Evaluation system and method for composite acid fracturing process parameters of dense dolomite reservoir

Abstract

The invention provides a dense dolomite reservoir composite acid fracturing process parameter evaluation system and method.The system comprises a crack simulation device, an acid injection device and a circulating water injection device, the acid injection device comprises an acid storage container, an acid injection pipeline and an acid discharge pipeline, and the two ends of the acid injection pipeline are communicated with the acid storage container and an acid injection liquid inlet hole of the crack simulation device; the acid discharge pipeline is communicated with an acid injection and liquid discharge hole of the crack simulation device; the circulating water injection device comprises a water storage container, a water inlet pipeline and a circulating water drainage pipeline, two ends of the water inlet pipeline are communicated with the water storage container and a water injection and liquid inlet hole of the crack simulation device, and two ends of the circulating water drainage pipeline are communicated with a water injection and liquid drainage hole of the crack simulation device and the water storage container. The composite acid fracturing process parameters such as the prepad fluid / acid fluid viscosity ratio, the pump injection displacement and the alternating series are evaluated by taking the fractal dimension, the used acid volume, the flow conductivity and the acid fluid filtration loss as targets, and the evaluation is more specific and systematic.

Description

Technical Field

[0001] The present application relates to the technical field of hydraulic fracturing of continental shale reservoirs, and in particular to an evaluation system and method for composite acid fracturing process parameters of tight dolomite reservoirs. Background Art

[0002] Carbonate reservoirs generally have developed natural fractures and caves, and are severely heterogeneous. Fracture-cave systems are the main places for oil and gas storage and seepage. Due to the low pore throat matching, poor connectivity, and low matrix permeability of the reservoir, the oil and gas seepage channels are mainly fractures. Theory and practice have proved that acid fracturing is the main process measure to increase the production of carbonate oil and gas reservoirs. Through acid fracturing transformation, the acid dissolves in an uneven manner, and the acid-etched fractures with a certain length and conductivity can still be maintained when the fractures are closed, so as to communicate and connect the oil and gas seepage channels and oil storage space, and achieve the purpose of increasing production, thereby ensuring the normal production of oil wells and high and stable production. The tidal flat dolomite reservoir in Pingqiao area has a gas-bearing area of ​​350km2 and a resource volume of 321×108m3, which has great exploration and development potential. However, its reservoir is deeply buried, has severe reservoir heterogeneity, and has strong density. It is difficult to achieve ideal results by relying on a single acid fracturing process or acid system, so composite acid fracturing is considered for reservoir transformation. Combining non-reactive high-viscosity liquid with various acids with different characteristics, a multi-stage injection composite acid fracturing technology of different types and scales is formed. The process method is: pre-fluid + acid (multi-stage alternating injection) + displacement fluid, which is mainly used in low-permeability and ultra-low-permeability carbonate reservoirs.

[0003] The Chinese invention patent with the authorization announcement number CN110397430B discloses a method for predicting the distribution of conductivity of carbonate rock pre-fluid acid fracturing fractures. The patent simulates the fracture extension and expansion process during the fracturing period based on the two-dimensional PKN fracture extension and expansion model, predicts the distribution of conductivity of acid-etched fractures along the fracture length, and optimizes the carbonate rock pre-fluid acid fracturing design scheme. Its shortcoming is that the distribution of conductivity is predicted only by numerical simulation, without experimental verification, and there are errors in the authenticity of the model.

[0004] The Chinese invention patent with the authorization announcement number CN112502685B discloses a method for optimizing the number of alternating acid fracturing stages in carbonate reservoirs considering thermal effects. The patent optimizes the number of alternating stages in the acid fracturing process based on acid etching experiments and conductivity experiments. Its shortcomings are that the acid fracturing effect cannot be fully verified by only acid etching experiments and conductivity experiments, and the acid fracturing parameters cannot be fully verified by only optimizing the number of alternating stages, and the acid fracturing design guidance is of little significance.

[0005] The Chinese invention patent with the authorization announcement number CN110630240B discloses a method for optimizing the displacement of multi-stage alternating acid fracturing in carbonate reservoirs. The patent optimizes the displacement parameters of multi-stage alternating acid fracturing based on the surface area ratio, thereby improving the acid fracturing effect. Its shortcomings are that the construction displacement is optimized only by the surface area ratio, the maximum construction displacement that the reservoir can withstand, and the pressure bearing capacity of the ground equipment, without analyzing the impact of the construction displacement on the fluid in the actual formation, and without considering the impact of other construction parameters on the acid fracturing effect.

