Method for optimizing concentration of gel breaker of reservoir fracturing fluid

Abstract

The invention belongs to the technical field of oil and gas field development, and particularly discloses a reservoir fracturing fluid gel breaker concentration optimization method which comprises the following steps: (1) selecting a representative core of a reservoir, measuring the volume fraction phi of carbonate minerals, and determining the maximum crack width d and the fracturing fluid action depth h; (2) preparing fracturing fluid with the volume of V, adding calcium carbonate powder with the mass of m g, adding gel breakers with different concentrations, and heating to the reservoir temperature; (3) carrying out thorough gel breaking at the reservoir temperature, and filtering a gel breaking solution; and (4) selecting the concentration range of the gel breaker according to the relationship between the concentration of the gel breaker and the relative molecular mass, the residue amount, the residue particle size and the sugar content of the filtrate of the degradation product of the gel breaking liquid. The relationship between the concentration of the gel breaking solution and the relative molecular mass distribution of the degradation product of the gel breaking solution, the sugar content of the filtrate and the residue content is established by considering the carbonate mineral content of the reservoir, the gel breaking performance of the gel breaker is analyzed more comprehensively, and the requirement for the performance of the gel breaking solution at present is met.

Description

Technical Field

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

[0002] Fracturing fluid is one of the key links in fracturing construction, known as the "blood" of fracturing. As the conventional oil and gas reservoirs move towards unconventional oil and gas reservoirs, higher requirements are placed on the performance of fracturing fluids. The continuous improvement of fracturing fluid performance is the prerequisite for improving fracturing technology. A large amount of polymer damage occurs during hydraulic fracturing. The thickener used in fracturing fluids is mainly polysaccharide polymers, which have the characteristics of thickening, transporting proppants, suspending, controlling filtration and interlayer isolation. After the operation is completed, it is difficult to degrade and often causes polymer damage. Therefore, fracturing operations require the addition of debonding agents to degrade polymers to reduce their molecular weight and reduce the viscosity of fracturing fluids, so that the fracturing fluid can be more completely returned, while leaving the proppants in the cracks to form efficient seepage channels to achieve the purpose of fracturing production increase. Especially for ultra-low permeability oil fields, it is very important to screen debonding agents suitable for the reservoir fracturing fluid system.

[0003] At present, the evaluation of the performance of the breaker is mainly based on the viscosity of the breaker and the residue content, and it does not take into account that the breaker will react with the carbonate minerals at the reservoir temperature. As oil and gas development continues to move towards the unconventional oil and gas field, evaluating the breaker performance only from the viscosity of the breaker and the residue content cannot comprehensively analyze and evaluate the adaptability of the breaker to the reservoir and the fracturing fluid system, and cannot meet the current requirements for the performance of the breaker.

[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 need to solve urgently. Summary of the invention

[0005] In view of this, the present invention establishes the 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 solution:

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

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

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

[0010] (3) performing complete gel breaking at reservoir temperature and filtering the gel breaking liquid;

[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 the 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 fracturing fluid, cm; φ is the volume fraction of carbonate minerals, %; ρ is the density of calcium carbonate minerals, g / cm3.

[0015] Preferably, the 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 step (3) in a thermostat at 80°C, and the gel is considered to be completely broken when 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 principle 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 the existing methods ignore the influence of reservoir minerals on the breaking effect of the breaker. This method combines the actual carbonate mineral content of the reservoir and the performance evaluation of the breaker to further adjust the breaker concentration, which can effectively improve the reservoir fracturing 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 effect. Currently, the evaluation of the breaker effect is mainly based on the viscosity of the breaker liquid 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 drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0024] Figure 1 The relationship curve between the relative molecular weight of the degradation products of the breaker at different concentrations and the breaker liquid;

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

[0026] Figure 3 It is the relationship curve between different concentrations of breaker and the amount of residue in the breaker liquid;

[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 technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Example 1

[0030] According to the method of the present invention, a specific implementation case is given as follows for the organic-rich shale reservoir in Block X:

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

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

[0033] (2) Prepare 2.5L of 0.4% guar gum fracturing fluid for field use, divide it into 5 equal parts, add 0.02%, 0.03%, 0.04%, 0.05%, 0.06% of gel breaker (ammonium persulfate) by mass percentage, that is, add 0.1g, 0.15g, 0.2g, 0.25g, 0.3g of ammonium persulfate respectively. The concentrations are set as a, b, c, d, 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 corresponding calcium carbonate powder added, 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 penetration depth of the fracturing fluid, cm; is the volume fraction of carbonate minerals in the reservoir, %; ρ is the density of calcium carbonate minerals, g / cm 3 .

