A method for evaluating the dissolving performance of dissolvable materials for fracturing tools

Abstract

The application discloses a method for evaluating the dissolving performance of soluble material for a fracturing tool. The soluble material sample is put into a clamping sleeve and dissolved in a dissolving solution. A curve is drawn according to the remaining mass of the sample measured in the dissolving process and the dissolving time. The dissolving rate is calculated according to the slope of the straight line segment in the middle part of the curve. The stable dissolving state of the soluble material sample is obtained, so that the dissolving performance of the soluble material is accurately evaluated.

Description

Technical Field

[0001] This invention relates to a method for evaluating the solubility of soluble materials in fracturing tools such as soluble bridge plugs and soluble ball seats for oil and gas field fracturing. Background Technology

[0002] In oil and gas reservoir stimulation tools, the application of soluble materials is becoming increasingly widespread. They can simplify the tool structure and dissolve quickly after fracturing, shortening the production cycle of oil and gas wells. Therefore, the key indicator of soluble materials used in fracturing tools is their solubility. Specifically, the dissolution rate of soluble materials and the risk of wellbore blockage by dissolution products need to be evaluated. The existing methods for evaluating the solubility of soluble materials have the following problems: (1) The evaluation methods mainly conduct whole-machine dissolution tests on tools made of soluble materials. The evaluation focuses on the time for complete dissolution of the tool and does not provide the dissolution rate or other solubility properties of soluble materials; (2) The test methods for the solubility of soluble materials do not consider the influence of specimen shape and surface oxide layer on the dissolution rate in the early stage of dissolution. For example, the diameter of a spherical specimen changes continuously during the dissolution process, and the dissolution rate also changes continuously, resulting in inaccurate test methods; (3) After the specimen begins to dissolve, the surface appears granular. The accurate dissolution rate cannot be obtained by directly measuring the size of the specimen after dissolution; (4) The dissolution products are not evaluated. For soluble fracturing tools, excessive dissolution products may block the wellbore.

[0003] To address the aforementioned issues, it is necessary to establish a more effective method for testing and evaluating the solubility performance of soluble materials. For example, Chinese patent CN206832717U eliminates the influence of the processed shape of the soluble material by setting up a clamp and using a standard cylindrical specimen, and allows for weighing the remaining soluble material in the specimen at any time during the dissolution process. However, this patent still has certain problems, including: the dissolution rate of the soluble material varies during the dissolution process due to factors such as the surface oxide layer in the early stage of dissolution and the accumulation of flocculent dissolution products in the later stage, making it difficult to accurately evaluate the solubility performance of the soluble material. Summary of the Invention

[0004] The purpose of this invention is to provide an accurate, rapid, and comprehensive method for evaluating the solubility of soluble materials used in fracturing tools, thereby facilitating the comparison and selection of soluble materials during the processing of fracturing tools.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] The method for evaluating the solubility of soluble materials includes the following steps:

[0007] Step 1: Place a cylindrical soluble material specimen with volume V and end cross-sectional area S into the heated solution, ensuring that only the two end faces of the cylindrical soluble material specimen can contact the heated solution.

[0008] Step 2 begins when the cylindrical soluble material specimen undergoes stable dissolution in the heated solution. The volume of the dissolved cylindrical soluble material specimen is measured at fixed intervals, and the dissolution time t and the volume of the dissolved cylindrical soluble material specimen are recorded.

[0009] Step 3: Plot a stable dissolution curve with dissolution time t as the abscissa and the volume of the dissolved cylindrical soluble material specimen as the ordinate. Then calculate the ratio of K to S. The absolute value of this ratio (K / S) is the dissolution rate of the soluble material, where K is the slope of the stable dissolution curve.

[0010] Preferably, the dissolving solution is a KCl solution with a mass fraction of 0.2%-0.5%.

[0011] Preferably, the soluble material is selected from soluble metals, etc.

[0012] Preferably, the heating temperature of the solution can be determined according to the specific formation temperature requirements, for example, a heating temperature of 40-60℃.

