nuclear magnetic resonance spectrometer in the alternate imbibition process; (4) determining an oil wet pore volume and a water wet pore volume through a fluid calibration relation; and (5) calculating a main pore wettability index according to the content of the imbibition fluid, and then evaluating the wettability of the shale.

Description

Technical Field

[0001] The present invention relates to a method for evaluating shale wettability based on alternating spontaneous imbibition, and belongs to the field of unconventional oil and gas exploration and development. Background Art

[0002] The molecular state of the surface layer of a substance is different from its internal molecular state, and the energy of the surface layer molecules is higher than that of its internal molecules. When a solid substance comes into contact with a liquid substance, once an interface is formed, an adsorption phenomenon that reduces the surface energy will occur, and the liquid substance will spread out on the surface of the solid substance. This phenomenon of a liquid spreading on a solid surface is called wetting, and the ability of a liquid to spread on a solid surface is called the wettability of the liquid to the solid.

[0003] The wettability of reservoir rocks is a key factor affecting oil recovery. Wettability affects relative permeability, capillary pressure, oil and gas migration, and ultimate recovery. If the wettability of the reservoir is wrongly assumed, it may cause irreversible damage to the reservoir.

[0004] Yan Jienian et al. published "Evaluation of the Wetting Effect of Completion Fluids and Their Components on Sandstone by the Amott / USBM Method" in the 5th issue of Petroleum Exploration and Development in 1993. In the article, the authors used the Amott / USBM method to evaluate the influence of some commonly used domestic water-based and oil-based completion fluid systems and components on the wettability of natural sandstone reservoir rock samples in order to avoid the damage to the reservoir caused by the wettability reversal of reservoir rocks caused by external fluids such as completion fluids as much as possible. However, due to the extremely low permeability of shale, usually in the nanodarcy range, and strong heterogeneity, the evaluation of shale wettability is complicated. Traditional wettability testing methods such as the Amoott method and the U.S.B.M method are no longer applicable to the evaluation of shale wettability.

[0005] Currently, the commonly used methods for evaluating shale wettability include the contact angle method. For example, Chinese Patent Application CN 116067844A discloses a method, system, electronic device, and storage medium for discriminating shale wettability. The method for discriminating shale wettability includes: obtaining the mineral types and the content of each type of mineral in the target shale, as well as the organic matter content of the target shale; obtaining the target fluid contact angles of each type of mineral contained in the target shale and the target fluid contact angle of the organic matter contained in the target shale; based on the mineral types and the content of each type of mineral in the target shale, as well as the organic matter content of the target shale, combining the target fluid contact angles of each type of mineral contained in the target shale and the target fluid contact angle of the organic matter contained in the target shale, determining the target fluid contact angle of the overall target shale. Another example is that Chinese Patent Application CN 110132796A discloses a system for evaluating the three-dimensional contact angle and wetting heterogeneity of shale, including: a constant temperature box; a liquid dropping device; the liquid dropping device includes a support platform, a first moving mechanism arranged on the support platform, a slider, and a liquid dropping component located on the slider; the liquid dropping component is used for dropping liquid onto the shale sample; the first moving mechanism can move the slider in horizontal and vertical directions; a polishing device; the polishing device includes a sample platform, a sample kettle arranged on the sample platform, a heating component, a polishing machine, and a second moving mechanism; the second moving mechanism can move the shale sample in horizontal and vertical directions; the polishing machine is used for polishing the shale sample to form a measurement plane; an imaging device for photographing the liquid drop in contact with the shale sample; a controller; the controller can calculate the spatial contact angle and wetting heterogeneity parameters of the shale sample according to a predetermined rule.

[0006] The technical solutions of the above contact angle method all have the following deficiencies: they are greatly affected by the surface roughness and heterogeneity of the sample, and it is difficult to reflect the true wettability of the sample.

[0007] Currently, the commonly used methods for evaluating shale wettability also include the flotation method. However, the flotation method also has some deficiencies: using crushed shale samples destroys the in-situ pore network structure of the shale and exposes minerals that originally did not contribute to the in-situ pore connectivity. Its results are not sufficient to characterize the pore wetting characteristics of the sample. In addition, due to the presence of particles suspended at the oil-water interface, it is difficult to quantitatively analyze the wettability of mixed-wet shale by the flotation method.

[0008] Therefore, establishing a scientific, well-founded, and operable method for evaluating shale wettability has become a technical problem that urgently needs to be solved, which has important guiding significance for the exploration and development of shale. Summary of the Invention

[0009] Objective of the Invention: To solve the above-mentioned technical problems faced by the existing shale wettability evaluation, the present invention provides a method for evaluating shale wettability based on alternating spontaneous imbibition. The present invention mainly reflects the affinity of the pore network of shale for oil and water through the alternating spontaneous imbibition process, and combines two-dimensional nuclear magnetic resonance to characterize the change of fluid content during the imbibition process, so as to quantitatively characterize the wettability of shale.

