A preferred method for determining the concentration of a reservoir fracturing fluid breaker

Abstract

The application belongs to the technical field of oil and gas field development, and particularly discloses a method for optimizing the concentration of a reservoir fracturing fluid gel breaker, which comprises the following steps: (1) selecting a representative core of a reservoir, measuring the volume fraction of carbonate minerals, and determining the maximum fracture width and the fracturing fluid action depth; (2) preparing a fracturing fluid with a volume of V, adding calcium carbonate powder with a mass of m grams, adding gel breakers with different concentrations, and heating to the reservoir temperature; (3) completely breaking the gel at the reservoir temperature, and filtering the gel breaking liquid; and (4) selecting the concentration range of the gel breaker according to the relationship between the gel breaker concentration and the relative molecular mass of the gel breaking liquid degradation product, the residue amount, the residue particle size, and the sugar content of the filtrate. The application establishes the relationship between the gel breaking liquid concentration and the relative molecular mass distribution of the gel breaking liquid degradation product, the sugar content of the filtrate, and the residue content by considering the content of carbonate minerals in the reservoir, more comprehensively analyzes the gel breaking performance of the gel breaker, and meets the current demand for the performance of the gel breaking liquid.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas field development, and particularly relates to a method for optimizing the concentration of a reservoir fracturing fluid breaker. Background Art

[0002] Fracturing fluid, often called the "blood" of fracturing, is a critical component of hydraulic fracturing operations. With the shift from conventional to unconventional oil and gas reservoirs, higher performance requirements are being placed on fracturing fluids. Continuous improvement of fracturing fluid performance is a prerequisite for advancements in fracturing technology. Hydraulic fracturing processes can cause significant polymer damage. The thickeners used in fracturing fluids are primarily polysaccharide polymers, which offer properties such as thickening, proppant transport, suspension, fluid loss control, and interlayer isolation. However, these polymers are difficult to degrade after the operation, often causing polymer damage. Therefore, fracturing operations require the addition of breakers to degrade the polymers, reducing their molecular weight and lowering the viscosity of the fracturing fluid, enabling more complete flowback of the fluid while retaining the proppant within the fractures, creating efficient seepage pathways and achieving the goal of increased production from fracturing. For ultra-low permeability oilfields, selecting a breaker suitable for the reservoir's fracturing fluid system is particularly crucial.

[0003] Currently, the performance of breaker fluids is mainly evaluated based on the viscosity and residue content of the breaker fluid, without considering that the breaker will react with carbonate minerals at reservoir temperature. As oil and gas development continues to move towards unconventional oil and gas fields, evaluating the breaker performance based solely on the viscosity and residue content of the breaker fluid cannot comprehensively analyze and evaluate the breaker's adaptability to reservoirs and fracturing fluid systems, and cannot meet current requirements for breaker performance.

[0004] Therefore, how to provide a method for comprehensively analyzing and evaluating the performance of a breaker is a technical problem that those skilled in the art urgently need to solve. Summary of the Invention

[0005] In view of this, the present invention establishes a relationship between the concentration of the breaker and the relative molecular mass distribution of the degradation products of the breaker, the sugar content of the filtrate and the residue content by considering the carbonate mineral content of the reservoir, so as to more comprehensively analyze the breaker performance and meet the current demand for the performance of the breaker.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for optimizing the concentration of a reservoir fracturing fluid breaker comprises the following steps:

[0008] (1) Select representative reservoir cores, measure the carbonate mineral volume fraction φ, and determine the maximum fracture width d and the fracturing fluid action depth h;

[0009] (2) preparing a fracturing fluid of volume V, adding m grams of calcium carbonate powder and different concentrations of breakers, and heating to the reservoir temperature;

[0010] (3) Perform complete gel breaking at reservoir temperature and filter the broken gel solution;

[0011] (4) According to the relationship between the concentration of the breaker and the relative molecular mass of the degradation products of the breaker liquid, the amount of residue, the particle size of the residue, and the sugar content of the filtrate, the concentration range of the breaker is selected.

[0012] Preferably, the mass m of added calcium carbonate powder is calculated according to Formula 1:

[0013]

[0014] Where m is the mass of calcium carbonate powder, g; V is the volume of fracturing fluid, cm3; d is the maximum fracture width of the reservoir, cm; h is the invasion depth of the fracturing fluid, cm; φ is the volume fraction of carbonate minerals, %; ρ is the density of calcium carbonate minerals, g / cm3.

[0015] Preferably, the gel breaker is ammonium persulfate.

[0016] Preferably, the fracturing fluid is guar gum fracturing fluid.

[0017] Preferably, the reservoir is an organic-rich shale reservoir.

[0018] Preferably, the gel is broken in a thermostat at 80° C. in step (3), and the gel is considered to be completely broken if the viscosity of the gel-breaking liquid is less than 5 mPa·s at room temperature.

