A method for establishing water quality standards for oilfield injection

Abstract

This invention belongs to the field of oilfield development technology and discloses a method for establishing water quality standards for oilfield injection, including the following steps: 1) obtaining characteristic data on permeability and micropore structure of the oilfield injection area; 2) dividing the injection area into injection blocks with different permeability levels based on permeability; 3) further subdividing the injection blocks with strong heterogeneity based on micropore structure; 4) conducting water injection compatibility studies, sensitivity studies, and corrosivity studies for each injection block, determining the standard values ​​of each water quality indicator, and establishing water quality standards for each injection block. This invention comprehensively considers the micropore characteristics of the oilfield, and through fine zoning, precise sampling, and targeted research, formulates more instructive water quality standards for each injection block, thereby accurately controlling the water quality of each block, achieving fine water injection in the oilfield, and improving the energy efficiency and economy of oilfield development.

Description

Technical Field

[0001] This invention belongs to the field of oilfield development technology, and specifically relates to a method for establishing water quality standards for oilfield injection. Background Technology

[0002] In oilfield development both domestically and internationally, water injection remains the dominant method. To achieve ideal water injection development results, while continuously optimizing the water injection process, it is also necessary to recognize the decisive role of water quality in water injection. Oilfield water injection needs to be matched with the oilfield's physical properties. Inappropriate water quality control can cause blockage of seepage channels, leading to increased injection pressure and deterioration of water drive performance. Furthermore, it can cause extensive corrosion and scaling of surface pipelines, severely reducing their remaining strength, pressure-bearing capacity, service life, and reliability. Therefore, water quality control is a key factor in improving oilfield water drive recovery and maintaining the stability of the surface system.

[0003] Currently, the industry generally adopts SY / T 5329-2012 "Water Quality Indicators and Analysis Methods for Water Injection in Clastic Rock Reservoirs" as the water quality standard for water injection in oilfield production processes. This standard is based on the fundamental principle that low-permeability oil layers have small pore and throat sizes, while high-permeability oil layers have large pore and throat sizes. It is divided into five levels according to permeability, and different water quality standards are implemented for different permeability levels. The water quality requirements for injection layers with low permeability are high, while the water quality requirements for injection layers with high permeability are low. This standard has certain guiding significance for water injection production in oilfields.

[0004] However, in actual engineering practice, there are still some problems in implementing the above-mentioned water quality standards for injection. After undergoing sedimentation, diagenesis, and a long geological evolution process, oilfields exhibit extremely uneven spatial distribution and various internal properties of the reservoirs; this phenomenon is called heterogeneity. Low-permeability reservoirs with a permeability of less than 50 mD generally exhibit strong heterogeneity. The microscopic pore structure, especially the geometry, size, and distribution of pore throats, varies greatly even at the same permeability. If water is injected using the same water quality standards, it is very easy to cause blockage of small pores, resulting in abnormally high injection pressure or inability to continue injection.

[0005] For medium-to-high permeability reservoirs, the homogeneity is relatively good, and permeability and porosity generally exhibit an exponential positive correlation. The microstructure of the pores has little impact on the extraction effect, which is conducive to oilfield development. Extraction in such areas is less difficult, and water quality requirements can be relaxed. However, the current water quality standards for medium-to-high permeability reservoirs are more stringent than actual needs. Although from a technical perspective, better water quality leads to better development results, from an economic perspective, better water quality means higher requirements for the water treatment system and higher economic costs.

[0006] In summary, the current water injection quality standards are insufficient to fully meet the actual conditions of oilfield development. In addition, the suitability of water injection quality during the water drive process in oilfields is also affected by reservoir sensitivity, native microorganisms, and other factors. For different oilfields, it is necessary to establish targeted water injection quality standards based on their actual characteristics. Summary of the Invention

[0007] The technical problem to be solved by this invention is to address the problems existing in the implementation of the current water quality standards for oilfield injection. This invention provides a method for establishing water quality standards for oilfield injection. This method comprehensively considers the micro-pore characteristics of oilfields and formulates more instructive water quality standards for injection by dividing the area into blocks through fine zoning, precise sampling and targeted research.

