A discriminant method for the intensity of wellbore-rock reaction of on-site shale gas wells

Abstract

The present invention discloses a method for discriminating the water-rock reaction intensity of on-site shale gas well water, comprising the following steps: Step 1: Collect the fracturing flowback fluid of the shale gas horizontal well after fracturing, and test the geochemical parameter characteristics of the fracturing flowback fluid and production water samples; Step 2: Obtain the linear relationship between the fracturing fluid ion concentration and the production water ion concentration according to the reference ions; Step 3: Plot the chemical characteristic parameters obtained in Step 1 on the chart formed by the linear relationship obtained in Step 2; the ion source of the water-rock reaction is above the line, and the sedimentation effect during the flowback process is below the line; Step 4: Calculate the water-rock reaction intensity coefficient I, and judge the water-rock reaction intensity according to the calculated water-rock reaction intensity coefficient; The method of the present invention analyzes the geochemical properties of the on-site fracturing flowback fluid, and quantitatively characterizes the difference in the strength of the water-rock reaction between wells on-site based on the flowback characteristics of the fracturing flowback fluid.

Description

Technical Field

[0001] The present invention relates to the technical field of petroleum geological exploration, and particularly relates to a method for discriminating the water-rock reaction intensity of on-site shale gas well water. Background Art

[0002] Shale reservoirs are dense and need to rely on hydraulic fracturing technology to create artificial fracture networks to improve the seepage capacity of shale gas and achieve the exploration and development of shale gas. During the fracturing process, the fracturing fluid will inevitably come into contact with the shale reservoir rock, and complex water-rock reaction effects are bound to occur, resulting in changes in the geochemical properties of the fracturing flowback fluid of the shale well. The stronger the water-rock reaction, the larger the contact area between the fracturing fluid and the reservoir rock, and the stronger the ion exchange effect, thereby revealing the quality of the fracturing effect. Therefore, the geochemical properties of the fracturing flowback fluid of the shale gas well can indirectly reflect the water-rock reaction intensity, and then guide the development of shale gas.

[0003] The main reasons for the changes in the geochemical property characteristics of the fracturing flowback fluid of shale gas wells are mainly the water-rock reaction effect and the mixing of reservoir pore water. Clearly understanding the strength of the water-rock reaction is also beneficial to understanding the proportion of pore water in the fracturing flowback fluid of shale gas wells, which is of great significance for guiding the construction of on-site flowback measures and the treatment of fracturing flowback fluid. At present, the methods for judging the strength of the water-rock reaction are mainly qualitative methods, mainly from laboratory experimental means (including temperature, pressure, fluid properties, etc.), and it is difficult to achieve quantitative characterization of the difference in the strength of the water-rock reaction between wells on site. Summary of the Invention

[0004] The present invention provides a method for discriminating the water-rock reaction intensity of on-site shale gas well water that can quantitatively characterize the strength of the water-rock reaction between wells in view of the problems existing in the prior art.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for discriminating the water-rock reaction intensity of on-site shale gas well water, comprising the following steps:

[0007] Step 1: Collect the fracturing flowback fluid of the shale gas horizontal well after fracturing, and test the geochemical parameter characteristics of the fracturing flowback fluid and the produced water samples;

[0008] Step 2: Obtain the linear relationship between the fracturing fluid ion concentration and the produced water ion concentration according to the reference ion;

[0009] Step 3: Plot the chemical characteristic parameters obtained in Step 1 on the chart formed by the linear relationship obtained in Step 2; the ion source of the water-rock reaction is above the line, and the sedimentation effect during the flowback process is below the line;

[0010] Step 4: Calculate the water-rock reaction intensity coefficient I;

[0011]

[0012] Among them: C R is the increased ion concentration after the water-rock reaction, and C * is the standard ion concentration in the fracturing flowback fluid;

[0013] C R is obtained by subtracting the standard ion concentration C0 under the ideal state from the standard ion concentration in the fracturing flowback fluid;

[0014] Step 5: According to the calculated water-rock reaction intensity coefficient, judge the water-rock reaction intensity. The larger the value of the water-rock reaction intensity coefficient, the stronger the water-rock reaction intensity.

