A method for identifying water invasion in the formation water of a gas reservoir

Abstract

The present invention relates to the field of geological exploration, and specifically relates to a method for identifying water invasion in formation water of gas reservoirs, which specifically includes the following steps: S1. Select a water-bearing gas reservoir as the target area; S2. Measure the concentrations of major anions and cations in the formation water, and perform linear fitting on the ion concentrations and salinity; S3. Convert the measured formation water ion concentrations into equivalent concentrations, and superimpose and plot the formation water data on the Norvak phase diagram; S4. Calculate the average value and standard deviation of each ion in the formation water to obtain the formation water coefficient of variation; S5. Combine the above indicators for comprehensive analysis to determine whether water invasion has occurred in the formation water. The present invention uses the analysis of the concentrations of major anions and cations in the formation water, combined with the superimposed drawing of the Norvak phase diagram, to intuitively and quickly determine whether formation water invasion has occurred. The identification is highly scientific and accurate, and can well guide the gas reservoir exploration and development work in areas with high incidence of formation water invasion problems.

Description

Technical Field

[0001] The present invention relates to the field of geological exploration, and particularly to a method for identifying water invasion in formation water of gas reservoirs, specifically a method for identifying water invasion in formation water during oil and gas exploration, evaluation, and development. Background Art

[0002] In sedimentary basins, formation water and oil and gas coexist in the pores or fractures of underground rocks and are participants in oil and gas generation. In particular, physical and chemical processes related to oil and gas migration, accumulation, and reservoir formation occur within the formation water medium, which contains rich information on diagenesis, sedimentation, reservoir formation, etc. During the basin evolution process, whether it is rock cementation and dissolution, or organic matter degradation reactions and their mixing effects, etc., formation water provides the carrier and driving force, and these processes are inseparably linked to oil and gas accumulation.

[0003] During the gas reservoir production process, the produced water includes two types: formation water and condensate water. Condensate water is mixed in natural gas underground, has a low mineral ion content, and a small produced water volume, and is not likely to cause blockage of natural gas wells. Formation water is mainly distributed in the pores of sandstone layers, has a high mineral ion content, and a high chloride ion concentration, and has strong corrosiveness. If formation water invasion occurs, it will seriously damage the production work of natural gas wells, and appropriate disposal measures must be taken, otherwise it will cause a reduction in gas well production.

[0004] In addition, the prior art often uses well logging curve analysis, characteristics of neutron and acoustic curves, etc. for formation water prediction, but the effects are not very ideal. After the gas reservoir is produced, as natural gas flows, the interaction between gas and water changes, and the water production rate and produced water volume change rapidly, resulting in a rapid decrease in prediction accuracy.

[0005] Produced water often appears simultaneously or successively as formation water and condensate water. Therefore, if the formation water and condensate water cannot be accurately distinguished during gas reservoir production, the gas reservoir production work will be greatly restricted.

[0006] In 1996, American hydrogeologist Novak first proposed the Novak coefficient determination method to determine the source of formation water. This method uses the ionic equivalent relationships of common cations in formation water to form 6 groups of discriminant coefficients, namely Na / Ca, Na / Mg, Mg / K, Ca / Mg, (Na + K) / (Ca + Mg), and Na / K, and determines the source of formation water by comparing the shapes of the phase diagrams they present, which has strong scientificity and practicality. Moreover, formation water is one of the important factors affecting the production capacity development of gas fields. When a gas well encounters water, the gas-phase permeability of the reservoir will decrease, resulting in a rapid decline in gas production, seriously restricting the improvement of the single-well production capacity. Summary of the Invention

[0007] The object of the present invention is to overcome the problem in the prior art that there is a lack of effective means for judging whether water invasion occurs in the formation water of gas reservoirs, and to provide a method for superimposing and representing the Novak phase diagram of formation water in gas reservoirs and a method for judging formation water invasion, so as to provide a theoretical basis for whether formation water in gas reservoirs undergoes water invasion.