[0010] Technical Solution: A method for evaluating shale wettability based on alternating spontaneous imbibition specifically includes the following steps:

[0011] Step (1): Obtain the apparent volume of the sample to be tested, then apply sealant around the sample to be tested, let it stand, and wait for it to dry before entering step (2);

[0012] Step (2): Based on the low-field nuclear magnetic resonance method, calibrate the relationship between the mass of the imbibing fluid and the nuclear magnetic signal quantity, where:

[0013] The imbibing fluid is dodecane and 8wt% KCl aqueous solution;

[0014] Step (3): Alternately imbibe dodecane and 8wt% KCl aqueous solution into the sample to be tested processed in step (1), and monitor the weight change of the imbibing fluid imbibed into the sample to be tested by a two-dimensional nuclear magnetic resonance instrument during the alternating imbibition process;

[0015] Step (4): Determine the oil-wet pore volume and water-wet pore volume through the fluid calibration relationship;

[0016] Step (5): Calculate the main pore wettability index through the content of the imbibing fluid, and then evaluate the shale wettability.

[0017] Further, obtaining the apparent volume of the sample to be tested in step (1) is based on one of the following methods:

[0018] (a) Drainage method;

[0019] (b) Blue light scanning method.

[0020] Furthermore, the steps of obtaining the apparent volume of the sample to be tested based on the drainage method in step (1) are as follows:

[0021] (11) Select a plug sample drilled parallel to the shale bedding plane as the sample to be tested;

[0022] (12) Weigh the weight M of the plug sample exposed to the air;

[0023] (13) With the help of a device for measuring volume by the suspension method, weigh the mass m of the plug sample immersed in the suspension fluid, then

[0024] The apparent volume V of the plug sample表 The formula is as follows:

[0025]

[0026] Where: M represents the weight of the plunger sample exposed to air, in g;

[0027] m represents the mass of the plunger sample immersed in the suspension fluid, in g;

[0028] ρ 液 represents the density of the suspension fluid, in g / cm 3 .

[0029] Furthermore, the suspension fluid is pure water or n-dodecane.

[0030] Further, the sealant in step (1) is one of epoxy resin, silicone sealant, polyurethane sealant, polysulfide sealant, acrylic sealant, butyl sealant, and chloroprene sealant.

[0031] Furthermore, the sealant in step (1) is epoxy resin.

[0032] Further, the steps of step (2) are as follows:

[0033] (21) Weigh different masses of the imbibition fluid into a test container without hydrogen signal in sequence;

[0034] (22) Place the test container in a low-field nuclear magnetic instrument and measure the nuclear magnetic resonance T2 spectrum of the imbibition fluid using the CPMG sequence;

[0035] (23) Invert the nuclear magnetic resonance T2 spectrum obtained in step (22), and obtain the nuclear magnetic signal corresponding to the imbibition fluid with different weights each time according to the obtained inversion result.

[0036] Furthermore, the test container without hydrogen signal is a glass bottle or a polytetrafluoroethylene bottle, preferably a glass bottle.

[0037] Furthermore, at least four different masses of the imbibition fluid are weighed into the test container without hydrogen signal in sequence in step (21).

[0038] Further, the steps of step (3) are as follows:

[0039] Complete four spontaneous imbibitions of the sample to be tested coated with sealant according to the imbibition sequence A or the imbibition sequence B to obtain the change result of the imbibition fluid content monitored by two-dimensional nuclear magnetic resonance, where:

[0040] The imbibition sequence A is to imbibe n-dodecane, 8wt% KCl aqueous solution, n-dodecane, and 8wt% KCl aqueous solution in sequence;

[0041] The imbibition sequence B is to imbibe 8 wt% KCl aqueous solution, dodecane, 8 wt% KCl aqueous solution, and dodecane in sequence.

[0042] Furthermore, the steps of each spontaneous imbibition are as follows:

[0043] Place the sample to be tested coated with sealant into the imbibition fluid. After a period of time, take out the sample to be tested, dry the imbibition fluid on the surface, and then place it in a low-field nuclear magnetic resonance. Measure the sample to be tested using the IR-CPMG sequence to obtain a T1T2 two-dimensional nuclear magnetic resonance spectrum. Invert the T1T2 two-dimensional nuclear magnetic resonance spectrum. For the inversion result of the two-dimensional spectrum, and based on the two-dimensional nuclear magnetic resonance identification map of hydrogen-containing fluids, identify the change in the signal amount of the imbibition fluid in the sample to be tested, and then place the sample to be tested back into the imbibition fluid;

[0044] After the fluid signal changes between two adjacent times become stable, replace the imbibition fluid.

[0045] Even further, the stability of the fluid signal changes between two adjacent times means that the difference in the fluid signals between two adjacent times is within 20 a.u.

[0046] Further, the specific steps of step (4) are as follows:

[0047] Based on the result of the change in the imbibition fluid content monitored by the two-dimensional nuclear magnetic resonance obtained in step (3), combined with the calibration relationship between the mass of the imbibition fluid and the nuclear magnetic signal amount, obtain the imbibition volume V C12 and V KCl液 The change reflects the change in the pore volume occupied by two types of fluids with different wettabilities during the imbibition process of the sample V 油湿 and V 水湿 , where:

[0048]

[0049]

[0050] In the formula:

[0051] V 油湿 —— Oil-wet pore volume, cm 3 ;

[0052] V C12 —— The volume of n-dodecane imbibed into the sample to be tested, cm 3 ;

[0053] m C12 —— The mass of n-dodecane imbibed into the sample to be tested, g;

[0054] ρ C12 —— The density of n-dodecane, g / cm3 ;