[0015] Furthermore, in the step 4, the standard ion concentration in the fracturing flowback fluid is calculated according to the ion types above the deviation from the linear relationship; the standard ion concentration under the ideal state is calculated according to the linear relationship.

[0016] Furthermore, it is characterized in that the reference ion is chloride ion.

[0017] Furthermore, the chemical parameter characteristics include the following ion concentrations:

[0018] Ca 2+ 、Mg 2+ 、K + 、Na + 、Mn 2+ 、Sr 2+ 、Ba 2+ 、Cl + 、Br - 、F - 、SO4 2- 。

[0019] The beneficial effects of the present invention are:

[0020] The method of the present invention analyzes the geochemical properties of the on-site fracturing flowback fluid, and quantitatively characterizes the difference in the strength of the water-rock reaction between wells according to the flowback characteristics of the fracturing flowback fluid. The method is simple and suitable for on-site calculation and discrimination. Description of the Drawings

[0021] Figure 1 is the distribution diagram of the ion concentration (Ca 2+ ) of the fracturing flowback fluid and the mixed model in the embodiment of the present invention.

[0022] Figure 2 is the distribution diagram of the ion concentration (Mg 2+ ) of the fracturing flowback fluid and the mixed model in the embodiment of the present invention.

[0023] Figure 3This is the distribution diagram of the ion concentration (Sr 2- ) of the fracturing flowback fluid and the mixing model in the embodiment of the present invention.

[0024] Figure 4 This is the distribution diagram of the ion concentration (Ba 2+ ) of the fracturing flowback fluid and the mixing model in the embodiment of the present invention.

[0025] Figure 5 This is the distribution diagram of the ion concentration (Mn 2+ ) of the fracturing flowback fluid and the mixing model in the embodiment of the present invention.

[0026] Figure 6 This is the cross plot of the chloride ion and δ 18 O ratio in the embodiment of the present invention.

[0027] Figure 7 This is the relationship curve diagram of the water-rock reaction intensity and the reflux time in two wells in the embodiment of the present invention. Detailed implementation manners

[0028] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0029] A method for discriminating the water-rock reaction intensity of on-site shale gas well water includes the following steps:

[0030] Step 1: Collect the fracturing flowback fluid of the shale gas horizontal well after fracturing, and test the geochemical parameter characteristics of the fracturing flowback fluid and the produced water samples;

[0031] First, collect the fracturing fluid, and then continuously collect the fracturing flowback fluid of the shale gas well for 40 - 50 days starting from the well opening, once at 9:00 every morning; select the produced water samples from the adjacent well that has been produced for 1 year as the reference basis for the geochemical information of the formation water. The geochemical parameter characteristics mainly include the following cation and anion concentrations:

[0032] Ca 2+ , Mg 2+ , K + , Na + , Mn 2+ , Sr 2+ , Ba 2+ , Cl + , Br - , F - , SO4 2- .

[0033] Step 2: Obtain the linear relationship between the ionic concentration of the fracturing fluid and the ionic concentration of the produced water based on the reference ion; Since chloride ions are relatively stable during the fracturing and flowback processes, they can be used as conservative ions to judge the absorption and release of other ions. Establish a homogeneous mixing model between the fracturing fluid and the formation water, that is, the linear relationship between the ionic concentration of the fracturing fluid and the ionic concentration of the produced water (formation water).

[0034] Step 3: Plot the chemical characteristic parameters obtained in Step 1 on the chart formed by the linear relationship obtained in Step 2; The ionic source of the water-rock reaction is above the line, and the sedimentation during the flowback process is below the line.

[0035] Consider the change relationship between the concentration of each ion and the reference ion respectively. When finally calculating the water-rock reaction intensity coefficient, only one of the ions needs to be determined.

[0036] Step 4: Calculate the water-rock reaction intensity coefficient I;

[0037]

[0038] Where: C R is the increased ionic concentration after the water-rock reaction,

[0039] C * is the standard ionic concentration in the fracturing flowback fluid; Select a certain ion above the deviation model as the standard ion type for calculating the water-rock reaction intensity, and record its concentration value. Under ideal conditions, the standard ionic concentration is the selected reference ion Cl - , and the standard ionic concentration under ideal conditions can be obtained according to the linear mixing model.