[0008] To address the technical problem proposed in this invention, this invention provides a method for establishing water quality standards for oilfield injection, comprising the following steps:

[0009] 1) Obtain characteristic data on permeability and micropore structure in the water injection area of ​​the oilfield;

[0010] 2) Based on the permeability, the water injection area is divided into water injection blocks with different permeability levels;

[0011] 3) Based on the microscopic pore structure, determine the degree of heterogeneity of each water injection block, retain the water injection blocks with weak heterogeneity, and further subdivide the water injection blocks with strong heterogeneity; the subdivision method is to divide the water injection blocks with strong heterogeneity into multiple (≥2) water injection blocks with weak heterogeneity, or to separate the weak heterogeneous part and the strong heterogeneous part of the water injection blocks with strong heterogeneity as different water injection blocks;

[0012] 4) Conduct water injection compatibility studies, sensitivity studies, and corrosion studies for all the final water injection blocks, determine the standard values ​​for each water quality indicator, and establish water injection water quality standards for each water injection block.

[0013] In the above scheme, the method of classifying the permeability level is to divide it into five levels according to the permeability size: ≤10mD, 10-50mD, 50-500mD, 500-1500mD, and >1500mD.

[0014] In the above scheme, the method for judging the strength of heterogeneity is to calculate the micro-homogeneity coefficient, which is the ratio of the average pore throat radius to the maximum pore throat radius. The ratio is between 0 and 1. The closer it is to 0, the stronger the heterogeneity, and the closer it is to 1, the weaker the heterogeneity.

[0015] Furthermore, a microhomogeneity coefficient < 0.25 indicates strong heterogeneity, while a microhomogeneity coefficient ≥ 0.25 indicates weak heterogeneity.

[0016] In the above scheme, the compatibility study, sensitivity study and corrosion study all use the actual rock cores and actual water sources of each water injection block, which can overcome the problem that artificial rock cores and simulated water sources cannot simulate reservoir sensitivity and original microorganisms.

[0017] Furthermore, the method for selecting actual core samples is to select core samples from the range of pore throat radii corresponding to a cumulative distribution percentage of 20%-80% based on the normal distribution curve of the pore throat radius of each water injection block, and use these core samples as the actual core samples for experimental research in each water injection block.

[0018] In the above scheme, the water quality indicators include controlling indicators and auxiliary indicators. During the implementation of water quality standards, if the controlling indicators meet the standards, there is no need to assess the auxiliary indicators; if the controlling indicators exceed the standards, the reasons for exceeding the standards are analyzed by examining the compliance status of the auxiliary indicators.

[0019] Furthermore, the control indicators include suspended solids content, median suspended solids particle size, oil content, sulfate-reducing bacteria, iron bacteria, saprophytic bacteria, average corrosion rate, average scaling rate, and total mineralization; auxiliary indicators include oxygen content, sulfur content, total iron content, and pH value.

[0020] In the above scheme, the compatibility study is to determine the standard values ​​of indicators such as suspended solids content, median suspended solids particle size, oil content, iron bacteria, saprophytic bacteria, and total iron content through core displacement experiments. Single-factor experiments are carried out for different indicators. When the displacement ratio is ≥50 times, the highest value of each indicator under the condition that the core permeability damage rate is ≤20% is used as the standard upper limit value of each indicator.

[0021] Furthermore, after determining the standard values ​​for suspended solids content, median suspended solids particle size, and oil content, a multi-factor verification experiment was conducted under these standard values. If the core permeability damage rate is ≤30% when the displacement ratio is ≥50, the verification is passed and the standard values ​​can be adopted. If the verification fails, the core permeability damage rate in the single-factor experiment is reduced, and the standard values ​​for these three indicators are re-determined.

[0022] In the above scheme, the sensitivity study is to determine the standard value of total mineralization through sensitivity flow experiments, and to carry out sensitivity flow experiments with different mineralization. The lowest mineralization under the condition that the core permeability change rate is ≤20% is taken as the standard lower limit value of total mineralization.

