Reservoir damage evaluation method suitable for fractured bedrock gas reservoir

Abstract

The invention discloses a reservoir damage evaluation method suitable for a fractured bedrock gas reservoir, and the evaluation method is used for evaluating the sensitivity of an ultra-low permeability reservoir matrix core and judging the damage evaluation of a well entering fluid to a fracture. Aiming at the problem that a systematic and comprehensive damage evaluation method is not formed in a fractured bedrock gas reservoir at present, the method integrates an unsteady-state permeability determination method applied to a tight gas reservoir and a method for simulating various in-well fluid damages by preparing fractures through Brazilian splitting, and is a comprehensive evaluation method giving consideration to fracture and matrix damage evaluation. The comprehensive systematic damage evaluation of the fractured bedrock gas reservoir from the sensitivity evaluation to the in-well fluid damage is completed.

Description

Technical Field

[0001] The invention belongs to the field of petroleum analysis, and in particular relates to a reservoir damage evaluation method suitable for fractured bedrock gas reservoirs. Background Art

[0002] Most of the bedrock reservoirs in the frontal belt of some mountain ranges are fractured metamorphic rocks with complex lithology, dense reservoirs, and core matrix permeability generally less than 0.01mD. At present, there are many studies on this type of reservoir in China in terms of gas reservoir recognition, resource evaluation, reservoir formation mechanism, and production enhancement technology, but there is no reference material for systematic and comprehensive reservoir damage evaluation. The reason is that there is no corresponding standard or specification for reservoir damage evaluation of dense low-permeability gas reservoirs, and even no suitable experimental method has been formed.

[0003] At present, the standard used for conventional reservoir sensitivity test evaluation technology is SY / T5358-2010 "Reservoir Sensitivity Flow Test Evaluation Method", which is mainly applicable to oil reservoirs. The evaluation scope is clastic rock samples with air permeability greater than 1mD. Liquid phase flow is used to measure the permeability when the pressure and flow rate at both ends of the core are stable to evaluate reservoir sensitivity. A large number of studies have shown that when conducting core flow test evaluation of tight gas reservoirs with ultra-low permeability less than 1mD, especially less than 0.01mD, according to this industry standard, there are problems such as long stable displacement time of pressure and flow rate at both ends of the core, high experimental pressure, inaccurate test results, or even no data can be measured.

[0004] In addition, since the matrix in this type of reservoir is the main body of the reservoir and the main body of natural gas storage, and the fractures are the channels for the flow of natural gas and various types of inflow fluids, the damage evaluation of this type of gas reservoir should also distinguish between matrix damage and fracture damage. This is because the damage mechanism and damage type of the two are very different, and there is currently little research in this area. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and provide a reservoir damage assessment method suitable for fractured bedrock gas reservoirs, so as to solve the problem that there are few evaluation methods for fractured metamorphic rock reservoirs, especially ultra-low permeability tight gas reservoirs less than 0.01mD in the prior art, and it is difficult to make objective and reasonable predictions for such reservoirs.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A reservoir damage assessment method applicable to fractured bedrock gas reservoirs comprises the following steps:

[0008] Step 1, respectively evaluating the sensitivity of the ultra-low permeability reservoir matrix core and the damage of the well fluid to the fracture;

[0009] The ultra-low permeability reservoir matrix core sensitivity evaluation process is: respectively judging the damage of each sensitivity factor to the ultra-low permeability reservoir matrix core to obtain reservoir sensitivity characteristics; the sensitivity factors include liquid phase sensitivity factors and gas phase sensitivity factors;

[0010] The process of evaluating the damage of the well fluid to the fracture is as follows: respectively judging the damage of each type of well fluid to the fracture, and obtaining the damage characteristics of the well fluid;

[0011] Step 2, combining the reservoir sensitivity characteristics and the damage characteristics of the fluid entering the well, to conduct a system reservoir damage evaluation, wherein the system reservoir damage evaluation is to prepare artificial fractures through the pressure recovery method, the pressure pulse attenuation method and the Brazilian splitting method to jointly conduct a damage evaluation on the system reservoir.

[0012] A further improvement of the present invention is:

[0013] Preferably, the ultra-low permeability reservoir is an ultra-low permeability fractured bedrock gas reservoir with a permeability of less than 0.01 mD.

[0014] Preferably, in step 1, the pressure recovery method is used to determine the damage of each liquid phase sensitivity factor to the ultra-low permeability reservoir matrix core.

[0015] Preferably, the liquid phase sensitivity factors include water sensitivity, salt sensitivity, acid sensitivity and alkali sensitivity.

[0016] Preferably, in step 1, the pressure pulse decay method is used to determine the damage of each gas phase sensitivity factor to the ultra-low permeability reservoir matrix core.

[0017] Preferably, the gas phase sensitivity factors include velocity sensitivity, stress sensitivity and liquid lock.

[0018] Preferably, the well fluid includes drilling fluid, completion fluid, well killing fluid, acidizing fluid and fracturing fluid.