[0013] Preferably, step 1 specifically includes the following steps: heating the solution to a specified temperature (e.g., 40-60℃), placing a cylindrical soluble material specimen with volume V and end face cross-sectional area S into a hollow cylindrical fixture, and then placing the cylindrical soluble material specimen along with the fixture into the solution (e.g., a KCl solution at a temperature of 40-60℃), thereby using the fixture to ensure that only the end face of the cylindrical soluble material specimen can contact the solution after being placed in it.

[0014] Preferably, the ratio of the diameter to the length of the cylindrical soluble material specimen is 0.4-0.5. This ratio ensures that the cylindrical soluble material specimen changes at a near-uniform rate during stable dissolution, thereby improving the accuracy of dissolution rate measurement.

[0015] Preferably, step 2 specifically includes the following steps: based on the change in the dissolution rate during the complete dissolution of the soluble material, after the cylindrical soluble material specimen (with the fixture) is placed in the dissolving solution (e.g., a KCl solution at a temperature of 40-60℃), the dissolved cylindrical soluble material specimen is taken out and weighed according to the set corresponding time intervals (e.g., a shorter time interval is used in the dissolution stage where the initial dissolution rate changes rapidly, and a longer time interval is used in the dissolution stage where the dissolution rate stabilizes and eventually slows down), to obtain the remaining mass G of the specimen. Then, the dissolution time t at each time interval and the ratio V1 of the remaining mass G of the specimen to the density of the soluble material are recorded.

[0016] Preferably, in step 2, the dissolved cylindrical soluble material specimen is dried (e.g., oven-dried) before weighing.

[0017] Preferably, in step 3, the stable dissolution curve refers to the line connecting any two points within two boundary points (inflection points) of the dissolution curve plotted according to the dissolution time t recorded during the complete dissolution process and the ratio V1 of the remaining mass G of the specimen to the density of the soluble material. This line connects any two points within the boundary points (inflection points) of the dissolution curve, where the slope calculated sequentially from adjacent points in the dissolution curve no longer changes significantly.

[0018] Preferably, the method for evaluating the solubility performance of soluble materials further includes the following steps: after a cylindrical soluble material specimen with volume V and end cross-sectional area S is completely dissolved, the volume V2 of the flocculent dissolution product is measured, and then the ratio of V2 to V is calculated. This ratio (V2 / V) is the expansion rate. The flocculent dissolution product is a flocculent precipitate formed by the dissolution product of the soluble material (i.e., a cylindrical soluble material specimen with volume V and end cross-sectional area S) and the dissolving liquid (e.g., a KCl solution at a temperature of 40-60℃). The larger the volume of the flocculent dissolution product, the less complete the dissolution (the higher the expansion rate), and the greater the impact on subsequent production (e.g., it is easier to form blockages). Therefore, the smaller the volume of the flocculent dissolution product, the better.

[0019] The beneficial effects of this invention are reflected in:

[0020] This invention uses the stable portion of the dissolution curve to obtain the slope K of the straight line, which can eliminate the influence of inconsistent changes in the dissolution rate of soluble materials in the initial and final stages of dissolution, thereby accurately evaluating the dissolution performance of soluble materials after they are used to process fracturing tools.

[0021] Furthermore, the present invention plots a dissolution curve using the ratio of the remaining mass G of the specimen to the density V1 of the soluble material, which can eliminate the influence of material density and obtain the true volume change of the specimen, which is more accurate than directly measuring the geometric dimensions of the specimen.

[0022] Furthermore, the cylindrical soluble material specimen used in this invention has a moderate diameter-to-length ratio, which helps to limit the specimen to dissolving only from the end face. In addition, the dissolving solution used is a KCl solution with a concentration of 0.2%-0.5%, which enables the soluble material (specifically soluble metals, etc.) in the specimen to dissolve at a medium to low rate, thereby ensuring that the dissolution process is close to or reaches a uniform rate change, making the measurement results of the dissolution rate more accurate.

[0023] Furthermore, by sequentially calculating the boundary points (inflection points) on the dissolution curve during the complete dissolution process, this invention helps to improve the calculation speed of the dissolution rate of soluble materials. Attached Figure Description

[0024] Figure 1This is a schematic diagram of the fixture (specimen tooling) structure and clamping of soluble metal specimens; in the diagram: 1. clamping sleeve; 2. specimen; 3. waterproof layer.