[0023] In the above scheme, the corrosion study is to determine the standard values ​​of indicators such as oxygen content, sulfur content, and sulfate-reducing bacteria through dynamic corrosion experiments using the rotating corrosion plate method. Single-factor experiments are carried out for different indicators, and the highest value of each indicator under the condition of average corrosion rate ≤ 0.076 mm / a is taken as the standard upper limit value of each indicator.

[0024] In the above scheme, the average corrosion rate, average scaling rate and pH value adopt the industry-standard standard, that is, the average corrosion rate ≤0.076mm / a, the average scaling rate ≤0.1mm / a, and the pH value 6.5-7.5.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] (1) In the process of establishing the water quality standard for water injection in this invention, not only are the permeability characteristics considered and water injection blocks are divided according to the permeability level, but also the micro-pore structure characteristics are fully considered. Blocks with strong heterogeneity are further subdivided, and water quality standards are established for each block separately. Water is injected in different areas and the water quality of each block is precisely controlled. On the one hand, this can avoid the problem that small channels in blocks with strong heterogeneity are easily blocked and cannot be injected. On the other hand, it can reduce the area of ​​water injection blocks with strong heterogeneity, thereby reducing the amount of water treated by the surface water treatment system and improving the efficiency of water drive development.

[0027] (2) This invention selects representative rock samples based on the characteristics of reservoir pore structure and conducts research on water injection compatibility, sensitivity, and corrosivity. This allows for the formulation of more accurate water quality standards, avoids the problem of excessively strict water quality requirements for some medium-to-high permeability reservoirs, optimizes the scientific nature of water quality management, water quality improvement strategies and the treatment efficiency of surface water treatment systems, and improves the economic efficiency of oilfield exploitation.

[0028] (3) Based on the complexity of oil fields, this invention uses actual rock cores and actual water injection sources to conduct research and determine specific assessment standards. This can overcome the problem that artificial rock cores and simulated water sources cannot simulate reservoir sensitivity and original microorganisms, minimize the error between experiments and reality, and have more guiding significance for actual mining. Detailed Implementation

[0029] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0030] Example

[0031] A method for establishing water quality standards for oilfield injection includes the following steps:

[0032] 1) Obtain characteristic data on permeability and micropore structure in the water injection area of ​​the oilfield;

[0033] By collecting geological and development data of the oilfield, we can gain a comprehensive understanding of the reservoir permeability characteristics, micropore structure characteristics, and the basic laws between the two in the water injection area of ​​the oilfield, which will provide a basis for the subsequent division of water injection blocks.

[0034] 2) Based on the permeability, the water injection area is divided into water injection blocks with different permeability levels;

[0035] Based on permeability levels of ≤10mD, 10-50mD, 50-500mD, 500-1500mD, and >1500mD, and combined with actual geological conditions, the water injection area is divided into several water injection blocks with different permeability levels.

[0036] 3) Based on the microscopic pore structure, determine the degree of heterogeneity of each water injection block, retain the water injection blocks with weak heterogeneity, and further subdivide the water injection blocks with strong heterogeneity.

[0037] The degree of heterogeneity of the water injection block is judged by the micro-homogeneity coefficient. A micro-homogeneity coefficient < 0.25 indicates strong heterogeneity, and a micro-homogeneity coefficient ≥ 0.25 indicates weak heterogeneity. The water injection block with strong heterogeneity is divided into multiple water injection blocks with weak heterogeneity, or the weak heterogeneous part of the water injection block with strong heterogeneity is separated from the strong heterogeneous part as different water injection blocks.

[0038] 4) For all the final water injection blocks, conduct water injection compatibility studies, sensitivity studies and corrosivity studies, determine the standard values ​​of each water quality index, and establish water injection water quality standards for each water injection block.

[0039] Based on the normal distribution curve of the pore throat radius of each water injection block, actual core samples were collected from the pore throat radius range corresponding to the cumulative distribution percentage of 20%-80%, and actual water injection sources were collected at the same time. Experimental studies were carried out in each water injection block.