[0019] 8. A reservoir damage assessment method suitable for fractured bedrock gas reservoirs according to claim 7, characterized in that solid phase particle blockage or liquid phase filtration damage method is used to conduct test judgment on drilling fluid, completion fluid and well killing fluid.

[0020] Preferably, the acidified liquid is judged using acidity evaluation standards and methods.

[0021] Preferably, the fracturing fluid is judged by residue damage to fractures, or by filtrate damage to fractures and relief research.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention discloses a reservoir damage evaluation method suitable for fractured bedrock gas reservoirs. The evaluation method selects a suitable flow experiment evaluation method for matrix damage evaluation to complete the determination of ultra-low permeability; and for fracture damage evaluation, in the case of difficulty in drilling cores containing natural fractures, artificial fractures are prepared to evaluate the type, mechanism and degree of damage to the fractures caused by different well-entering fluids after entering the fractures. Ultimately, it is necessary to take both fractures and matrix into consideration to carry out damage evaluation in order to obtain a comprehensive conclusion and understanding of reservoir damage evaluation, so as to solve the technical difficulties existing in the development process of such reservoirs. This method aims at the problem that there is no systematic and comprehensive damage evaluation method for fractured bedrock gas reservoirs at present. It integrates the non-steady-state permeability determination method applied to tight gas reservoirs, and the method of using Brazilian splitting to prepare fractures to simulate various types of well-entering fluid damage, and invents a comprehensive evaluation method that takes into account the "fracture + matrix" damage evaluation, and completes a comprehensive and systematic damage evaluation of fractured bedrock gas reservoirs from sensitivity evaluation to well-entering fluid damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart of the evaluation method for reservoir sensitivity and wellbore fluid damage in fractured bedrock gas reservoirs. DETAILED DESCRIPTION

[0025] See also Figure 1 On the one hand, the present invention provides a reservoir damage evaluation method applicable to fractured bedrock gas reservoirs. The method is aimed at ultra-low permeability fractured bedrock gas reservoirs with a permeability of less than 0.01mD. When conducting damage evaluation, the method evaluates from two aspects, including: ultra-low permeability reservoir matrix core sensitivity evaluation and well fluid damage evaluation to fractures. Specifically, it includes the following two steps:

[0026] Step 1: Conduct core sensitivity evaluation of ultra-low permeability reservoir matrix and damage evaluation of well fluid to fractures, and obtain reservoir sensitivity characteristics and well fluid damage characteristics for the reservoir.

[0027] In the first part, reservoir sensitivity evaluation was carried out on the matrix core of ultra-low permeability fractured bedrock gas reservoirs. By comprehensively comparing the various low-porosity and low-permeability tight gas reservoir permeability determination methods currently used, different types of liquid and gas phase damage media were distinguished, and the pressure recovery method in the non-steady-state method was used to determine the damage caused by the liquid phase, such as water sensitivity, salt sensitivity, acid sensitivity and alkali sensitivity; the pressure pulse attenuation method in the non-steady-state method was used to determine the damage caused by the gas phase, such as velocity sensitivity, stress sensitivity, and liquid lock.

[0028] Finally, the sensitivity characteristics of ultra-low permeability fractured bedrock gas reservoirs are found from water sensitivity, salt sensitivity, acid sensitivity, alkali sensitivity, liquid lock, velocity sensitivity and stress sensitivity;

[0029] Specifically, during the experiment, these factors are changed in turn, and the pressure recovery method or the pressure pulse attenuation method is used in single factor tests to determine whether it is the sensitivity characteristic of this type of gas reservoir.

[0030] In the second part, the damage caused by fluid entering the well is evaluated for bedrock fractures.

[0031] Since the micro-cracks in the former bedrock reservoir are well developed, they are the channels for the flow of natural gas and various wellbore fluids (drilling fluid, completion fluid, well killing fluid, acidizing fluid, well killing fluid), and it is necessary to carry out damage evaluation on bedrock cracks. In the case that the core with natural cracks is difficult to drill, artificial cracks are prepared indoors, and the width of the artificial cracks is set to the same as the natural crack width by controlling the confining pressure to simulate the natural cracks, and then the mechanism of damage caused by different wellbore fluids to reservoir cracks is explored, and different types of damage are simulated to evaluate the degree of damage caused by wellbore fluids.

[0032] Specifically, the drilling fluid, completion fluid and well killing fluid are tested and judged by the solid particle blockage or liquid phase filtration damage method, the acidizing fluid is judged by the acidity evaluation standard and method, and the fracturing fluid is judged by the residue damage to the fracture or the filtrate damage to the fracture and the relief research and judgment. By using the above-mentioned test or judgment methods, the damage characteristics of the fluid entering the well are screened out.