[0055] N C12实测 —— The nuclear magnetic signal quantity of the actually measured n - dodecane volume of the sample to be measured, a.u;

[0056] A C12 —— The nuclear magnetic signal quantity of dodecane per unit mass, a.u / g;

[0057] V 水湿 —— The water - wet pore volume, cm 3 ;

[0058] V KCl液 —— The volume of the 8wt% KCl aqueous solution imbibed into the sample to be measured, cm 3 ;

[0059] m KCl液 —— The mass of the 8wt% KCl aqueous solution imbibed into the sample to be measured, g;

[0060] ρ KCl液 —— The density of the 8wt% KCl aqueous solution, g / cm 3 ;

[0061] N KCl液实测 —— The nuclear magnetic signal quantity of the actually measured 8wt% KCl aqueous solution volume of the sample to be measured, a.u;

[0062] A KCl液 —— The nuclear magnetic signal quantity of the 8% KCl solution per unit mass, a.u / g.

[0063] Furthermore, step (5) includes the following steps:

[0064] (51), Calculate the mixed wetting porosity of the sample to be measured after the alternate imbibition ends The strong oil - wet porosity of the sample to be measured after the alternate imbibition ends The strong water - wet porosity of the sample to be measured after the alternate imbibition ends Among them:

[0065]

[0066]

[0067] Among them:

[0068] V 混合 represents the volume occupied by the mixed wetting pore part, that is, the fluid volume converted from the increment of the oil signal or water signal in the latter two imbibition stages;

[0069] V 水It represents the volume occupied by the strongly water-wet pore part, that is, the fluid volume converted when the water signal is stable during the water absorption stage in the latter two imbibition stages;

[0070] V 油 It represents the volume occupied by the strongly oil-wet pore part, that is, the fluid volume converted when the oil signal is stable during the oil absorption stage in the latter two imbibition stages;

[0071] V 表 It represents the apparent volume of the plunger sample;

[0072] (52) Calculate the main pore wettability index DWI, and its calculation formula is:

[0073]

[0074]

[0075] In the formula: —— The final porosity of the sample to be measured after the alternating imbibition, %;

[0076] —— The mixed wetting porosity of the sample to be measured after the alternating imbibition, %;

[0077] —— The strongly oil-wet porosity of the sample to be measured after the alternating imbibition, %;

[0078] —— The strongly water-wet porosity of the sample to be measured after the alternating imbibition, %;

[0079] DWI—— The main pore wettability index of the sample to be measured, dimensionless;

[0080] When the DWI index is equal to 0, it reflects that the oil-wet and water-wet pores of the sample to be measured are equivalent;

[0081] (53) Judge and evaluate the wettability of shale:

[0082] When the DWI index is greater than 0, it reflects that the water-wet pores of the sample dominate, and the larger the value, the stronger the water-wetting;

[0083] When the DWI index is less than 0, it reflects that the oil-wet pores of the sample dominate, and the smaller the value, the stronger the oil-wetting.

[0084] Beneficial effects: A method for evaluating the wettability of shale based on alternating spontaneous imbibition disclosed by the present invention has the following beneficial effects:

[0085] 1. It realizes the evaluation of the wettability of shale, divides the wettability of shale into three grades, and quantifies three types of wetting pore spaces;

[0086] 2. The result of this method has strong reliability, sufficient basis, and is easy to operate, which has important guiding significance for shale oil exploration and development. Description of the Drawings

[0087] Figure 1 It is a flowchart of a method for evaluating shale wettability based on alternating spontaneous imbibition disclosed by the present invention.

[0088] Figure 2 It is a calibration diagram of the nuclear magnetic resonance signal of n-dodecane.

[0089] Figure 3 It is a calibration diagram of the nuclear magnetic resonance signal of 8 wt% KCl aqueous solution.

[0090] Figure 4 It is a diagram of the change in the imbibition porosity of the sample. Detailed Embodiments:

[0091] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0092] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0093] A method for evaluating shale wettability based on alternating spontaneous imbibition specifically includes the following steps:

[0094] Step (1): Obtain the apparent volume of the sample to be tested, then apply sealant around the sample to be tested, let it stand, and after it dries, proceed to step (2);

[0095] Step (2): Based on the low-field nuclear magnetic resonance method, calibrate the relationship between the mass of the imbibition fluid and the amount of nuclear magnetic resonance signal, where:

[0096] The imbibition fluid is dodecane and 8 wt% KCl aqueous solution;

[0097] Step (3): Alternately imbibe the sample to be tested processed in step (1) with dodecane and 8 wt% KCl aqueous solution, and monitor the weight change of the imbibed fluid imbibed into the sample to be tested by using a two-dimensional nuclear magnetic resonance instrument during the alternate imbibition process;

[0098] Step (4): Determine the oil-wet pore volume and water-wet pore volume through the fluid calibration relationship;

[0099] Step (5): Calculate the main pore wettability index through the content of the imbibed fluid, and then evaluate the shale wettability.

[0100] Furthermore, the apparent volume of the sample to be tested obtained in step (1) is based on one of the following methods:

[0101] (a) Drainage method;

[0102] (b) Blue light scanning method.