[0006] Conventional acid fracturing technology faces limitations such as fast acid-rock reaction speed, limited acid penetration depth, and rapid decline in acid-etched fracture conductivity. It also faces problems such as rapid decline after fracturing and difficulty in stabilizing production, which prevents the full utilization of reserves and greatly affects the economic development of similar reservoirs. The above-mentioned prior art construction parameter optimization evaluation does not analyze the fluid loss in the actual formation, and does not fully consider the limitation of the acid fracturing construction parameters that only optimize a single parameter. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide an evaluation system and method for the composite acid fracturing process parameters of tight dolomite reservoirs in response to the above-mentioned problems. By adopting reverse thinking, the composite acid fracturing process parameters such as the pre-fracturing fluid / acid viscosity ratio, pumping displacement and alternating stages are evaluated with fractal dimension, acid volume, conductivity and acid loss as targets.

[0008] The embodiment of the present application is implemented as follows:

[0009] The embodiment of the present application provides an evaluation system for the composite acid fracturing process parameters of a dense dolomite reservoir, including a simulated fracture device, an acid injection device and a circulating water injection device, wherein the simulated fracture device includes a fracture rock plate, a front frame and a rear cover plate, wherein the fracture rock plate is clamped and fixed by the front frame and the rear cover plate, a front slit is formed between the front end face of the fracture rock plate and the front frame, and a rear slit is formed between the rear end face and the rear cover plate, an acid injection inlet hole and a water injection inlet hole are respectively provided at positions corresponding to the front slit on both sides of the front frame, and a water injection inlet hole is respectively provided at positions corresponding to the rear slit on both sides of the rear cover plate. An acid injection drainage hole and a water injection drainage hole are provided, and the characteristic is that the acid injection device comprises an acid storage container, an acid injection pipeline and an acid discharge pipeline, the two ends of the acid injection pipeline are connected to the acid storage container and the acid injection inlet hole of the simulated crack device, and the acid discharge pipeline is connected to the acid injection drainage hole of the simulated crack device; the circulating water injection device comprises a water storage container, a water inlet pipeline and a circulating drainage pipeline, the two ends of the water inlet pipeline are connected to the water storage container and the water injection inlet hole of the simulated crack device, and the two ends of the circulating drainage pipeline are connected to the water injection drainage hole of the simulated crack device and the water storage container.

[0010] In some optional embodiments, the water storage container is a constant temperature water bath, and the acid storage container is immersed in the constant temperature water bath.

[0011] In some optional embodiments, the acid injection pipeline is provided with an acid injection constant flow pump, an acid injection back pressure valve and an acid injection valve in sequence, and the acid discharge pipeline is provided with an acid discharge valve and an acid discharge back pressure valve in sequence.

[0012] In some optional implementation schemes, a water injection constant flow pump and a water injection back pressure valve are sequentially provided on the water inlet pipeline, and a drainage valve and a drainage back pressure valve are sequentially provided on the circulating drainage pipeline.

[0013] In some optional embodiments, the acid injection valve, acid discharge valve and water drain valve are all multi-way valves, and each channel of the multi-way valve is connected to the corresponding acid injection inlet hole, acid injection discharge hole and water injection discharge hole one by one.

[0014] In some optional embodiments, the acid injection back pressure valve, water injection back pressure valve, acid discharge back pressure valve and water discharge back pressure valve are respectively connected to the back pressure multi-way valve, and the back pressure multi-way valve is connected to the back pressure manual pump; the acid discharge back pressure valve and the water discharge back pressure valve are respectively connected to the acid discharge back pressure gauge and the water discharge back pressure gauge.