[0019] Preferably, the breaker concentration range is selected based on the principles of low residue amount, low residue relative molecular weight, low filtrate sugar content, and the median particle size of the breaker residue being greater than or equal to 2 / 3 of the maximum throat diameter of the reservoir, and the particle size D90 of the breaker residue being less than 1 / 3 of the maximum mesh proppant guide gap diameter.

[0020] It can be seen from the above technical solutions that compared with the prior art, the present invention has the following excellent effects:

[0021] (1) This method takes into account the reaction between the breaker and carbonate minerals at reservoir temperature, while existing methods ignore the influence of reservoir minerals on the breaker's breaking effect. This method combines the actual carbonate mineral content of the reservoir with the evaluation of breaker performance to further adjust the breaker concentration, which can effectively improve the reservoir fracturing and breaking effect.

[0022] (2) This method uses the relative molecular weight distribution of the residue and the sugar content of the filtrate as evaluation indicators for the breaker's breaking effect. Currently, the evaluation of breaker effectiveness is mainly based on the viscosity of the breaker solution and the residue content, without considering the relationship between the breaker concentration and the relative molecular weight distribution of the breaker degradation products and the sugar content of the filtrate. The higher the relative molecular weight of small molecules, the less damage to the reservoir permeability. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0024] Figure 1 The curves are related to the relative molecular weight of the degradation products of the breaker at different concentrations;

[0025] Figure 2 This is the relationship curve between different concentrations of breaker and filtrate sugar content;

[0026] Figure 3 The relationship curve between different concentrations of breaker and the amount of residue in the breaker solution;

[0027] Figure 4 This is the relationship curve between different concentrations of breaker and the particle size of the breaker liquid residue. DETAILED DESCRIPTION

[0028] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0029] Example 1

[0030] The following specific implementation case is given for the organic-rich shale reservoir in Block X according to the method of the present invention:

[0031] (1) A typical representative core was taken on site and analyzed by X-ray diffraction. The volume fraction of carbonate minerals in the reservoir was φ36%, and the density of carbonate minerals was ρ2.8g / cm 3 Based on the use of 20 / 40 mesh ceramsite as a proppant on site, the maximum fracture width d in the reservoir was calculated to be 0.255 cm. Mercury intrusion injection on typical cores revealed a rock throat radius of 0.3 to 5 μm.

[0032] (2) The maximum particle size of carbonate minerals in the core was measured using a scanning electron microscope to be 0.0012 cm, which is the penetration depth h of the fracturing fluid.

[0033] (2) Prepare 2.5 L of 0.4% guar gum fracturing fluid for field use. Divide the fluid into five equal portions and add 0.02%, 0.03%, 0.04%, 0.05%, and 0.06% of a gel breaker (ammonium persulfate) at mass percentages, i.e., 0.1 g, 0.15 g, 0.2 g, 0.25 g, and 0.3 g of ammonium persulfate, respectively. The concentrations are grouped as a, b, c, d, and e, from low to high.

[0034] (3) Calculate the mass of calcium carbonate powder required according to the following formula:

[0035]

[0036] Where m is the mass of the added calcium carbonate powder, g; V is the volume of the prepared fracturing fluid, cm 3 ; d is the maximum fracture width of the reservoir, cm; h is the fracturing fluid invasion depth, cm; is the volume fraction of carbonate minerals in the reservoir, %; ρ is the density of calcium carbonate minerals, g / cm 3 .

[0037] It was calculated that 4.74 g of calcium carbonate powder needed to be added to each portion of fracturing fluid.

[0038] (4) The gel was broken in an 80°C thermostat. The viscosity of the broken solution was measured after 2, 3, 4, 6, and 8 hours. The gel was considered completely broken if the viscosity was less than 5 mPa·s at room temperature. Group a was completely broken after 8 hours, groups b and c were completely broken after 6 hours, and groups d and e were completely broken after 4 hours.

[0039] (5) After the gel-breaking liquid was completely broken, the degradation products of the gel-breaking liquid treated with 10k ultrafiltration membrane were semi-quantitatively analyzed by mass spectrometry. The high molecular weights of the degradation products of the five groups of gel-breaking liquid were measured, and the relationship curves between different concentrations of gel-breaking agents and relative molecular weights were established, such as Figure 1 shown.

[0040] (6) Filter the broken gel solution, use anthrone colorimetric method to determine the sugar content of the filtrate, use visible light spectrophotometer to determine the transmittance of the reaction product solution at a wavelength of 625 nm, and calculate the sugar content of the filtrate based on the transmittance-sugar concentration standard curve. Establish the relationship curve between different concentrations of breaker and the sugar content of the filtrate, such as Figure 2 shown.