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

[0038] (4) The gel was broken in a thermostatic box at 80°C, and the viscosity of the broken gel solution was measured after 2 h, 3 h, 4 h, 6 h, and 8 h. The broken gel solution was considered to be completely broken when 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 relative molecular weight of the degradation products of the gel-breaking liquid treated with the 10k ultrafiltration membrane was semi-quantitatively analyzed by mass spectrometry. The relative molecular weights of the high molecular weights of the degradation products of the five groups of gel-breaking liquids were measured, and the relationship curve between the different concentrations of the gel-breaking agent and the relative molecular weight was established, such as Figure 1 shown.

[0040] (6) Filter the degelling liquid, use anthrone colorimetric method to determine the sugar content of the filtrate, use a 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 a relationship curve between different concentrations of degelling agent and the sugar content of the filtrate, such as Figure 2 shown.

[0041] (7) After filtering, transfer all the residues of the degelling liquid into a dried centrifuge tube, centrifuge at a speed of 3000 r / min for 30 min, stir and wash the residue sample with a glass rod, centrifuge for another 20 min, pour out the supernatant, place the centrifuge tube in a constant temperature electric drying oven for baking, and dry it to a constant weight at a temperature of 105°C. After the experiment, use a Malvern laser particle size analyzer to measure the particle size distribution characteristics of the residue, obtain the particle volume or mass fraction within different particle size ranges, and then clarify the content of the residue, and establish a relationship curve between different concentrations of degelling agent and the amount of degelling liquid residue, such as 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 of the gel breaking fluid with different concentrations of gel breaking agent was established, such as Figure 4 As shown. The particle size range of the residual gelling liquid is required to be: the median particle size of the residual gelling liquid is greater than or equal to 2 / 3 of the maximum throat diameter of the reservoir, and the particle size D90 of the residual gelling liquid is less than 1 / 3 of the maximum mesh proppant guide gap diameter. According to Horsfield's closest packing theory, it can be calculated that the pore diameter formed by 20 / 40 mesh ceramsite after filling in the cracks is 0.122-0.298 mm. When the breaker concentration is 0.04%, the median particle size of the residual gelling liquid is 4 μm, which is greater than 2 / 3 of the maximum throat diameter of the reservoir (2 / 3×5=3.33 μm), and the particle size D90 of the residual gelling liquid is 20 μm, which is less than 1 / 3 of the maximum mesh proppant guide gap diameter (1 / 3×122=41.67 μm). The particle size of the residual gelling liquid after gelling with other concentrations of breaker is greater than 1 / 3 of the maximum mesh proppant gap diameter, which does not meet the particle size range requirements of the breaker.

[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 the actual application process, a shale oil well in Block X began to use a breaker with a concentration of 0.02%, and the daily production of shale oil was 14.2 tons. The optimal concentration of the breaker was calculated by this method to be 0.04%, and the daily production of shale oil was 14.7 tons, an increase of 3.5% year-on-year, and the residue amount, relative molecular mass and filtrate sugar content in the return fluid were significantly reduced.

[0045] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be 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 will not be limited to the embodiments shown herein, but rather 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 cores from the reservoir and determine the volume fraction of carbonate minerals Specify 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) performing complete gel breaking at reservoir temperature and filtering 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. A method for optimizing the concentration of a reservoir fracturing fluid breaker according to claim 1, characterized in that: 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 penetration depth of the fracturing fluid, cm; is the volume fraction of carbonate minerals, %; ρ is the density of calcium carbonate minerals, g / cm 3 .

3. A method for optimizing the concentration of a reservoir fracturing fluid breaker according to claim 1, characterized in that: The debonding agent is ammonium persulfate.

4. A method for optimizing the concentration of a reservoir fracturing fluid breaker according to claim 1, characterized in that: 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, characterized in that: The reservoir is an organic-rich shale reservoir.

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

7. The method for preparing 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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