[0015] An evaluation method for an evaluation system of composite acid fracturing process parameters of a tight dolomite reservoir, characterized by comprising the following steps:

[0016] Step a, acid fingering experiment in the fracture: according to the experimental design requirements, a fracturing fluid of corresponding viscosity and an acid of corresponding viscosity are prepared, stored in a corresponding acid storage container, and water injection pressure test is performed in the front slit and the rear slit respectively to check the sealing condition. After the inspection, a pre-fluid is injected into the front slit until the fracture is full, and then the acid is injected. The fingering evolution morphology and related experimental parameters in the simulated fracture are recorded, the viscosity ratio of the fracturing fluid and the acid, the pumping displacement and the alternating series are adjusted, different experimental schemes are designed, and the above experiments are repeated to evaluate and optimize the process parameters;

[0017] Step b, acid-etched fracture conductivity experiment: rock samples are collected to prepare rock plates for use in the standard diversion chamber of the American Petroleum Institute, the roughness of the rock plates before acid etching is obtained using a laser scanning device, a fracturing fluid of corresponding viscosity and an acid of corresponding viscosity are prepared according to the parameters optimized after the acid fingering experiment in the fracture, the acid is injected into the rock plates to obtain acid-etched rock plates, the roughness of the acid-etched fracture surface is obtained using a laser scanning device, and the conductivity of the rock plates after etching is tested using a conductivity testing device;

[0018] Step c, natural fracture acid loss experiment: rock samples are collected to prepare standard cores, artificial fractures are constructed, and confining pressure is applied to displace acid into the cores to simulate on-site construction, and relevant pressure and flow data are collected to conduct displacement acidification experiments;

[0019] Step d, rock plate strength damage experiment based on continuous strength test: collect rock samples to prepare rock plates for American Petroleum Institute standard diversion chamber, perform scratching tests on the rock plates at multiple different positions on the X-axis, measure the original surface strength of the rock plates, and take the average value of multiple locations as the standard. After the rock plates are acid-etched, perform multiple scratching tests on the flat position of the rock plates, and take the average value as the surface strength of the rock plates after acid etching.

[0020] In some optional embodiments, the artificial fracture structure in step c includes the following contents: the core is vertically split along the diameter direction of the core using artificial fracture-splitting technology, and an artificial groove is etched in the middle of the split half of the core along the length direction according to a pre-designed fracture network morphology with a carving knife to simulate the fracture network in the carbonate reservoir. The depth and width of the artificially carved groove are controlled within 1 mm. The two halves of the core are merged and fixed by wrapping tape on the side of the core.

[0021] In some optional embodiments, the rock plate described in step d is coated with super copper gasket glue on four sides before the scraping test, air-dried at room temperature for 24 hours, and vacuum-saturated with simulated formation water for 6 hours.

[0022] In some optional embodiments, the relevant experimental parameters in step a include fractal dimension, area ratio of the fingering region, and acid volume.

[0023] The beneficial effects of the present application are as follows: the present application provides a system and method for evaluating composite acid fracturing process parameters of tight dolomite reservoirs, which evaluates composite acid fracturing process parameters with fractal dimension, acid volume, conductivity, and acid loss as targets, and the evaluation method is more specific and systematic; on the basis of the traditional acid fracturing process parameter evaluation method which only evaluates composite acid fracturing process parameters such as pre-fracturing fluid / acid viscosity ratio, pumping displacement, and alternating stage through in-fracture acid fingering experiments and acid-etched fracture conductivity experiments, a natural fracture acid loss experiment is added to comprehensively consider the influence of on-site composite acid fracturing process parameters on the flow response and loss of acid in a single fracture; a rock plate strength damage experiment based on continuous strength testing is added to comprehensively consider the influence of on-site composite acid fracturing process parameters on the surface strength of the rock plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0025] Figure 1 A schematic diagram of a physical device for the acid finger penetration experiment in a crack according to an embodiment of the present application;

[0026] Figure 2 A schematic diagram of the acid finger penetration experiment process in the seam of the embodiment of the present application;

[0027] Figure 3 A schematic diagram of the results of the acid fingering experiment analysis in the seam of the embodiment of the present application;

[0028] Figure 4 This is a schematic diagram of a rock plate before the acid etching reaction of an embodiment of the present application;

[0029] Figure 5 This is a schematic diagram of a rock plate after acid etching reaction according to an embodiment of the present application;

[0030] Figure 6 This is a schematic diagram of the results of an acid-etched crack conductivity test in an embodiment of the present application;

[0031] Figure 7 This is a schematic diagram of an artificial fracture in a core of an embodiment of the present application;

[0032] Figure 8 This is a schematic diagram of the core after the displacement acidification experiment in the embodiment of the present application;

[0033] Fig. 9 This is a box diagram showing the effect of the number of alternating stages on the compressive strength of the rock slab according to an embodiment of the present application. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0035] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for which protection is sought, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0036] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0037] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, or the positions or positional relationships in which the product of the application is usually placed when in use. They are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific position, be constructed and operated in a specific position, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0038] In addition, the terms "horizontal", "vertical", "overhanging" and the like do not mean that the components are required to be absolutely horizontal or overhanging, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0039] In the description of this application, it should also be noted that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0040] In the present application, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0041] The features and performance of the present application are further described in detail below in conjunction with the embodiments.