[0103] Even further, the steps for obtaining the apparent volume of the sample to be tested based on the drainage method in step (1) are as follows:

[0104] (11) Select a plug sample drilled parallel to the shale bedding plane as the sample to be tested;

[0105] (12) Weigh the weight M of the plug sample exposed to the air;

[0106] (13) With the aid of a device for measuring volume by the suspension method, weigh the mass m of the plug sample immersed in the suspension fluid, then

[0107] The apparent volume V 表 of the plug sample is calculated by the following formula:

[0108]

[0109] where: M represents the weight of the plug sample exposed to the air, with the unit of g;

[0110] m represents the mass of the plug sample immersed in the suspension fluid, with the unit of g;

[0111] ρ 液 represents the density of the suspension fluid, with the unit of g / cm 3 .

[0112] Even more further, the suspension fluid is pure water or n-dodecane.

[0113] Furthermore, the sealant in step (1) is one of epoxy resin, silicone sealant, polyurethane sealant, polysulfide sealant, acrylic sealant, butyl sealant, and chloroprene sealant.

[0114] Furthermore, the sealant in step (1) is epoxy resin.

[0115] Explain step (1) in detail:

[0116] Place the plunger sample on a tray exposed to air and then on a tray immersed in a suspension fluid (pure water or n-dodecane) successively to obtain the corresponding masses M and m of the plunger sample in the two states. Then, the buoyancy force F_buoy of the plunger sample immersed in the fluid is:

[0117] F 浮 = M·g - m·g

[0118] Combined with Archimedes' law, the buoyancy force acting on an object is equal to the weight of the fluid displaced by the object with volume V 排 i.e.:

[0119] F 浮 = ρ 液 ·V 排 ·g

[0120] The apparent volume of the sample is the volume of the fluid displaced by the plunger sample:

[0121]

[0122] In the formula: V 表 —— Apparent volume of the plunger sample, cm 3 ;

[0123] V 排 —— Volume of the fluid displaced by the plunger sample, cm 3 ;

[0124] F 浮 —— Buoyancy force acting on the plunger sample, N;

[0125] ρ 液 —— Density of the suspension fluid used, g / cm 3 ;

[0126] g —— Acceleration due to gravity, m / s 2 ;

[0127] M —— Mass of the plunger sample in air, g;

[0128] m —— Mass of the plunger sample completely immersed in the fluid, g.

[0129] When the sample to be measured in step (1) is a plunger sample, the circumference side of the sample to be measured is its circumferential side, that is, no sealant is applied to the bottom and top surfaces;

[0130] The purpose of applying sealant around the sample to be measured is to ensure that the sample to be measured undergoes imbibition along the unidirectional parallel bedding direction during the subsequent alternating spontaneous imbibition process.

[0131] Further, the steps of step (2) are as follows:

[0132] (21) Weigh different masses of the imbibition fluid into a test container without hydrogen signal in sequence;

[0133] (22) Place the test container in a low-field nuclear magnetic resonance instrument, and measure the nuclear magnetic resonance T2 spectrum of the imbibition fluid using the CPMG sequence;

[0134] (23) Invert the nuclear magnetic resonance T2 spectrum obtained in step (22), and obtain the nuclear magnetic resonance signal corresponding to the imbibition fluid with different weights each time according to the obtained inversion result.

[0135] Furthermore, the test container without hydrogen signal is a glass bottle or a polytetrafluoroethylene bottle, preferably a glass bottle.

[0136] Furthermore, in step (21), at least four different masses of the imbibition fluid are weighed into a test container without hydrogen signal in sequence.

[0137] The following is an explanatory note for step (2):

[0138] The nuclear magnetic resonance instrument used in step (2) is the MicroMR23-060H-1 type nuclear magnetic resonance instrument produced by Suzhou Niumag Co., Ltd., with a resonance frequency of 21.36 MHz, a magnet strength of 0.5 T, a coil diameter of 25 mm, the inversion software is Niumag nuclear magnetic data analysis software V1.1, and the inversion method is the BRD inversion;

[0139] Through the calibration of the nuclear magnetic resonance signals of the imbibition fluid with four or more different masses, the conversion relationship between the mass of the imbibition fluid and the amount of nuclear magnetic resonance signal is obtained by fitting:

[0140]

[0141]

[0142] In the formula: m C12 —— The mass of dodecane, g;

[0143] N C12 —— The signal amount of dodecane, a.u;

[0144] A C12 —— The nuclear magnetic resonance signal amount per unit mass of dodecane, a.u / g;

[0145] m KCl液 —— The mass of 8wt% KCl aqueous solution, g;

[0146] N KCl液 —— The signal amount of 8wt% KCl aqueous solution, a.u;

[0147] A KCl液 —— The nuclear magnetic signal quantity of the 8 wt% KCl aqueous solution per unit mass, a.u / g.

[0148] Furthermore, the steps of step (3) are as follows:

[0149] Place the sample to be tested coated with sealant according to the imbibition sequence A or the imbibition sequence B to complete four spontaneous imbibitions to obtain the change result of the imbibition fluid content monitored by two-dimensional nuclear magnetic resonance, where:

[0150] The imbibition sequence A is to imbibe dodecane, 8 wt% KCl aqueous solution, dodecane, 8 wt% KCl aqueous solution in sequence;

[0151] The imbibition sequence B is to imbibe 8 wt% KCl aqueous solution, dodecane, 8 wt% KCl aqueous solution, dodecane in sequence.