[0041] (7) All the residues of the filtered gel-breaking liquid were transferred to a dried centrifuge tube, centrifuged at a speed of 3000 r / min for 30 min, stirred and washed with a glass rod, and centrifuged for another 20 min. The supernatant was poured out, and the centrifuge tube was placed in a constant temperature electric drying oven for baking. It was dried to a constant weight at a temperature of 105 ° C. After the experiment, the particle size distribution characteristics of the residue were measured using a Malvern laser particle size analyzer to obtain the particle volume or mass fraction within different particle size ranges, thereby clarifying the content of the residue and establishing a relationship curve between different concentrations of gel-breaking agent and the amount of gel-breaking liquid residue. Figure 3 shown.

[0042] (8) The particle size of the residue was measured using a Malvern laser particle size analyzer, and the characteristics of the fracturing fluid residue after gel breaking were analyzed using a scanning electron microscope. The relationship curve between the particle size distribution of the residue after gel breaking with different concentrations of gel breaker was established, such as Figure 4 As shown. The particle size range requirements for the breaker liquid residue are: the median particle size of the breaker liquid residue is greater than or equal to 2 / 3 of the maximum throat diameter of the reservoir, and the particle size D90 of the breaker liquid residue is less than 1 / 3 of the maximum mesh proppant diversion gap diameter. According to Horsfield's closest packing theory, it can be calculated that the pore diameter formed after 20 / 40 mesh ceramsite is filled in the crack is 0.122-0.298mm. When the breaker concentration is 0.04%, the median particle size of the breaker liquid residue is 4μm, which is greater than 2 / 3 of the maximum throat diameter of the reservoir (2 / 3×5=3.33μm), and the particle size D90 of the breaker liquid residue is 20μm, which is less than 1 / 3 of the maximum mesh proppant diversion gap diameter (1 / 3×122=41.67μm). After breaker breaking with other concentrations, the particle size of the breaker liquid residue is greater than 1 / 3 of the maximum mesh proppant gap diameter, which does not meet the breaker liquid particle size range requirements.

[0043] (9) Taking into account the requirements of low residue amount, low residue relative molecular weight, low filtrate sugar content and the particle size range of the breaker residue, the optimal breaker concentration range is 0.04%.

[0044] In actual application, a shale oil well in Block X began using a 0.02% breaker concentration, achieving a daily shale oil production of 14.2 tons. Using this method, the optimal breaker concentration was calculated to be 0.04%, resulting in a daily shale oil production of 14.7 tons, a year-on-year increase of 3.5%. The residual amount, relative molecular weight, and filtrate sugar content in the flowback fluid were also significantly reduced.

[0045] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for optimizing the concentration of a reservoir fracturing fluid breaker, characterized in that: The following steps are involved: (1) Select representative reservoir cores and determine the volume fraction of carbonate minerals Clarify the maximum crack width d and the depth of fracturing fluid action h; (2) preparing a fracturing fluid of volume V, adding m grams of calcium carbonate powder and different concentrations of breakers, and heating to the reservoir temperature; (3) Perform complete gel breaking at reservoir temperature and filter the broken gel solution; (4) According to the relationship between the concentration of the breaker and the relative molecular mass of the degradation products of the breaker liquid, the amount of residue, the particle size of the residue, and the sugar content of the filtrate, the concentration range of the breaker is selected.

2. The method for optimizing the concentration of a reservoir fracturing fluid breaker according to claim 1, wherein: The mass m of added calcium carbonate powder is calculated according to formula 1: Where m is the mass of calcium carbonate powder, g; V is the volume of fracturing fluid, cm 3 ; d is the maximum fracture width of the reservoir, cm; h is the fracturing fluid invasion depth, cm; is the volume fraction of carbonate minerals, %; ρ is the density of calcium carbonate minerals, g / cm 3 .

3. The method for optimizing the concentration of a reservoir fracturing fluid breaker according to claim 1, wherein: The gel breaker is ammonium persulfate.

4. The method for optimizing the concentration of a reservoir fracturing fluid breaker according to claim 1, wherein: The fracturing fluid is guar gum fracturing fluid.

5. The method for optimizing the concentration of a reservoir fracturing fluid breaker according to claim 1, wherein: The reservoir is an organic-rich shale reservoir.

6. A method for optimizing the concentration of a breaker in a reservoir fracturing fluid according to any one of claims 1 to 5, characterized in that: In step (3), the gel is broken in a thermostat at 80° C., and the gel is considered to be completely broken if the viscosity of the gel-breaking liquid is less than 5 mPa·s at room temperature.

7. The preparation method of a reservoir fracturing fluid breaker concentration optimization method according to any one of claims 1 to 5, characterized in that: The breaker concentration range is selected based on the principles of low residue amount, low residue relative molecular weight, low filtrate sugar content, and the median particle size of the breaker residue being greater than or equal to 2 / 3 of the maximum reservoir throat diameter, and the particle size D90 of the breaker residue being less than 1 / 3 of the maximum mesh proppant diversion gap diameter.

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