[0042] The present invention provides an evaluation system and method for composite acid fracturing process parameters of tight dolomite reservoirs. The system adopts reverse thinking and takes fractal dimension, acid volume, conductivity and acid fluid loss as targets to evaluate composite acid fracturing process parameters such as pre-fracturing fluid / acid fluid viscosity ratio, pumping displacement and alternating stages.

[0043] The embodiment of the present application is implemented as follows:

[0044] like Figure 1 As shown, the embodiment of the present application provides an evaluation system for the composite acid fracturing process parameters of a dense dolomite reservoir, including a simulated fracture device 1, an acid injection device and a circulating water injection device. The simulated fracture device includes a fracture rock plate, a front frame and a rear cover plate. The fracture rock plate is clamped and fixed by the front frame and the rear cover plate. A front slit is formed between the front end face of the fracture rock plate and the front frame, and a rear slit is formed between the rear end face and the rear cover plate. An acid injection inlet hole and a water injection inlet hole are respectively provided at positions corresponding to the front slit on both sides of the front frame, and a rear slit is formed between the rear end face and the rear cover plate. Acid injection drainage holes and water injection drainage holes are respectively provided at the positions of the cracks. The acid injection device comprises an acid storage container 2, an acid injection pipeline 3 and an acid discharge pipeline 4. Both ends of the acid injection pipeline are connected to the acid storage container and the acid injection inlet hole of the simulated crack device, and the acid discharge pipeline is connected to the acid injection drainage hole of the simulated crack device. The circulating water injection device comprises a water storage container 5, a water inlet pipeline 6 and a circulating drainage pipeline 7. Both ends of the water inlet pipeline are connected to the water storage container and the water injection inlet hole of the simulated crack device, and both ends of the circulating drainage pipeline are connected to the water injection drainage hole of the simulated crack device and the water storage container.

[0045] In some optional embodiments, the water storage container is a constant temperature water bath, and the acid storage container is immersed in the constant temperature water bath.

[0046] In some optional embodiments, the acid injection pipeline is provided with an acid injection constant flow pump 8, an acid injection back pressure valve 9 and an acid injection valve 10 in sequence, and the acid discharge pipeline is provided with an acid discharge valve 11 and an acid discharge back pressure valve 12 in sequence. The water inlet pipeline is provided with a water injection constant flow pump 13 and a water injection back pressure valve 14 in sequence, and the circulating drainage pipeline is provided with a drainage valve 15 and a drainage back pressure valve 16 in sequence.

[0047] In some optional embodiments, the acid injection valve, the acid discharge valve and the water drain valve are all multi-way valves, and each channel of the multi-way valve is connected to the corresponding acid injection inlet hole, acid injection discharge hole and water injection discharge hole one by one.

[0048] The acid injection back pressure valve, water injection back pressure valve, acid discharge back pressure valve and drainage back pressure valve are respectively connected to the back pressure multi-way valve 17, and the back pressure multi-way valve is connected to the back pressure manual pump 18. The acid discharge back pressure valve and drainage back pressure valve are respectively connected to the acid discharge back pressure gauge 19 and the drainage back pressure gauge 20. The back pressure manual pump is turned on and adjusts the channels between the back pressure multi-way valve and each back pressure valve to perform pressure regulation control.

[0049] The evaluation system is used to evaluate the composite acid fracturing process parameters of tight dolomite reservoirs, including the following steps:

[0050] Step a, acid fingering experiment in the fracture: according to the experimental design requirements, prepare the corresponding viscosity of the fracturing fluid and the corresponding viscosity of the acid, store them in the corresponding acid storage container, respectively perform water injection pressure test in the front slit and the rear slit, check the sealing condition, and after the inspection, inject the front slit with the pre-fluid until the fracture is full, and then inject the acid, record the fingering evolution morphology and related experimental parameters in the simulated fracture, adjust the viscosity ratio of the fracturing fluid and the acid, the pumping displacement and the alternating series, design different experimental schemes, repeat the above experiments, and evaluate and optimize the process parameters.