[0008] Therefore, in view of the current difficult problems, the present invention utilizes the corresponding evolutionary reactions that occur between the water body and sedimentary rock masses during the migration process, and the chemical composition of formation water shows corresponding changes for major ions (K + 、Na + 、Ca 2+ 、Mg 2+ 、HCO - 3、SO 2- 4、Cl - ). A method for superimposing and representing the Novak phase diagram of formation water in gas reservoirs and a method for judging formation water invasion are provided, so as to provide a theoretical basis for whether formation water in gas reservoirs undergoes water invasion.

[0009] In order to achieve the above object of the invention, the present invention provides the following technical solutions:

[0010] A method for identifying formation water invasion in gas reservoirs, comprising the following steps:

[0011] S1. Select a water-bearing gas reservoir as the target area;

[0012] S2. Measure the concentrations of major anions and cations in the formation water, and perform linear fitting on the ion concentrations and salinity;

[0013] S3. Convert the measured formation water ion concentrations into equivalent concentrations, and superimpose and plot the formation water data on the Novak phase diagram;

[0014] S4. Calculate the average value and standard deviation of each ion in the formation water to obtain the coefficient of variation of the formation water;

[0015] S5. Combine the above indicators for comprehensive analysis to judge whether water invasion occurs in the formation water.

[0016] The method for identifying formation water invasion in gas reservoirs of the present invention uses the concentration analysis of major anions and cations in formation water, combines with the Novak phase diagram for superimposing and plotting, and reflects the characteristics of formation water invasion into the chart through the plotting method, so as to intuitively and quickly judge whether formation water invasion occurs. This method has the advantages of excellent identification scientificity and high identification accuracy, and can well guide the gas reservoir exploration and development work in areas with high incidence of formation water invasion problems.

[0017] Furthermore, in step S1, the salinity of the formation water in the target area is greater than 10,000 mg / L. Formation water with a lower salinity needs to be excluded. When formation water with a lower salinity undergoes water invasion, there are significant interference errors in the variation law of the ionic concentration of the formation water, and the accuracy of finally identifying the water invasion of the formation water will be reduced.

[0018] Formation water with a lower salinity needs to be excluded. If the salinity of the formation water ≤ 10,000 mg / L, it may be condensate water, and there may be experimental errors and omissions in the ionic concentrations of its major anions and cations (K + , Na + , Ca 2+ , Mg 2+ , HCO - 3, SO 2- 4, Cl - ), so it is not applicable to the method of the present invention.

[0019] The salinity of formation water refers to the amount of inorganic salts contained in formation water, with the imperial unit PPM and the international unit mg / L.

[0020] Furthermore, in step S2, the concentrations of the major anions and cations in the formation water are measured following the method of the petroleum and natural gas industry standard.

[0021] Preferably, the petroleum and natural gas industry standard is SY / T 5523-2016 "Analysis Method of Oilfield Water".

[0022] Furthermore, in step S2, the major anions and cations include potassium ions, sodium ions, calcium ions, magnesium ions, bicarbonate ions, sulfate ions, and chloride ions.

[0023] The major anions and cations include major anions and major cations, specifically including K + , Na + , Ca 2+ , Mg 2+ , HCO - 3, SO 2- 4, Cl - and other ions.

[0024] Furthermore, in step S2, the formation water concentration may be below the detection limit of the laboratory test instrument.

[0025] Furthermore, in step S2, the major cations are determined by ICP 7400 inductively coupled plasma emission spectrometry. Preferably, the major cations include K + , Na + , Ca 2+ , Mg 2+ . Preferably, the determination error of the inductively coupled plasma emission spectrometry is less than 2%.

[0026] Further, in step S2, for chloride ions and sulfate ions (Cl - , SO 2- 4) among the constant anions, an ICS2000 ion chromatograph is used for determination. Preferably, the measurement error of the ICS2000 ion chromatograph is 0.2 - 0.3% or less.

[0027] Further, in step S2, for bicarbonate ions (HCO - 3) among the constant anions, volumetric analysis is used for determination. Preferably, hydrochloric acid titration is used for measurement. Preferably, the measurement error of volumetric analysis is 0.2 - 0.3% or less.