[0152] Even further, the steps of each spontaneous imbibition are as follows:

[0153] Place the sample to be tested coated with sealant in the imbibition fluid. After an interval of time (such as 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 16 h, 20 h, 24 h, 28 h, 36 h, 44 h, 58 h, 70 h, 82 h, 94 h, 106 h, 118 h, 130 h, 145 h, 160 h, 175 h, 190 h, 205 h...), take out the sample to be tested, dry the imbibition fluid on the surface, and then place it in low-field nuclear magnetic resonance. Use the IR-CPMG sequence to measure the sample to be tested to obtain the T1T2 two-dimensional nuclear magnetic resonance spectrum. Invert the T1T2 two-dimensional nuclear magnetic resonance spectrum, and based on the two-dimensional spectrum inversion result and the two-dimensional nuclear magnetic identification map of hydrogen-containing fluid, identify the change in the signal quantity of the imbibition fluid in the sample to be tested, and then place the sample to be tested in the imbibition fluid again;

[0154] After the fluid signal changes between two adjacent times are stable, replace the imbibition fluid.

[0155] Even more further, the stability of the fluid signal changes between two adjacent times means that the difference between the fluid signals of two adjacent times is within 20 a.u.

[0156] The following is an explanation of step (3):

[0157] The time of each spontaneous imbibition is at least 200 hours. The interval time before performing two-dimensional nuclear magnetic resonance is relatively small because the change amplitude is relatively large in the early stage, and the interval time in the later stage can be appropriately increased. For example, the following times can be used for detection:

[0158] 1h, 2h, 4h, 6h, 8h, 10h, 12h, 16h, 20h, 24h, 28h, 36h, 44h, 58h, 70h, 82h, 94h, 106h, 118h, 130h, 145h, 160h, 175h, 190h, 205h…;

[0159] Of course, each spontaneous imbibition can also be at other detection times, as long as it satisfies the principle of "smaller interval time in the front and appropriately larger interval time in the back".

[0160] The nuclear magnetic resonance instrument used is the MicroMR23 - 060H - 1 type nuclear magnetic resonance instrument produced by Suzhou Niumai Co., Ltd., with a resonance frequency of 21.36 MHz, a magnet strength of 0.5 T, a coil diameter of 25 mm, the inversion software being Niumai nuclear magnetic resonance data analysis software V1.1, and the inversion method being 2D inversion.

[0161] Furthermore, the specific steps of step (4) are as follows:

[0162] For the result of the change in the imbibition fluid content monitored by two - dimensional nuclear magnetic resonance obtained in step (3), combined with the calibration relationship between the mass of the imbibition fluid and the nuclear magnetic signal amount, the fluid imbibition volume V C12 and V KCl液 are obtained. The changes in V 油湿 and V 水湿 reflect the changes in the pore volume occupied by two types of fluids with different wettabilities during the imbibition process of the sample, where:

[0163]

[0164]

[0165] In the formula:

[0166] V 油湿 —— Oil - wet pore volume, cm 3 ;

[0167] V C12 —— The volume of n - dodecane imbibed into the sample to be measured, cm 3 ;

[0168] m C12 —— The mass of n - dodecane imbibed into the sample to be measured, g;

[0169] ρ C12 —— The density of n - dodecane, g / cm 3 ;

[0170] N C12实测 —— The nuclear magnetic signal amount of the actually measured n - dodecane volume in the sample to be measured, a.u;

[0171] A C12 —— Nuclear magnetic signal quantity of dodecane per unit mass, a.u / g;

[0172] V 水湿 —— Water-wet pore volume, cm 3 ;

[0173] V KCl液 —— Volume of 8 wt% KCl aqueous solution imbibed into the sample to be measured, cm 3 ;

[0174] m KCl液 —— Mass of 8 wt% KCl aqueous solution imbibed into the sample to be measured, g;

[0175] ρ KCl液 —— Density of 8 wt% KCl aqueous solution, g / cm 3 ;

[0176] N KCl液实测 —— Nuclear magnetic signal quantity of the actually measured 8 wt% KCl aqueous solution volume in the sample to be measured, a.u;

[0177] A KCl液 —— Nuclear magnetic signal quantity of 8% KCl solution per unit mass, a.u / g.

[0178] Furthermore, in step (5), both imbibition sequence A and imbibition sequence B go through four alternating imbibition stages. Through the analysis of the changes in the porosity of the two types of fluids during the imbibition process, it is found that:

[0179] There is a mixed-wet pore part in the pore space of the sample. The mixed-wet pore part can both imbibe oily n-dodecane and water-absorbing 8 wt% KCl aqueous solution. During the alternating imbibition process, the fluid imbibed into this part of the pore space in the previous stage is displaced by the fluid being imbibed. Therefore:

[0180] The pore network in the sample to be measured is divided into a strongly oil-wet pore part, a strongly water-wet pore part, and a mixed-wet pore part.