[0040] Single-factor experiments on suspended solids content were conducted, and the suspended solids content in the injection water was adjusted to different concentrations. For example, core displacement experiments were carried out at concentration gradients of 3 mg / L, 5 mg / L, 10 mg / L, and 20 mg / L. When the displacement ratio was 50 times, the highest concentration of suspended solids content under the condition that the core permeability damage rate was ≤20% was taken as the standard upper limit value of suspended solids content.

[0041] A single-factor experiment on the median suspended solids particle size was conducted to adjust the median suspended solids particle size in the injected water to different sizes. For example, core displacement experiments were carried out according to the size gradients of 0.5μm, 1μm, 3μm, and 5μm. When the displacement ratio was 50 times, the maximum size of the median particle size under the condition that the core permeability damage rate was ≤20% was taken as the standard upper limit value of the median suspended solids particle size.

[0042] Single-factor experiments were conducted on oil content and iron content, respectively. The oil content or iron content in the injection water was adjusted to different concentrations, for example, core displacement experiments were carried out at concentration gradients of 0.5 mg / L, 1 mg / L, 2 mg / L, 5 mg / L, and 10 mg / L. When the displacement ratio was 50 times, the highest concentration of oil content or iron content under the condition that the core permeability damage rate was ≤20% was taken as the standard upper limit value of oil content or iron content.

[0043] Single-factor experiments were conducted on iron bacteria and saprophytic bacteria separately, adjusting the content of iron bacteria or saprophytic bacteria in the injected water to different levels, for example, 2.5 × 10⁻⁶. 0 cells / mL, 2.5×10 1 cells / mL, 2.5×10 2 cells / mL, 2.5×10 3 cells / mL, 2.5×10 4 Core displacement experiments were conducted using a concentration gradient of 100 cells / mL. When the displacement ratio was 50 times, the highest concentration of iron bacteria or saprophytes under the condition that the core permeability damage rate was ≤20% was taken as the standard upper limit value of iron bacteria or saprophytes.

[0044] Multi-factor experiments were conducted on suspended solids content, median suspended solids particle size, and oil content. These three indicators in the injected water were adjusted to the upper limit of the standard obtained from the single-factor experiment. Core displacement experiments were carried out. When the displacement ratio was 50 times, the core permeability damage rate was ≤30%, indicating that the upper limit of the standard for these three indicators can be adopted.

[0045] A single-factor experiment on mineralization was conducted, and the mineralization in the injected water was adjusted to different concentrations. Sensitive flow experiments were carried out in accordance with the "SY / T5358-2010 Evaluation Method for Reservoir Sensitivity Flow Test". The lowest mineralization under the condition of core permeability change rate ≤20% was taken as the standard lower limit value of total mineralization.

[0046] Single-factor experiments were conducted on oxygen content, sulfur content, and sulfate-reducing bacteria, respectively. Dynamic corrosion experiments were carried out using the rotating corrosion plate method according to SY / T 5273-2014 "Performance Inhibitors for Oilfield Produced Water Treatment". The highest value of each index under the condition of average corrosion rate ≤0.076mm / a was taken as the standard upper limit value of each index.

[0047] The average corrosion rate, average scaling rate, and pH value adopt industry-standard criteria, namely, average corrosion rate ≤ 0.076 mm / a, average scaling rate ≤ 0.1 mm / a, and pH value 6.5-7.5.

[0048] Based on the standard values ​​determined by experimental studies of each water injection block, water quality standards for each water injection block are established. The standards include control indicators and auxiliary indicators. The control indicators include suspended solids content, median suspended solids particle size, oil content, sulfate-reducing bacteria, iron bacteria, saprophytic bacteria, average corrosion rate, average scaling rate, and total mineralization. The auxiliary indicators include oxygen content, sulfur content, total iron content, and pH value.

[0049] During the implementation of water quality standards, if the control indicators meet the standards, there is no need to assess the auxiliary indicators; if the control indicators exceed the standards, the reasons for exceeding the control indicators are analyzed by examining the compliance status of the auxiliary indicators.

