Comprehensive evaluation method for gel breaking degree of gel fracturing fluid

Abstract

The invention belongs to the technical field of oil and gas field development, and particularly relates to a comprehensive evaluation method for the gel breaking degree of gel fracturing fluid. According to the method, a calculation model of the integrity psi after gel breaking of the gel fracturing fluid is established on the basis of the mass of the residual gel separated after gel breaking or the mass of the gel breaking clear liquid in combination with the viscosity of the gel breaking clear liquid, and the gel breaking degree is quantified. According to the method, through weighted calculation of the residual colloid mass or the colloid breaking clear liquid mass and the colloid breaking clear liquid viscosity, a calculation model of the integrity psi after gel breaking of the gel fracturing fluid in different gel breaking stages is established, the gel breaking degree is quantified, and the problem that a traditional single viscosity index cannot be measured or measurement interferes is solved; the problems that the amount of dried solid residues is small, weighing is extremely inconvenient, and large errors are easily caused are solved. The method can adapt to the characteristic difference of different gel breaking stages, the evaluation reliability is remarkably improved, and data support is provided for optimizing the performance of the controlled-release gel breaker.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas field development, and particularly relates to a comprehensive evaluation method for the gel-breaking degree of gel fracturing fluid. Background Art

[0002] Incomplete gel breaking of gel fracturing fluid can cause many problems. For example, it can lead to residue blocking of fracture pores and proppant filling layers, significantly reducing fracture permeability; directly affecting the viscosity of fracturing fluid, and the retention of high-viscosity residual liquid will exacerbate water lock damage, etc. The evaluation of the gel-breaking degree of gel fracturing fluid is the core link to balance reservoir protection, engineering efficiency, economy and environmental protection, and directly affects the stimulation effect and full-cycle cost control. By evaluating the gel-breaking effect, it can be ensured that the gel-breaking time and the viscosity of the residual liquid meet the construction requirements, thereby reducing the damage to the reservoir and improving the flowback efficiency. At the same time, by evaluating the gel-breaking effect, the formula and dosage of the gel breaker can be optimized to adapt to different working conditions. For example, in a formation containing hydrogen sulfide, gel breaking and neutralization of H2S gas need to be synchronized to ensure construction safety and environmental protection requirements. In addition, the gel-breaking evaluation method promotes the synergistic effect between the plugging agent and the fracturing fluid system, improves the plugging efficiency and self-gel-breaking flowback ability, and provides technical support for efficient and low-cost operations in complex formations.

[0003] At present, traditional gel-breaking evaluation usually relies on the measurement of the viscosity of the gel-breaking fluid and the mass of the solid residue after drying, and has the following defects: (1) When the residual colloid is excessive, the amount of the gel-breaking fluid is insufficient and the viscosity measurement cannot be completed; (2) The adhesion of the residual colloid to the rotor of the rotational viscometer causes the "climbing rod effect", interfering with the accuracy of the viscosity test; (3) When conducting traditional gel-breaking evaluation in the laboratory, the amount of the solid residue after drying is very small (recorded in mg), and weighing is extremely inconvenient and prone to large errors. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention proposes a comprehensive evaluation method for the gel-breaking degree of gel fracturing fluid. Based on the mass of the residual colloid or the mass of the gel-breaking clear liquid separated after gel breaking, combined with the viscosity of the gel-breaking clear liquid, the integrity Ψ after gel breaking of the gel fracturing fluid is established

[0005] a calculation model to quantify the gel-breaking degree. Ψ Based on the mass of the gel-breaking clear liquid separated after gel breaking, combined with the viscosity of the gel-breaking clear liquid, the integrity ; In the formula: Ψ ——The integrity of 200 g of gel fracturing fluid after being coated with resin and gel broken, 0 ≤ Ψ ≤ 1; m L [[ID=�6]]——The mass of the gel-breaking clear liquid, g; ν ——The viscosity of the gel-breaking clear liquid, mPa·s.

[0006] Based on the residual colloid mass separated after gel breaking and combined with the viscosity of the gel-breaking clear liquid, Ψ the calculation model is: ; In the formula: Ψ ——The integrity of 200 g of gelled fracturing fluid after being coated with resin and gel broken, 0 ≤ Ψ ≤ 1; m S ——The residual colloid mass, g; ν ——The viscosity of the gel-breaking clear liquid, mPa·s.