[0028] Furthermore, the ion concentration and the salinity are linearly fitted, and the R 2 ;

[0029] When R 2 ≥0.90, it indicates a close relationship between the ion concentration and the salinity;

[0030] When 0.90 > R 2 > 0.60, it indicates a good relationship between the ion concentration and the salinity;

[0031] When R 2 ≤0.60, it indicates a poor relationship between the ion concentration and the salinity.

[0032] Furthermore, step S3 is to convert the measured formation water ion concentration into equivalent concentration, and substitute the equivalent concentration ratio into the Novak phase diagram, so that the phase diagrams at different time periods are all superimposed and fall within the same coordinate system.

[0033] Making the phase diagrams at different time periods all superimposed and fall within the same coordinate system can more conveniently and intuitively display the changing trend of their inflection points. As the gas reservoir is exploited and formation water invasion occurs, the concentrations of some ions will change significantly. By superimposing the phase diagrams at different time periods on the same coordinate system, the situation of formation water invasion can be judged more quickly and intuitively.

[0034] Even further, in step S4, the formation water coefficient of variation is calculated through the following formula, and the specific formula is as follows:

[0035] Coefficient of variation = standard deviation ÷ average value

[0036] In the formula, the standard deviation is used as the numerator of the coefficient of variation, and the average value is used as the denominator. When formation water invasion occurs, the standard deviation will increase significantly, resulting in an increase in the difference in the measured ion concentrations of different gas wells, and thus it is possible to identify whether formation water invasion has occurred.

[0037] Further, in step S4, when calculating the standard variance of each ion in formation water, keep the variance constant. By setting the variance constant, the fluctuation is relatively stable, and the heteroscedasticity problem is eliminated.

[0038] Further, in step S4, select typical constant ion concentrations for calculation to obtain the coefficient of variation of formation water. Preferably, in an embodiment of the present invention, the typical constant ions are sodium ions and chloride ions.

[0039] Furthermore, in step S4, when selecting typical constant ion concentrations, select ions with R 2 ≥ 0.90 in the linear relationship. That is, ions with a close linear relationship with salinity, indicating that an increase or decrease in the concentration of this ion directly affects the magnitude of salinity.

[0040] Further, in step S5, the comprehensive analysis by combining the above indicators is performed using the coefficient of variation of formation water.

[0041] Further, in step S5, select abnormal inflection points according to the superimposed Novak phase diagram; calculate the coefficient of variation of the ions selected in combination with the above two conditions. When the coefficient of variation of the ions and salinity > 30%, it is considered that water invasion has occurred in the gas reservoir formation water.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] 1. The water invasion identification method for gas reservoir formation water of the present invention aims at the current difficult problems, utilizes the corresponding evolution reaction between the water body and sedimentary rock masses during the migration process, and the chemical composition (K + 、Na + 、Ca 2+ 、Mg 2+ 、HCO - 3、SO 2- 4、Cl - ) of formation water shows corresponding changes. Through the intuitive analysis and judgment of the formation water invasion discrimination method by superimposing the Novak phase diagram, the identification result has high accuracy, provides a theoretical basis for whether there is water invasion in gas reservoir formation water, and can effectively guide the exploration and exploitation of natural gas reservoirs.

[0044] 2. The method of the present invention applied to the exploration evaluation and development of the entire gas reservoir has the characteristics of being specific, effective, and feasible. It has practical significance for the identification of water invasion in gas reservoir formation water, can further provide empirical data and method references for future exploration evaluation and development of gas reservoirs, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a flowchart of the identification method for gas reservoir formation water of the present invention.

[0046] Figure 2It is a regional gas field distribution map of the formation water sedimentation in the Xinchang structural belt of the western Sichuan depression.

[0047] Figure 3 It is a graph showing the relationship between the contents of sodium ions and chloride ions and the salinity. Among them, the upper graph is the relationship graph between sodium ion salinity, and the lower graph is the relationship graph between chloride ion salinity.

[0048] Figure 4 It is an analysis chart of substituting the equivalent concentration ratio of ion contents into the Novak phase diagram.