[0181] Furthermore, step (5) includes the following steps:

[0182] (51), Calculate the mixed-wet porosity of the sample to be measured after the alternating imbibition ends The strongly oil-wet porosity of the sample to be measured after the alternating imbibition ends The strongly water-wet porosity of the sample to be measured after the alternating imbibition ends Wherein:

[0183]

[0184]

[0185] Among them:

[0186] V 混合 represents the volume occupied by the mixed-wet pore part, that is, the fluid volume converted from the increment of the oil signal or water signal in the latter two imbibition stages;

[0187] V 水 represents the volume occupied by the strongly water-wet pore part, that is, the fluid volume converted when the water signal is stable in the water imbibition stage of the latter two imbibition stages;

[0188] V 油 represents the volume occupied by the strongly oil-wet pore part, that is, the fluid volume converted when the oil signal is stable in the oil imbibition stage of the latter two imbibition stages;

[0189] V 表 represents the apparent volume of the plunger sample;

[0190] (52) Calculate the main pore wettability index DWI, and its calculation formula is:

[0191]

[0192]

[0193] In the formula: —— The final porosity of the sample to be measured after the alternating imbibition, %;

[0194] —— The mixed-wet porosity of the sample to be measured after the alternating imbibition, %;

[0195] —— The strongly oil-wet porosity of the sample to be measured after the alternating imbibition, %;

[0196] —— The strongly water-wet porosity of the sample to be measured after the alternating imbibition, %;

[0197] DWI—— The main pore wettability index of the sample to be measured, dimensionless;

[0198] When the DWI index is equal to 0, it reflects that the oil-wet and water-wet pores of the sample to be measured are equivalent;

[0199] (53) Judge and evaluate the wettability of shale:

[0200] When the DWI index is greater than 0, it reflects that the water-wet pores of the sample dominate, and the larger the value, the stronger the water-wetting;

[0201] When the DWI index is less than 0, it reflects that the oil-wet pores of the sample dominate, and the smaller the value, the stronger the oil-wetting.

[0202] Example 1:

[0203] This example uses shale parallel samples A and B. Both samples A and B are plug samples drilled in the direction of parallel bedding.

[0204] As Figure 1 shown, a method for evaluating shale wettability based on alternating spontaneous imbibition specifically includes the following steps:

[0205] Step (1), obtain the apparent volume of the sample to be tested, then apply sealant around the sample to be tested, let it stand, and wait for it to dry before entering step (2);

[0206] Step (2), based on the low-field nuclear magnetic resonance method, calibrate the relationship between the mass of the imbibition fluid and the nuclear magnetic signal amount, where:

[0207] The imbibition fluid is dodecane and 8wt% KCl aqueous solution;

[0208] Step (3), alternately imbibe dodecane and 8wt% KCl aqueous solution into the sample to be tested processed in step (1), and monitor the weight change of the imbibition fluid imbibed into the sample to be tested by using a two-dimensional nuclear magnetic resonance instrument during the alternating imbibition process;

[0209] Step (4), determine the oil-wet pore volume and the water-wet pore volume through the fluid calibration relationship;

[0210] Step (5), calculate the main pore wettability index through the content of the imbibition fluid, and then evaluate the shale wettability.

[0211] Further, in step (1): Based on the drainage method, first obtain the apparent volumes of samples A and B. The liquid displaced is n-dodecane, and its density is 0.753 g / cm 3 .

[0212] The mass M of sample A in air is measured to be 14.8350 g, and the mass m when immersed in n-dodecane is 10.4464 g. The apparent volume V of sample A is calculated by the apparent volume calculation formula 表 to be 5.8282 cm 3 .

[0213] Sample B is a parallel sample of sample A. Its mass in air is 13.4031 g. According to the apparent volume measured by the drainage method of sample A, the apparent volume of sample B is converted to 5.4182 cm 3 .

[0214] Further, the sealant in step (1) is epoxy resin.

[0215] Further, in step (2):

[0216] The imbibition fluids used in this alternating imbibition experiment are n-dodecane representing an oily fluid and an 8 wt% KCl aqueous solution representing an aqueous fluid, respectively.

[0217] By weighing different masses of the two fluids to carry out the measurement of nuclear magnetic resonance signals, the experimental data are shown in Table 1, and then the calibration relationship between the fluid mass and the nuclear magnetic resonance signal amount is obtained, as Figure 2 and Figure 3 shown.

[0218] Table 1 Imbibition fluid calibration data

[0219]

[0220]

[0221] Further, in step (3):

[0222] Carry out an alternating imbibition experiment on sample A coated with epoxy resin on all sides according to sequence A (imbibing n-dodecane, 8 wt% KCl aqueous solution, n-dodecane, 8 wt% KCl aqueous solution in sequence);

[0223] Carry out an alternating imbibition experiment on sample B according to sequence B (imbibing 8 wt% KCl aqueous solution, n-dodecane, 8 wt% KCl aqueous solution, n-dodecane in sequence).

[0224] Measure the two-dimensional nuclear magnetic resonance signals of the samples at regular intervals during the imbibition process to monitor the change in the content of the imbibition fluid. The fluid change results after nuclear magnetic resonance signal extraction are shown in Table 2.

[0225] Table 2 Change in fluid content during the imbibition process of the samples

[0226]

[0227]

[0228]

[0229] Further, in step (4):

[0230] For the change results of the imbibition fluid content monitored by two-dimensional nuclear magnetic resonance, combined with the calibration relationship between the fluid content and the nuclear magnetic resonance signal amount and the apparent volume of the samples, calculate the change in the pore space content occupied by the two types of fluids during the imbibition process of the two samples, as shown in Table 3, and plot it in combination with time Figure 4 shown. Figure 4 Mix-wet is the abbreviation of mixed wetting porosity. In order to make the drawing clearer, the percentage sign is omitted in the representation of porosity.