[0007] The specific steps are as follows: (1) Separate the solid and liquid of the mixture after gel breaking to obtain the residual colloid and the gel-breaking clear liquid; (2) Weigh the residual colloid mass m S and the mass of the gel-breaking clear liquid m L ; (3) Select the corresponding L or m S interval to select the corresponding Ψ value calculation model and output the quantitative index of gel-breaking degree.

[0008] The Ψ is the integrity of 200 g of gelled fracturing fluid after being coated with resin and gel broken, Ψ the smaller the

[0009] value indicates the higher the degree of hydration gel breaking. According to the "Technical Requirements for Water-Based Fracturing Fluids" (SY / T 7627-2021) and the "General Technical Conditions for Fracturing Fluids" (SY / T 6376-2008), it is required that the viscosity of the water-based gelled fracturing fluid after gel breaking is lower than 5 mPa·s, and the filtered residual colloid and solid residue are less than 600 mg·L -1 . For 200 g of gelled fracturing fluid prepared from a 0.5% thickener solution, the standard of the dried residue of 600 mg·L -1 can be converted to a residual colloid mass of 24 g. That is, when the viscosity of the gel-breaking liquid of 200 g of gelled fracturing fluid is less than 5 mPa·s and the separated residual colloid mass is less than 24 g, it is considered to meet the gel-breaking standard. Calculated by Equation 1-2, the integrity Ψ of the gelled fracturing fluid at this time is 0.106. Therefore, take Ψ 0 = 0.1 as the judgment basis for whether the gelled fracturing fluid is completely gel broken when selecting the gel breaker. Ψ ≤ 0.1 means meeting the gel-breaking standard.

[0010] The viscosity of the gel-breaking clear liquid is the viscosity value measured at a rotational speed of 60 r / min of the No. 1 rotor of a digital display viscometer.

[0011] In step (1), a filter is used for solid-liquid separation, and the filter pore size is ≤1mm.

[0012] Compared with the prior art, the present invention is based on the residual colloid mass (m S ) or the mass of the gel-breaking supernatant (m L ) and the viscosity of the gel-breaking supernatant ( ν ) is calculated by weighted calculation to establish the integrity of the frozen fracturing fluid after gel breaking at different gel breaking stages. Ψ The calculation model quantifies the degree of gel breaking, solving the problem that the traditional single viscosity index cannot be measured or measurement interference occurs; it also solves the problem that the amount of solid residue after drying is very small, which is extremely inconvenient to weigh and easily causes large errors; the present invention can adapt to the characteristic differences at different gel breaking stages, significantly improves the reliability of the evaluation, and provides data support for optimizing the performance of controlled-release breakers. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 For gel integrity Ψ 3D network diagram of residual colloid mass and viscosity of broken gel supernatant; Figure 2 The gel-breaking effect of different oxidative breakers (ammonium persulfate and potassium persulfate) on boron cross-linked guar jelly fracturing fluid was studied. Figure 3 For gel integrity Ψ Curve of value changing with breaker dosage; Figure 4 The gel-breaking effect of different acid breaker dosages on boron cross-linked guar jelly fracturing fluid; Figure 5 For gel integrity Ψ The value changes with the amount of acid breaker added. DETAILED DESCRIPTION

[0014] Example 1 In order to establish the integrity of the gel fracturing fluid after gel breaking Ψ A large number of experiments were first conducted to systematically study the residual colloid mass m S and viscosity of the supernatant ν Completeness Ψ During the experiment, the residual colloid mass m after gel breaking under different conditions was accurately measured. S and viscosity of the supernatant ν and through observation and evaluation, the corresponding completeness is determined Ψ After collecting a large amount of experimental data, the data were analyzed and summarized in detail. The experiment found that Ψ With m S andν There is an obvious piecewise linear relationship. Specifically, when m S is within different ranges, Ψ the variation rules are also different. Based on these observations, the present invention proposes a piecewise linear mathematical model to describe this relationship. The inventors found that when 0 ≤ m S ≤ 50, it is mainly affected by m S and ν jointly; when 50 < m S ≤ 150, it is mainly affected by m S ; when 150 < m S ≤ 200, it is still mainly affected by m S , but the variation trend changes. Through further mathematical analysis and parameter estimation, the coefficients and parameters in the model are finally determined, and a complete calculation model is obtained, that is, formula (1-2). Through m S + m L = 200 g, formula (1-1) is obtained.