[0051] The above-mentioned relevant experimental parameters include: fractal dimension, area ratio of finger penetration region, and acid volume used.

[0052] Step b, acid-etched crack conductivity test:

[0053] Rock samples were collected to prepare rock plates for the American Petroleum Institute standard diversion chamber. The roughness of the rock plates before acid etching was obtained using a laser scanning device. Fracturing fluid with corresponding viscosity and acid with corresponding viscosity were prepared according to the parameters optimized after the acid fingering experiment in the fracture. The acid was injected into the rock plates to obtain acid-etched rock plates. The roughness of the acid-etched crack surface was obtained using a laser scanning device. The conductivity of the rock plates after etching was obtained using a conductivity testing device.

[0054] The laser scanning device adopts a three-dimensional four-eye scanner, and the image obtained by scanning is programmed and processed to obtain the crack surface roughness coefficient JRC (Joint roughness coefficient).

[0055] Step c, natural fracture acid filtration experiment: rock samples are collected to prepare standard cores, and artificial fractures are constructed. The depth and width of the artificially carved grooves should be controlled within 1 mm, and confining pressure is applied to displace acid into the core to simulate on-site construction, and relevant pressure and flow data are collected to conduct displacement acidification experiments.

[0056] Through the acid loss experiment of natural fractures, the core acidification pictures were obtained, the dissolution of acid in the main fractures and natural fractures was compared, and the influence of the on-site composite acid process parameters on the acid flow response and loss in a single fracture was evaluated.

[0057] Step d, rock plate strength damage experiment based on continuous strength test: collect rock samples to prepare rock plates for American Petroleum Institute standard diversion chamber, perform scratching tests on the rock plates at multiple different positions on the X-axis, measure the original surface strength of the rock plates, and take the average value of multiple locations as the standard. After the rock plates are acid-etched, perform multiple scratching tests on the flat position of the rock plates, and take the average value as the surface strength of the rock plates after acid etching.

[0058] The influence of composite acid fracturing process parameters on the surface strength of rock slabs was tested by rock slab strength damage experiment based on continuous strength test, and the influence of composite acid fracturing process parameters on conductivity was evaluated as an auxiliary measure. In order to prevent the acid from leaking between the rock slab and the API inner groove, Permatex super copper gasket glue was applied around the rock slabs in the rock slab strength damage experiment based on continuous strength test, and then air-dried at room temperature for 24 hours after application; before acid etching, all rock slabs were vacuum-saturated with simulated formation water (2wt% KCl) for six hours.

[0059] Example 1

[0060] A method for evaluating process parameters of composite acid fracturing of tight dolomite reservoirs includes the following steps:

[0061] S1. Acid finger penetration test in seams

[0062] 1) According to the requirements of the experimental design, prepare fracturing fluid of corresponding viscosity and acid solution of corresponding concentration, and then store them in corresponding liquid storage containers;

[0063] 2) Use corresponding pipelines to connect the corresponding instruments (meters) and accessories. Pay special attention to the fact that when assembling the simulated crack, the sealing gaskets at each location must be placed flat, and the sealing bolts should be slowly screwed in when assembling the back cover;

[0064] 3) Start the water injection constant flow pump, pump clean water into the rear slit for pressure test, and carefully check the pipeline and the sealing of the rear slit; if there is a leak, turn off the power of the constant flow pump, unload the back pressure, drain the water, and return to the previous step;

[0065] 4) Perform pressure test on the front slit according to the similar procedure in the previous step, and carefully check the sealing of the slit and the acid pipeline;

[0066] 5) After closing the acid injection valve, inject the pre-fluid into the front slit. When injecting the pre-fluid, open all the exhaust holes to remove the air in the slit as much as possible so that the pre-fluid can quickly and evenly fill the crack;

[0067] 6) After the pre-pad fluid fills the fracture, start the constant flow pump to inject acid and record the fingering evolution morphology and other relevant parameters in the simulated fracture;

[0068] 7) According to the corresponding viscosity of the fracturing fluid and the corresponding concentration of the acid in the on-site construction, adjust the pre-fluid / acid viscosity ratio, pumping displacement and alternating stage design experimental plan, repeat the above steps 1) to 6) to evaluate and optimize the process parameters.