[0025] Figure 2 The figure shows the dissolution curve of a soluble metal specimen; in the figure, the dashed line represents the stable dissolution portion with a slope of K.

[0026] Figure 3 This is a schematic diagram of the dissolved products; in the diagram: the volume of the flocculent colloid precipitate V2 is the volume of the flocculent dissolved products, which can be read directly. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0028] This embodiment provides a method for evaluating the solubility of soluble metals (specifically soluble aluminum alloys) used in fracturing tools, including the following steps:

[0029] 1) For oilfield fracturing tools, due to the low formation temperature, a 0.2wt% KCl solution (volume V0, V0 = 2000mL) was heated to 40℃ and kept constant. A soluble metal specimen 2 (a cylinder with a diameter-to-length ratio of 0.4 and a volume V = 15700mm²) with a diameter of 2cm and a length of 5cm was used. 3 Placed in such Figure 1 The fixture shown has an outer cylindrical clamping sleeve 1 and an inner rubber diaphragm (as a waterproof layer 3) attached to the inner wall of the clamping sleeve 1. The rubber diaphragm is used to tightly wrap the side of the soluble metal specimen 2 placed in the fixture, ensuring that only the end face of the soluble metal specimen 2 can contact the solution.

[0030] 2) In the initial stage of dissolution of soluble metal specimen 2, the specimen was taken out and dried every 0.5 hours, the remaining mass of the specimen was measured, and the ratio of the remaining mass of the specimen to the density of the material (soluble metal) was calculated, i.e. the true remaining volume of the specimen V1. The corresponding dissolution time and the true remaining volume of the specimen V1 were recorded. The results are shown in Table 1.

[0031] 3) After dissolving for 2 hours, measure the remaining mass of the specimen every 1-2 hours and calculate the ratio of the remaining mass to the density of the material (soluble metal). Record the corresponding dissolution time and the actual remaining volume V1 of the specimen until it is completely dissolved. The results are shown in Table 1.

[0032] Table 1. Record of dissolution time and remaining volume of soluble metal specimens

[0033]

[0034] 4) After the soluble metal specimen 2 is completely dissolved, measure the volume V2 of the flocculent dissolution product in the KCl solution. Figure 3 As shown, the flocculent dissolution product is a flocculent colloidal mixture of dissolved solid residue and water, so its volume V2 > (total volume of the dissolved system - V0). Calculating V2 / V, the expansion ratio, can be used to determine whether the dissolved products from the fracturing tool will clog the wellbore. Generally, when V2 / V > 10, there is a risk of wellbore blockage. This is because the dissolved products solidify into lumps at high formation temperatures. When V2 / V > 10, the formation pressure cannot dislodge the solid lumps in the wellbore, causing blockage. In this example, the selected material (soluble metal) has a V2 / V ratio of 1:6, indicating that it will not clog the wellbore.

[0035] 5) Plot the relationship curve between dissolution time and V1, such as... Figure 2 As shown, taking the stable portion of the curve's slope (i.e., the straight line segment in the middle of the curve), the slope K is calculated to be -1475 mm. 3 / h, which is the volume of the specimen dissolved per hour, and then the absolute value of K / S is calculated. The resulting 1.17mm / h is the dissolution rate of the soluble metal. This dissolution rate is used to characterize the dissolution performance of soluble metals. It means the thickness of the soluble metal dissolved per unit time and unit area under specified temperature and solution conditions, and the unit is mm / h.