[0040] C R is obtained by subtracting the standard ionic concentration C0 under ideal conditions from the standard ionic concentration in the fracturing flowback fluid;

[0041] Step 5: Judge the water-rock reaction intensity according to the calculated water-rock reaction intensity coefficient. The larger the value of the water-rock reaction intensity coefficient, the stronger the water-rock reaction intensity.

[0042] In the embodiments of the present invention, two wells, Z202H1 and Z203, in the western part of the Sichuan Basin are taken as examples. The same fracturing fluid formulation system is used for the fracturing of the two wells. Therefore, 500 mL of fracturing fluid was collected before fracturing, and its geochemical properties were measured for reference. After the shale gas horizontal well was fractured and soaked for a period of time, it flowed back. 500 ml of fracturing flowback fluid for experimental analysis was collected at the site at 22:30 every day, sealed in plastic bottles, and taken back to the laboratory. Wells Z202H1 and Z203 were sampled for 108 days and 38 days respectively. Continuous sampling was carried out in the first 15 days, and then sampling was carried out on the next day. When sampling, the first time after shutting in the well should be removed, and then normal sampling should be carried out. A total of 82 experimental analysis samples, including fracturing fluid samples, were collected. The chemical composition and stable isotope ratio analysis of the collected fracturing flowback fluid samples must be completed within 3 days to avoid the mutual conversion between CO3 2- and HCO 3- . In order to better analyze whether formation water exists in the fracturing flowback fluid, the produced water of adjacent wells after 380 days in the study area was collected.

[0043] The test method for ion concentration is carried out by using existing known methods.

[0044] The stable isotope composition ratio, ion concentration and total salinity of the fracturing fluid and produced water after 380 days are shown in Table 1. The collected fracturing fluid and fracturing flowback fluid samples need to be filtered through a 0.45 μm filter membrane before ion testing, and then the filtered samples are divided into three equal parts for cation testing, anion testing and stable isotope ratio testing respectively.

[0045] Table 1. Statistical data of stable isotope ratios, ion compositions and total salinities of fracturing fluid and formation water

[0046]

[0047] In order to accurately calculate the content of fracturing fluid in the fracturing flowback fluid, it is necessary to know which ions increase or decrease after the water-rock reaction. Chloride can be regarded as a conservative tracer for the in-situ formation water to flush the fracturing fluid. During the fracturing and flowback process, if the water-rock reaction is not considered, there are only two components in the fracturing flowback fluid, formation water and fracturing fluid. As the flowback time increases, the total salinity and stable isotope ratio will become larger and larger, indicating that the formation water content is getting higher and higher, and the fracturing flowback fluid will eventually evolve into formation water. It is considered that the produced water after one year of production can be approximately regarded as formation water. Therefore, the fluid during the fracturing and flowback process can be regarded as a mixing process of fracturing fluid and formation water. The ions mainly come from the mixture of formation water and water-rock reaction. A uniform mixing model between the geochemical properties of fracturing fluid and formation water can be established as the basis for the absorption or release of ions during the water-rock reaction process.

[0048] Since chloride ions are relatively stable and basically do not participate in chemical reactions during the reflux process, the source can be determined by the variation relationship between different ion concentrations and Cl - as shown below. Figures 1 to 5 As shown in the figure, the straight line is the connection line between the fracturing fluid and the produced water, which can be regarded as the uniform mixing between the fracturing fluid and the formation water. Stable isotopes mainly indicate the water source, and other water sources can be basically ignored. As can be seen from Figure 6 , the fracturing flowback fluid is distributed near the mixing model, indicating that chloride ions are relatively stable during the fracturing flowback process.