[0050] The water quality standards for water injection in the Jianghan Oilfield were established according to the method of this invention, and the water quality standards for some water injection blocks are shown in the table below:

[0051]

[0052] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for establishing water quality standards for oilfield water injection, characterized by guiding oilfield water injection and exploitation by developing differentiated water quality standards for different regions and blocks. Includes the following steps: 1) Obtain characteristic data on permeability and micropore structure in the water injection area of ​​the oilfield; 2) Based on the permeability, the water injection area is divided into water injection blocks with different permeability levels; 3) Determine the degree of heterogeneity of each water injection block based on the microscopic pore structure; the method for determining the degree of heterogeneity is to calculate the microscopic homogeneity coefficient, which is the ratio of the average pore throat radius to the maximum pore throat radius. A microscopic homogeneity coefficient < 0.25 indicates strong heterogeneity, and a microscopic homogeneity coefficient ≥ 0.25 indicates weak heterogeneity. The water injection blocks with weak heterogeneity are retained, and the water injection blocks with strong heterogeneity are further subdivided. The subdivision method is to divide the water injection blocks with strong heterogeneity into multiple water injection blocks with weak heterogeneity, or to separate the weak heterogeneous part and the strong heterogeneous part of the water injection block with strong heterogeneity as different water injection blocks. 4) Conduct water injection compatibility studies, sensitivity studies, and corrosion studies for all the final water injection blocks, determine the standard values ​​for each water quality indicator, and establish water injection water quality standards for each water injection block. The compatibility study, sensitivity study and corrosion study all used actual rock cores and actual water sources. The method for selecting actual rock cores was to select rock cores with a cumulative distribution number of 20%-80% corresponding to the pore throat radius range based on the normal distribution curve of the pore throat radius of each water injection block, and use these as the actual rock cores of each water injection block. The compatibility study determined standard values ​​for suspended solids content, median suspended solids particle size, oil content, iron bacteria, saprophytic bacteria, and total iron content through core displacement experiments. Single-factor experiments were conducted for each indicator. When the displacement ratio was ≥50, the highest value of each indicator under the condition of core permeability damage rate ≤20% was used as the upper limit of the standard value for each indicator. After the standard values ​​for suspended solids content, median suspended solids particle size, and oil content were determined, multi-factor verification experiments were conducted under these standard values. When the displacement ratio was ≥50, if the core permeability damage rate was ≤30%, the verification was passed, and the standard values ​​of this set of standard values ​​could be adopted. If the verification failed, the core permeability damage rate in the single-factor experiments was reduced, and the standard values ​​of these three indicators were re-determined.

2. The method for establishing water quality standards for oilfield injection according to claim 1, characterized in that, The method for classifying the permeability levels is to divide them into five levels according to the permeability: ≤10 mD, 10-50 mD, 50-500 mD, 500-1500 mD, and >1500 mD.

3. The method for establishing water quality standards for oilfield injection according to claim 1, characterized in that, The water quality indicators include control indicators and auxiliary indicators. The control indicators include suspended solids content, median suspended solids particle size, oil content, sulfate-reducing bacteria, iron bacteria, saprophytic bacteria, average corrosion rate, average scaling rate, and total mineralization. The auxiliary indicators include oxygen content, sulfur content, total iron content, and pH value.

4. The method for establishing water quality standards for oilfield injection according to claim 1, characterized in that, The sensitivity study determines the standard value of total mineralization through sensitivity flow experiments. Sensitivity flow experiments with different mineralization are carried out, and the lowest mineralization under the condition of core permeability change rate ≤ 20% is taken as the standard lower limit value of total mineralization.

5. The method for establishing water quality standards for oilfield injection according to claim 1, characterized in that, The corrosion study determined the standard values ​​of oxygen content, sulfur content, and sulfate-reducing bacteria through dynamic corrosion experiments using the rotating corrosion plate method. Single-factor experiments were conducted for different indicators, and the highest value of each indicator under the condition of average corrosion rate ≤ 0.076 mm / a was used as the upper limit of the standard value for each indicator.

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