[0033] Step 2: Conduct a comprehensive reservoir damage assessment

[0034] The selected reservoir sensitivity characteristics and well fluid damage characteristics were tested, and the specific test methods included pressure recovery method, pressure pulse decay method and Brazilian splitting method. During the experiment, the pressure recovery method judged the damage caused by the liquid phase, such as water sensitivity, salt sensitivity, acid sensitivity and alkali sensitivity, the pressure pulse decay method judged the damage caused by the gas phase, such as velocity sensitivity, stress sensitivity and liquid lock, and the Brazilian splitting method prepared artificial fractures to simulate natural fractures, and carried out the evaluation of the degree of well fluid damage. The results of the three aspects were listed to obtain the final evaluation results of reservoir damage.

[0035] The technical solution of the invention is a comprehensive system sensitivity evaluation method for fractured bedrock reservoirs, taking into account "matrix + fractures" and the evaluation method of fluid damage entering the well. The specific implementation method points out that it is a comprehensive application of flow experimental evaluation methods such as pressure recovery method, pressure pulse attenuation method, Brazilian splitting method to prepare artificial fractures, etc.

[0036] Example

[0037] This method uses unconventional experimental methods such as pressure recovery method and pressure pulse attenuation method to complete a full set of reservoir sensitivity evaluation of low permeability tight reservoir cores with permeabilities ranging from 0.0007 to 0.168 mD in the three blocks of Dongping, Pingxi and Jianbei in the frontal bedrock reservoir of the Altun Mountains. Including velocity sensitivity, water sensitivity, salt sensitivity, acid sensitivity, alkali sensitivity, stress sensitivity, liquid lock sensitivity, a total of 57 groups. The experimental results show that its reservoir sensitivity is mainly concentrated in strong water lock sensitivity and stress sensitivity, while the degree of velocity sensitivity, water sensitivity, salt sensitivity and alkali sensitivity is weak to moderately weak, and the reservoir is not sensitive to hydrochloric acid or soil acid.

[0038] The Brazilian splitting method was used to artificially split the matrix core, and the width of the natural fracture was simulated by precisely controlling the confining pressure. On this basis, the damage process of the well fluid entering the reservoir fracture was simulated according to the possible damage types caused by different well fluids entering the reservoir fracture. A total of 66 groups of indoor studies were completed, including the damage and optimization of solid particles in the wellbore working fluid (drilling and completion fluid, well killing fluid), the adaptability of the acidizing fluid system, and the damage and relief of various well fluid filtrates to the fracture.

[0039] For the first time, a comprehensive and systematic evaluation of fractured bedrock gas reservoirs was completed, including reservoir matrix sensitivity and the damage of different wellbore fluids to fractures.

[0040] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs, characterized in that: The following steps are involved: Step 1, respectively evaluating the sensitivity of the ultra-low permeability reservoir matrix core and the damage of the well fluid to the fracture; The ultra-low permeability reservoir matrix core sensitivity evaluation process is: respectively judging the damage of each sensitivity factor to the ultra-low permeability reservoir matrix core to obtain reservoir sensitivity characteristics; The sensitivity factors include liquid phase sensitivity factors and gas phase sensitivity factors; The process of evaluating the damage of the well fluid to the fracture is as follows: respectively judging the damage of each type of well fluid to the fracture, and obtaining the damage characteristics of the well fluid; Step 2, combining the reservoir sensitivity characteristics and the damage characteristics of the fluid entering the well, to conduct a system reservoir damage evaluation, wherein the system reservoir damage evaluation is to prepare artificial fractures through the pressure recovery method, the pressure pulse attenuation method and the Brazilian splitting method to jointly conduct a damage evaluation on the system reservoir.

2. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 1, characterized in that: The ultra-low permeability reservoir is an ultra-low permeability fractured bedrock gas reservoir with a permeability of less than 0.01 mD.

3. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 1, characterized in that: In step 1, the pressure recovery method is used to determine the damage of each liquid phase sensitivity factor to the ultra-low permeability reservoir matrix core.

4. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 1, characterized in that: The liquid phase sensitivity factors include water sensitivity, salt sensitivity, acid sensitivity and alkali sensitivity.

5. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 1, characterized in that: In step 1, the pressure pulse decay method is used to determine the damage of each gas phase sensitivity factor to the ultra-low permeability reservoir matrix core.

6. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 5, characterized in that: The gas phase sensitivity factors include velocity sensitivity, stress sensitivity and liquid lock.

7. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 1, characterized in that: The well fluid includes drilling fluid, completion fluid, well killing fluid, acidizing fluid and fracturing fluid.

8. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 7, characterized in that: For drilling fluid, completion fluid and well killing fluid, solid phase particle blockage or liquid phase filtration damage method is used for test judgment.

9. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 7, characterized in that: The acidification fluid is judged using acidity evaluation standards and methods.

10. A reservoir damage assessment method applicable to fractured bedrock gas reservoirs according to claim 1, characterized in that: The fracturing fluid is judged by residue damage to the fractures, or by filtrate damage to the fractures and relief research.

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