[0015] To verify the accuracy and applicability of the model, the model is applied to the experimental data not involved in modeling, and further experimental verification is carried out.

[0016] Example 2 A gel-breaking experiment was carried out on the boron-crosslinked guar gum gel fracturing fluid using various gel breakers, in order to screen out the gel breaker with good gel-breaking effect and suitable for sustained release. The gel breakers involved in the preferred experiment of gel breakers include oxidative gel breakers (APS (ammonium persulfate), KPS (potassium persulfate)), acid gel breakers (oxalic acid, citric acid, AH (organic sulfonic acid), ML (organic carboxylic acid), ammonium bisulfate), and chelating agent gel breakers (sodium citrate, sodium succinate, sodium gluconate, EDTA-2Na, sodium tartrate, sodium pyrophosphate, sodium dihydrogen phosphate).

[0017] The gel-breaking effects of the above gel breakers were evaluated through a systematic and quantitative experimental plan: (1) Place several portions of 200 g of gel fracturing fluid in a 70 °C constant temperature water bath and carry out gel breaking for 2 h at different dosages (0.25 - 1.5 g) of the gel breaker; then perform solid-liquid separation on the gel-breaking fracturing fluid system to obtain the gel-breaking clear liquid and the residual colloid; (2) Use an NDJ-9S digital display viscometer to measure the viscosity of the gel-breaking clear liquid, and at the same time use an analytical balance to weigh the gel-breaking clear liquid and the residual colloid; (3) According to the range of the mass m L of the gel-breaking clear liquid or the mass m S of the residual colloid, select the corresponding Ψ value calculation model, output the quantitative index of gel-breaking degree, and evaluate the gel-breaking effect of the gel breaker on the gel fracturing fluid. For example: When m S = 43 g, ν = 20 mPa·s, then Ψ = 0.2 * 43 / 50 + 0.2 * 20 / 100 = 0.212; When m S = 118 g, then Ψ = 0.4 + 0.4 * (118 - 50) / 100 = 0.672; When m S = 176 g, then Ψ = 0.8 + 0.2 * (176 - 150) / 50 = 0.864; Based on the above measurement and calculation results, plot Figures 2 - 5 .

[0018] Figure 2 is the change in the viscosity of the broken - gel clear liquid and the residual colloid mass after 2 - hour gel - breaking in a 70 °C water bath with different dosages of boron - crosslinked guar - gum gel breaker (ammonium persulfate and potassium persulfate).

[0019] Figure 3 is the gel - integrity Ψ value change curve with the dosage of the gel breaker. It can be seen that ammonium persulfate and potassium persulfate have a very good gel - breaking effect on boron - crosslinked guar - gum gel. A dosage of 0.25 g can reduce the integrity of 200 g of boron - crosslinked guar - gum gel to Ψ below 0.

[0020] Figure 3 directly shows the change in the gel - breaking integrity of boron - crosslinked guar - gum gel fracturing fluid with the dosage of the oxidative gel breaker, while Figure 2 only shows the viscosity of the broken - gel clear liquid and the residual colloid mass. Gel - breaking integrity is a more direct indicator reflecting the integrity of the colloid structure and can more clearly illustrate the influence of the gel - breaking effect on the colloid structure. Figure 3 Uses a unified gel - breaking integrity index ( Ψ ), enabling the direct comparison of the effects of different gel breakers (KPS and APS), while Figure 2 the viscosity of the broken - gel clear liquid and the residual colloid mass in Figure 3 are two different indices and are not as direct for comparison. Ψ 0 = 0.1) is clearly marked in Figure 2 , allowing for an intuitive judgment of whether the colloid achieves an ideal gel - breaking effect at different dosages, while Figure 2 there is no similar threshold in Figure 3 . However, whether it is the viscosity of the broken - gel liquid and the residual colloid mass in Figure 2and Figure 3 It can be seen that the effect comparison trends of KPS and APS are consistent. KPS can achieve better gel breaking effect at a lower dosage, while APS requires a higher dosage to achieve similar effect.