[0231] According to the results, it can be seen that part of the pore space is always occupied by n - dodecane and is classified as the strongly oil - wet pore part;

[0232] Part of the pore space is always occupied by an 8 wt% KCl aqueous solution and is classified as the strongly water - wet pore part;

[0233] There is also part of the pore space during the alternating imbibition process, where the fluid contained before alternation can be driven and replaced by the fluid imbibed after alternation, and is classified as the mixed - wet pore part.

[0234] Table 3 Calculation results of fluid porosity

[0235]

[0236]

[0237]

[0238] Furthermore, in step (5):

[0239] According to the pore - space division results of the samples, the DWI index is calculated, as shown in Table 4, where:

[0240] The total porosity of sample A is 6.28%, the strongly oil - wet porosity is 4.65%, the strongly water - wet porosity is 0.98%, the mixed - wet porosity is 0.65%, and the DWI index is - 0.58;

[0241] The total porosity of sample B is 6.09%, the strongly oil - wet porosity is 4.44%, the strongly water - wet porosity is 0.96%, the mixed - wet porosity is 0.69%, and the DWI index is - 0.57. The DWI indices of the two parallel samples both reflect that the sample is an oil - wet shale.

[0242] Table 4 Pore division and DWI index results

[0243] Sample number Total porosity / % Strong oil-wet pores / % Strong water-wet pores / % Mixed wetting pores / % DWI A 6.28 4.65 0.98 0.65 -0.58 B 6.09 4.44 0.96 0.69 -0.57

[0244] The wettability results characterized by the two imbibition sequences carried out on the two parallel samples show good consistency, fully proving that this method has good repeatability, reliability, and feasibility in the process of characterizing the wettability of shale.

[0245] Example 2

[0246] It is substantially the same as Example 1, with the only difference being that the sealant in step (1) is silicone sealant.

[0247] Example 3

[0248] It is substantially the same as Example 1, with the only difference being that the sealant in step (1) is polyurethane sealant.

[0249] Example 4

[0250] It is substantially the same as Example 1, except that: the sealant in step (1) is polysulfide sealant.

[0251] Example 5

[0252] It is substantially the same as Example 1, except that: the sealant in step (1) is acrylic sealant.

[0253] Example 6

[0254] It is substantially the same as Example 1, except that: the sealant in step (1) is butyl sealant.

[0255] Example 7

[0256] It is substantially the same as Example 1, except that: the sealant in step (1) is neoprene sealant.

[0257] The above has made a detailed description of the embodiments of the present invention. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the gist of the present invention within the knowledge scope of those of ordinary skill in the art.

Claims

1. A method for evaluating shale wettability based on alternating spontaneous imbibition, characterized in that: The following steps are involved: Step (1), obtaining the apparent volume of the sample to be tested, then applying sealant around the sample to be tested, leaving it to stand, and then proceeding to step (2) after it is dried; Step (2), based on the low-field nuclear magnetic resonance method, calibrate the relationship between the mass of the imbibition fluid and the amount of nuclear magnetic resonance signal, wherein: The imbibition fluid is dodecane and 8wt% KCl aqueous solution; Step (3), alternately imbibing dodecane and 8 wt % KCl aqueous solution into the sample to be tested treated in step (1), and monitoring the weight change of the imbibed fluid into the sample to be tested by a two-dimensional nuclear magnetic resonance spectrometer during the alternating imbibition process; Step (4), determining the oil-wet pore volume and the water-wet pore volume through a fluid calibration relationship; Step (5), calculating the main pore wettability index by the content of imbibed fluid, and then evaluating the shale wettability.

2. A method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: The apparent volume of the sample to be tested in step (1) is obtained based on one of the following methods: (a), drainage method; (b) Blue light scanning method.

3. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: The steps of obtaining the apparent volume of the sample to be tested based on the liquid displacement method in step (1) are as follows: (11) Selecting a plunger sample drilled parallel to the shale bedding plane as the sample to be tested; (12) Weigh the plunger sample exposed to air, M; (13) Using a suspension volume measuring device, weigh the mass m of the plunger sample immersed in the suspension fluid, then Apparent volume V of the plunger sample 表 The formula is as follows: Where: M represents the weight of the plunger sample exposed to air, in g; m represents the mass of the plunger sample immersed in the suspension fluid, in g; ρ 液 Indicates the density of the suspension fluid in g / cm 3 .

4. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: The suspension fluid is pure water or n-dodecane.

5. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: The sealant in step (1) is one of epoxy resin, silicone sealant, polyurethane sealant, polysulfide sealant, acrylic sealant, butyl sealant and chloroprene sealant.

6. A method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 5, characterized in that: The sealant in step (1) is epoxy resin.

7. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: The steps of step (2) are as follows: (21) Sequentially weigh different masses of imbibition fluid into a test container without hydrogen signal; (22), placing the test container in a low-field nuclear magnetic resonance instrument, and measuring the nuclear magnetic resonance T2 spectrum of the imbibed fluid using a CPMG sequence; (23) Inverting the nuclear magnetic resonance T2 spectrum obtained in step (22), and obtaining the nuclear magnetic resonance signals corresponding to the different weights of the imbibition fluid each time according to the obtained inversion results.

8. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 7, characterized in that: The test container without hydrogen signal is a glass bottle or a polytetrafluoroethylene bottle, preferably a glass bottle.

9. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 7, characterized in that: In step (21), at least four imbibition fluids of different masses are weighed sequentially into a test container without hydrogen signal.

10. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: The steps of step (3) are as follows: The sample to be tested after applying the sealant is subjected to four spontaneous imbibitions according to the imbibition sequence A or the imbibition sequence B to obtain the imbibition fluid content change results monitored by two-dimensional nuclear magnetic resonance, where: The imbibition sequence A is to sequentially imbibe dodecane, 8 wt % KCl aqueous solution, dodecane, and 8 wt % KCl aqueous solution; The imbibition sequence B is to sequentially imbibe 8 wt % KCl aqueous solution, dodecane, 8 wt % KCl aqueous solution, and dodecane.

11. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 10, characterized in that: The steps for each spontaneous imbibition are as follows: The sample to be tested after being coated with sealant is placed in an imbibition fluid. After a certain period of time, the sample to be tested is taken out, the imbibition fluid on the surface is wiped off, and then the sample is placed in a low-field nuclear magnetic resonance. The sample to be tested is measured using an IR-CPMG sequence to obtain a T1T2 two-dimensional nuclear magnetic resonance spectrum, and the T1T2 two-dimensional nuclear magnetic resonance spectrum is inverted. Based on the inversion result of the two-dimensional spectrum and the two-dimensional nuclear magnetic resonance identification spectrum of the hydrogen-containing fluid, the change in the signal amount of the imbibition fluid in the sample to be tested is identified, and then the sample to be tested is placed in the imbibition fluid; After two adjacent fluid signal changes are stable, replace the imbibition fluid.

12. A method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 11, characterized in that: The changes of the fluid signals between two adjacent times are stable so that the difference between the fluid signals between two adjacent times is within 20 a.u.

13. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: The specific steps of step (4) are as follows: Based on the results of the imbibition fluid content change monitored by two-dimensional nuclear magnetic resonance obtained in step (3), combined with the calibration relationship between the imbibition fluid mass and the nuclear magnetic resonance signal quantity, the fluid imbibition volume V is obtained. C12 and V KCl液 The change of V reflects the change of the pore volume occupied by two types of fluids with different wettability during the sample infiltration process. 油湿 and V 水湿 ,in: Where: V 油湿 ——Oil-wet pore volume, cm 3 ; V C12 ——The volume of n-dodecane absorbed into the sample to be tested, cm 3 ; m C12 ——The mass of n-dodecane absorbed into the sample to be tested, g; ρ C12 ——density of n-dodecane, g / cm 3 ; N C12实测 ——NMR signal of the measured volume of n-dodecane of the sample to be tested, au; A C12 ——NMR signal of dodecane per unit mass, au / g; V 水湿 ——water-wet pore volume, cm 3 ; V KCl液 ——The volume of 8wt% KCl aqueous solution absorbed into the sample to be tested, cm 3 ; m KCl液 ——The mass of 8wt% KCl aqueous solution absorbed into the sample to be tested, g; ρ KCl液 ——Density of 8wt% KCl aqueous solution, g / cm 3 ; N KCl液实测 ——NMR signal volume of 8wt% KCl aqueous solution of the sample to be tested, au; A KCl液 ——NMR signal per unit mass of 8% KCl solution, au / g.

14. The method for evaluating shale wettability based on alternating spontaneous imbibition according to claim 1, characterized in that: Step (5) comprises the following steps: (51) Calculate the mixed wetting porosity of the sample after the alternating imbibition is completed. Strong oil-wet porosity of the sample to be tested after alternating imbibition Strong water-wet porosity of the sample to be tested after the alternating imbibition is completed in: in: V 混合 It represents the volume occupied by the mixed wet pores, that is, the fluid volume converted from the oil signal or water signal increment in the last two imbibition stages; V 水 It represents the volume occupied by the strongly water-wet pores, that is, the fluid volume converted when the water signal in the water absorption stage is stable in the last two imbibition stages; V 油 It represents the volume occupied by the strongly oil-wet pores, that is, the fluid volume converted when the oil signal is stable in the oil absorption stage in the last two imbibition stages; V 表 represents the apparent volume of the plunger sample; (52) Calculate the main pore wettability index DWI, and the calculation formula is: Where: ——The final porosity of the sample to be tested after the alternating imbibition is completed, %; ——Mixed wetting porosity of the sample to be tested after the alternating imbibition is completed, %; ——The strong oil-wet porosity of the sample to be tested after the alternating imbibition is completed, %; ——The strong water-wet porosity of the sample to be tested after the alternating imbibition is completed, %; DWI is the main pore wettability index of the sample to be tested, dimensionless; When the DWI index is equal to 0, it reflects that the oil-wet and water-wet pores of the sample to be tested are equivalent; (53) Determine and evaluate the wettability of shale: When the DWI index is greater than 0, it reflects that the water-wetted pores of the sample are dominant, and the larger the value, the stronger the water wetting; When the DWI index is less than 0, it reflects that the oil-wetted pores of the sample are dominant, and the smaller the value, the stronger the oil wetting.

Images