[0036] Compared to step 3, step 2 uses a shorter time interval for measuring the remaining mass of the specimen. This is because in the initial stage of dissolution, the surface of the soluble metal specimen 2 is relatively smooth and has a thin oxide layer. At this stage, the dissolution rate cannot reflect the true dissolution performance; it is necessary to find the inflection point of the dissolution curve. Therefore, the measurement time interval is shorter. The inflection point is indicated by the slope K of the line connecting points N and N-1. (N-1，N) The slope K of the line connecting points N-1 and N-2 (N-2，N-1) In comparison, the absolute value of the rate of change of the slope, i.e., |[K] (N-1，N) -K (N-2，N-1) ] / K (N-2，N-1) If ×100%|≤10%, then points N, N-1, and N-2 meet the true dissolution rate condition. (See also...) Figure 2 In this example, the first inflection point is the 6th point, the second inflection point is the 8th point, and the slope K of the line connecting the 6th and 7th points is... (6，7) -1435mm 3 / h, compared to the slope of the line connecting the first two points (points 5 and 6), the absolute value of the rate of change of the slope is 5%, and the slope K of the line connecting the 7th and 8th points is... (7，8) -1514mm 3 / h, compared with the slope of the line connecting the first two points (points 6 and 7), the absolute value of the slope change rate is 8%. Therefore, the slope of the line connecting any two points between points 6 and 8 can reflect the true solubility performance.

[0037] In this example, the slope K is taken as K (6，7) and K (7，8) The average value is -1475mm 3 / h. But actually, it can also be K. (6，7) Using K as a reference allows for faster measurement of solubility.

Claims

1. A method for evaluating the solubility of soluble materials used in fracturing tools, characterized in that: The method for evaluating the solubility of soluble materials includes the following steps: 1) Place a cylindrical soluble material specimen with volume V and end cross-sectional area S in the heated solution, ensuring that only the two end faces of the cylindrical soluble material specimen are in contact with the heated solution; 2) Starting from the stable dissolution of the cylindrical soluble material specimen in the heated solution, the volume of the dissolved cylindrical soluble material specimen is measured at fixed time intervals, and the dissolution time t and the volume of the dissolved cylindrical soluble material specimen are recorded. 3) Plot a stable dissolution curve with dissolution time t as the abscissa and the volume of the dissolved cylindrical soluble material specimen as the ordinate. Then calculate the ratio of K to S. The absolute value of this ratio is the dissolution rate of the soluble material, where K is the slope of the stable dissolution curve. The method for evaluating the solubility of soluble materials further includes the following steps: after a cylindrical soluble material specimen with a volume of V and an end cross-sectional area of ​​S is completely dissolved, the volume of the flocculent dissolution product V2 is measured, and then the ratio of V2 to V is calculated. This ratio is the expansion rate. The flocculent dissolution product is a flocculent precipitate formed by the soluble material dissolution product and the solution.

2. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 1, characterized in that: The solution is a KCl solution with a mass fraction of 0.2%-0.5%.

3. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 1, characterized in that: The soluble material is selected from one of the soluble metals.

4. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 1, characterized in that: The heating temperature of the solution is 40-60℃.

5. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 1, characterized in that: Step 1 specifically includes the following steps: heating the solution to a specified temperature, placing a cylindrical soluble material specimen with a volume of V and an end cross-sectional area of ​​S into a hollow cylindrical fixture, and then placing the cylindrical soluble material specimen into the solution along with the fixture.

6. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 1 or 5, characterized in that: The ratio of the diameter to the length of the cylindrical soluble material specimen is 0.4-0.

5.

7. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 1, characterized in that: Step 2 specifically includes the following steps: based on the change in the dissolution rate during the complete dissolution of the soluble material, after the cylindrical soluble material specimen is placed in the dissolving liquid, the dissolved cylindrical soluble material specimen is taken out and weighed at the set corresponding time intervals to obtain the remaining mass G of the specimen. Then, the dissolution time t at each time interval and the ratio V1 of the remaining mass G of the specimen to the density of the soluble material are recorded.

8. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 7, characterized in that: In step 2, the dissolved cylindrical soluble material specimen is dried before weighing.

9. The method for evaluating the solubility of soluble materials used in fracturing tools according to claim 7, characterized in that: In step 3, the stable dissolution curve refers to the line connecting any two points within two boundary points of the dissolution curve plotted according to the dissolution time t recorded during the complete dissolution process and the ratio of the remaining mass G of the specimen to the density of the soluble material V1. This line connects any two points within the boundary points of the dissolution curve such that the slope calculated sequentially from adjacent points in the dissolution curve no longer changes significantly.

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