[0049] Figure 5 It is a graph of the coefficient of variation of Na ions, Cl ions and salinity selected. Specific implementation manners

[0050] The present invention will be further described in detail below in combination with test examples and specific implementation manners. However, it should not be understood that the scope of the above-mentioned subject matter of the present invention is limited to the following embodiments, and all technologies implemented based on the content of the present invention belong to the scope of the present invention.

[0051] Example 1

[0052] According to Figure 1 the flow chart of the identification method of formation water in the gas reservoir shown, the formation water in the gas reservoir is identified.

[0053] Specifically, it includes the following steps:

[0054] S1. Select a water-bearing gas reservoir as the target area; select the area with formation water: the salinity of the formation water in the gas reservoir in the target area should be greater than 10,000 mg / L, and the formation water with lower salinity should be excluded.

[0055] S2. Measure the ionic concentrations of the constant anions and cations (K + , Na + , Ca 2+ , Mg 2+ , HCO - 3, SO 2- , Cl - ) in the formation water according to the method of the petroleum and natural gas industry standard (SY / T5523 - 2016), and draw the linear relationship between the ionic concentration and the salinity.

[0056] Through the analysis of geological data, the Xinchang structural belt in the western Sichuan depression of the Sichuan Basin is considered to be the area of formation water sedimentation in the gas reservoir, specifically referring to Figure 2 . Measure the constant ions (K + , Na + , Ca 2+ , Mg 2+) It is determined by using an ICP7400 inductively coupled plasma emission spectrometer (error less than 2%); for major ions (Cl - 、SO 2- 4), it is determined by using an ICS2000 ion chromatograph (error 0.2 - 0.3%); for major ions (HCO - 3), volumetric method (hydrochloric acid titration) is used (error 0.2 - 0.3%), and the test results are shown in the following table.

[0057] Table 1 Ionic content of formation water in the implementation area (mg / L)

[0058] Time <![CDATA[K + > <![CDATA[Na + > <![CDATA[Ca 2+ > <![CDATA[Mg 2+ > <![CDATA[Cl - > <![CDATA[SO4 2- > <![CDATA[HCO 3- > TDS 2022.7.5 385.1 8260 674.5 112.9 14437.4 111.4 23981.3 2022.7.6 323.3 7406 612.3 104.8 12853.1 99.4 10 21408.9 2022.7.21 481.6 11462 758.4 121.8 20000 32823.8 2022.8.28 525.2 13072 855.8 117.94 21643.4 10 36224.34 2022.9.21 512.8 11920 870.8 109.38 22330.5 10 35753.48 2022.9.27 599 13382 908.6 117.72 24880.2 10 39897.52 2022.10.10 543.4 12680 844.8 106.42 23536.6 37711.22 2022.11.14 602.8 15926 986.2 111.04 27537 189.6 45352.64 2022.12.14 811 20700 1340 148 37600 266 10 60875 2022.12.21 823 21000 1360 150 37700 269 10 61312 2023.1.21 808 22225 1353 148.35 38551.5 186.7 63272.55 2023.1.22 819.5 22890 1412 152.45 38980.2 202.27 64456.42 2023.1.23 833.5 23010 1421 153.95 39458.2 202.27 65078.92 2023.1.24 836 23015 1431 154.6 39928.8 217.83 65583.23 2023.1.25 842 23080 1429.5 154.25 40152.6 233.39 65891.74 2023.1.26 841.5 22945 1419 152.95 40387.6 233.39 65979.44 2023.1.27 839.5 22955 1440.5 153.95 41119.6 248.95 66757.5 2023.1.28 829 22900 1433 153.75 40871 248.95 66435.7 2023.2.6 837 25480 1603.5 156.6 42739 443.44 71259.54 2023.2.15 891.5 27734.45 1694 168.95 43053.5 404.54 73946.94 2023.3.2 849.5 25145 1899.5 204.15 44882.2 72980.35 2023.3.2 854.5 25515 1926 207.3 43325.5 71828.3

[0059] Draw a relationship diagram between ionic content and salinity ( Figure 3 ), and it is found that the changes of sodium ions and chloride ions are closely related to salinity, and the linear relationship R 2 > 0.99.

[0060] S3. Convert the ionic concentration of formation water into equivalent concentration, and superimpose and draw the formation water data on the Norvak phase diagram.