[0049] As can be seen from Figure 1 and Figure 2 , the concentrations of Ca 2+ and Mg 2+ in the fracturing flowback fluid of Well Z202H1 increase rapidly, and the overall distribution position is above the mixing model, reflecting that not only the mixing of formation water exists during the fracturing flowback process, but also the additional supplement of water-rock reaction. As can be seen from Figure 3 and Figure 4 , the concentrations of Ba 2+ and Sr 2+ also show an increasing trend, but the characteristic distribution of the fracturing flowback fluid is below the mixing model, indicating that there are obvious other physical and chemical effects to reduce it, such as barite and celestite precipitation. As can be seen from Figure 5 , the cross results of trace ions Mn 2+ and Cl - show that the evolution characteristics of the ion concentrations of the fracturing flowback fluid are distributed near the mixing model, indicating that these ions mainly evolve along the direction of formation water and there is no additional source or other sources. During the reflux process, not only the mixing of formation water exists, but also water-rock reactions occur to generate other ions.

[0050] It can be seen that water-rock reactions will lead to an increase in Ca and Mg ions. The variation law of calcium or magnesium ion concentration can be selected to analyze the intensity of water-rock reactions.

[0051] Taking Ca ions as an example

[0052]

[0053] where C Ca is the concentration of Ca 2+ in the fracturing flowback fluid, is the concentration of Ca 2+ calculated by the model.

[0054] According to the uniform model (i.e., the straight line relationship in the attached figure), it can be expressed as:

[0055]

[0056] Where: C Cl is the concentration of chlorine in the fracturing flowback fluid, and k and b are constants.

[0057] Through the above equation, the water-rock reaction intensity coefficient I of well Z202H1 and well Z203 can be obtained. From Figure 7 it can be seen that in the first four days of the reflux, the I value of Z202H1 is relatively small, and then increases to a maximum of 67.85%. As the reflux time increases by 31 days, the formation water content in the fracturing flowback fluid gradually increases, and the water-rock reaction intensity decreases, and I finally stabilizes at about 18%. In Z203, due to the addition of acetone to remove blockage, the I value shows a negative value. The maximum value of I is 82.21%, and finally stabilizes at about 25%, which is much larger than that of Z202H1.

[0058] The present invention discriminates the water-rock reaction intensity of on-site shale gas well water through the geochemical property analysis of on-site fracturing flowback fluid and based on the flowback characteristics of the fracturing flowback fluid. It can easily realize the on-site quantitative characterization of the difference in the strength of the water-rock reaction between wells.

Claims

1. A method for discriminating the intensity of wellbore-rock reaction of on-site shale gas wells, characterized in that, It includes the following steps: Step 1: Collect the fracturing flowback fluid of the shale gas horizontal well after fracturing, and test the geochemical parameter characteristics of the fracturing flowback fluid and produced water samples; Step 2: Obtain the linear relationship between the fracturing fluid ion concentration and the produced water ion concentration according to the reference ion; the reference ion is chloride ion; Step 3: Plot the chemical characteristic parameters obtained in Step 1 on the chart formed by the linear relationship obtained in Step 2; the ion source of the water-rock reaction is above the line, and the sedimentation during the flowback process is below the line; Step 4: Calculate the water-rock reaction intensity coefficient I; Among them: C R is the increased ion concentration after the water-rock reaction, C * is the standard ion concentration in the fracturing flowback fluid; C R Obtained by subtracting the standard ion concentration in the fracturing flowback fluid from the standard ion concentration C0 under the ideal state; Standard ion concentration in the fracturing flowback fluid: Select a certain ion deviating above the model as the standard ion type for calculating the water-rock reaction intensity, and record its concentration value; The standard ion concentration under ideal conditions is obtained according to the linear mixing model; Step 5: Judge the water-rock reaction intensity according to the calculated water-rock reaction intensity coefficient. The larger the value of the water-rock reaction intensity coefficient, the stronger the water-rock reaction intensity.

2. The discriminant method for the water-rock reaction intensity of on-site shale gas well water according to claim 1, characterized in that The geochemical parameter characteristics include the following ion concentrations: Ca 2+ 、Mg 2+ 、K + 、Na + 、Mn 2+ 、Sr 2+ 、Ba 2+ 、Br - 、F - 、SO4 2- 。

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