[0021] Figure 4 The viscosity and residual gel mass of boron-crosslinked guar jelly after 2 hours of gel breaking in a 70°C water bath with different acid breakers (oxalic acid, citric acid, AH, ML, ammonium bisulfate, etc.) were measured. The residual gel mass of guar jelly after treatment with AH, oxalic acid, citric acid, and ammonium bisulfate was lower, while the viscosity of the gel breaking solution after treatment with citric acid and oxalic acid was higher.

[0022] Figure 5 Is the integrity of the gel ψ The value changes with the amount of acid breaker. It can be seen that there are great differences in the breaking effect of different acid breakers on guar gum jelly. Among them, AH and ammonium bisulfate have the best breaking effect. Only 0.5 g of the addition can reduce the integrity of 200 g guar gum jelly to Ψ 0 or less.

[0023] Figure 5 and Figure 4 Both indicate that the breaking effect will gradually increase with the increase of the amount of breaker added, and both show the difference in effect between different breakers. For example, AH and ammonium bisulfate have better breaking effects than other breakers. Figure 5 and Figure 4 compared to, Figure 5 Provides more comprehensive and intuitive information. Figure 5 The effects of different acid breakers (oxalic acid, citric acid, AH, ML, ammonium bisulfate and AH+ML) on the breaking integrity of boron cross-linked guar jelly were demonstrated. Figure 4 The effects of different breakers on the viscosity of the breaking liquid and the quality of the residual colloid are respectively demonstrated. Figure 5 The advantage of is that it can directly reflect the effect of the breaker on the integrity of the gel breaking. Figure 4 It is necessary to indirectly infer the breaking effect through the changes in the viscosity of the breaking liquid and the mass of the residual colloid. Figure 5 It can also more intuitively compare the differences in the effects of different breakers. Figure 4 You need to look at the graphs for each breaker separately for comparison.

Claims

1. A comprehensive evaluation method for the gel-breaking degree of gel fracturing fluid, characterized in that, Based on the mass of the residual colloid separated after gel breaking or the mass of the gel-breaking clear liquid, combined with the viscosity of the gel-breaking clear liquid, establish a calculation model for the integrity after gel breaking of the gelled fracturing fluid to quantify the degree of gel breaking; Ψ ​ Based on the quality of the gel-breaking clear liquid separated after gel breaking, combined with the viscosity and integrity of the gel-breaking clear liquid Ψ The calculation model is as follows: ； Based on the mass of the residual colloid obtained after gel breaking and combined with the viscosity and integrity of the gel-breaking clear liquid Ψ The calculation model is as follows: ； In the formula: Ψ —— Integrity of 200 g of gel fracturing fluid after being coated with resin and broken, 0 ≤ Ψ ≤ 1; m L —— Mass of the breaker solution, g; m S ——Residual colloid mass, g; ν ——Breaking gel clear liquid viscosity, mPa·s.

2. The comprehensive evaluation method for the gel-breaking degree of a gel fracturing fluid according to claim 1, characterized in that, Ψ ≤0.1 to meet the gel-breaking standard.

3. The comprehensive evaluation method for gel breaking degree of gel fracturing fluid according to claim 1, characterized in that: The specific steps are as follows: (1) Separating the liquid and solid of the mixed liquid after gel breaking to obtain residual colloid and gel-breaking clear liquid; (2)Weigh the mass \(m\) of the residual colloid S and the mass \(m\) of the gel-breaking clear liquid L ; (3)According to m L or m S Select the corresponding Ψ value calculation model and output the quantification index of the gel breaking degree.

4. The comprehensive evaluation method for the gel-breaking degree of a gel fracturing fluid according to claim 1, characterized in that, The viscosity of the gel-breaking clear liquid is the viscosity value measured at a rotational speed of 60 r / min of the No. 1 rotor of a digital display viscometer.

5. The comprehensive evaluation method for the gel-breaking degree of a gel fracturing fluid according to claim 3, wherein In step (1), a filter screen with a pore size of ≤ 1 mm is used for liquid-solid separation.

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