[0061] Calculate and determine the equivalent of formation water in the target area, substitute the equivalent concentration ratio into the Norvak phase diagram, and it is easy to find that the inflection point of Na / Mg has an increasing trend after superimposing on the same coordinate plane, as Figure 4 shown.

[0062] S4. Calculate the average value and standard deviation of each ion in the formation water to obtain the coefficient of variation of the formation water.

[0063] For the ions selected in combination with the above two conditions, it is further verified that there are obvious anomalies in the cations Na and anions Cl, and calculate their coefficient of variation. The formula is as follows:

[0064] Coefficient of variation = standard deviation ÷ average value

[0065] Keep the variance constant, that is, make the fluctuation relatively stable and eliminate the heteroscedasticity problem.

[0066] S5. Conduct comprehensive analysis in combination with the above indicators to judge whether water invasion has occurred in the formation water and identify whether the water production source is formation water invasion.

[0067] According to the coefficient of variation of the selected Na ions, Cl ions and salinity > 30%, it is judged that water invasion has occurred in the formation water of the gas reservoir, as Figure 5 shown, and the identification result is consistent with the actual situation.

[0068] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for identifying water invasion in the formation water of a gas reservoir, characterized in that, It includes the following steps: S1. Select a water-bearing gas reservoir as the target area; S2. Measure the concentrations of the major anions and cations in formation water, perform a linear fit on the ion concentrations and the salinity, and calculate the R obtained from the linear fitting relationship 2 ; When R 2 ≥ 0.90, it indicates a close relationship between the ion concentration and the salinity; When 0.90 > R 2 > 0.60, it indicates that the relationship between ion concentration and salinity is relatively good; When R 2 ≤ 0.60, it indicates that the relationship between ion concentration and salinity is poor S3. Convert the measured concentrations of major anions and cations in formation water into equivalent concentrations, and superimpose and plot the formation water data on the Novak phase diagram; S4. Calculate the average value and standard deviation of each ion in the formation water to obtain the formation water coefficient of variation; Formation water coefficient of variation = standard deviation ÷ average value S5. Combine the Novak phase diagram and the formation water coefficient of variation for comprehensive analysis to determine whether water invasion has occurred in the formation water.

2. The method for identifying water invasion of formation water in a gas reservoir according to claim 1, wherein, In step S1, the salinity of the formation water in the gas reservoir of the target area is greater than 10000 mg / L.

3. The method for identifying water invasion of gas reservoir formation water according to claim 1, wherein In step S2, follow the method of the petroleum and natural gas industry standard to measure the concentrations of major anions and cations in formation water.

4. The method for identifying water invasion in the gas reservoir formation water according to claim 3, wherein, The petroleum and natural gas industry standard is SY / T5523-2016 "Analysis Method of Oilfield Water".

5. The method for identifying water invasion of gas reservoir formation water according to claim 4, wherein In step S2, the major anions and cations include potassium ion, sodium ion, calcium ion, magnesium ion, bicarbonate radical, sulfate radical, and chloride ion.

6. The method for identifying water invasion in the formation water of a gas reservoir according to claim 1, characterized in that Step S3 is to convert the measured concentrations of major anions and cations in formation water into equivalent concentrations, and substitute the equivalent concentration ratio into the Novak phase diagram so that the phase diagrams at different time periods are all superimposed and fall in the same coordinate system.

7. The method for identifying water invasion of gas reservoir formation water according to claim 1, characterized in that In step S4, select typical major ion concentrations for calculation to obtain the formation water coefficient of variation.

8. The method for identifying water invasion of gas reservoir formation water according to claim 7, wherein, In step S4, when calculating by selecting typical constant ion concentrations, ions with R 2 ≥ 0.90 in the linear relationship are selected.

9. The method for identifying water invasion in gas reservoir formation water according to claim 8, wherein, In step S5, abnormal inflection points are selected according to the superimposed Novak phase diagram; combined with the ions with R 2 ≥ 0.90 in the selected linear relationship, the coefficient of variation is calculated. When the coefficient of variation of ions and salinity > 30%, it is considered that water invasion occurs in the gas reservoir formation water.

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