[0069] Specifically, the on-site composite acid fracturing process parameters are: acid guar gum cross-linking liquid (50mPa·s) + 0.8% gelling agent gelling acid (15mPa·s), displacement 6m 3 / min. According to the above process, the acid fingering experiment in the crack was completed. The fingering start, fingering growth and fingering front edge reaching the boundary were as follows: Figure 2 The area proportion of the finger-entry region, fractal dimension and acid volume are shown in Table 1.

[0070] Table 1 Parameters of the acid fingering test results in the seam

[0071]

[0072] By adjusting the viscosity ratio of the pre-fluid / acid solution, the pumping displacement and the alternating series design experimental scheme, the on-site composite acid solution process parameters were evaluated and optimized. The experimental scheme is shown in Table 2.

[0073] Table 2 Slit acid fingering experimental plan

[0074]

[0075]

[0076] The fractal dimension, the area ratio of the fingering area and the volume of acid used were compared by the acid fingering experiment in the crack. The analysis results are as follows: Figure 3 As shown in the figure, the fractal dimension increases with the increase of viscosity ratio and displacement, and decreases with the increase of the order. In experiment 5, the fractal dimension is large when the front edge of the finger reaches the edge, the area of ​​the fingering area accounts for a small proportion, the amount of acid used is small, and the fingering effect is obvious.

[0077] S2. Acid-etched crack conductivity test

[0078] 1) Collect rock samples to prepare API rock plates, and use a laser scanning device to obtain the roughness of the rock plates before acid etching;

[0079] 2) According to the requirements of the experimental design, the composite acid fracturing process parameter comparison experimental scheme obtained by the evaluation and optimization of the acid fingering experiment in the fracture is used to prepare the corresponding viscosity of the fracturing fluid and the corresponding viscosity of the acid, and then store them in the corresponding liquid storage container;

[0080] 3) Use an acid-resistant pump to pump the prepared acid solution from the storage tank into the API diversion chamber. The acid solution reacts with the rock surface when flowing on the fracture wall, approximately simulating the chemical reaction process between the acid solution and the formation rock in the field acid fracturing to obtain the acid-etched rock plate;

[0081] 4) Using a laser scanning device to obtain the roughness of the acid-etched crack surface;

[0082] 5) Using a flow conductivity testing device to test the flow conductivity of the etched rock plate;

[0083] Specifically, the on-site composite acid fracturing process parameters are: acid guar gum cross-linking liquid (50mPa·s) + 0.8% gelling agent gelling acid (15mPa·s), displacement 6m 3 / min. According to the above process, the acid-etched rock plate is obtained, and the roughness of the acid-etched crack surface is obtained by using a laser scanning device. The conductivity of the etched rock plate is tested by using a conductivity testing device. The roughness before and after acid etching is as follows: Figure 4 , Figure 5 shown.

[0084] By adjusting the viscosity ratio of the pre-fluid / acid solution, the pumping displacement and the alternating series design experimental scheme, the on-site composite acid solution process parameters were evaluated and optimized. The experimental scheme is shown in Table 2 above. The flow capacity analysis is as follows: Figure 6 As shown in the figure. The curve generally shows that the higher the closure pressure, the smaller the conductivity. The conductivity values ​​of the gentle section under high closure pressure are mostly below 10D·cm, and a few are above 15D·cm. Among them, the conductivity of the rock samples in Experiments 5 and 7 is better, and the surface etching and groove support are strong. At 48.3MPa, the conductivity of Experiment 7 is the highest, which is 31.5D·cm. The conductivity of other rock samples is below 10D·cm.

[0085] Therefore, the fractal dimension, acid volume and conductivity were taken as the targets by the acid fingering experiment and the acid-etched fracture conductivity experiment to optimize and evaluate the composite acid fracturing process parameters such as the pre-fluid / acid viscosity ratio, pumping displacement and alternating stages. The final optimal result was Experiment 5. Experiment 5: Acid guar fracturing fluid (50 mPa·s) + 0.8% gelling agent gelled acid (15 mPa·s), displacement 9 m 3 / min.

[0086] S3. Natural fracture acid loss test

[0087] 1) Collect rock samples and prepare standard cores;

[0088] 2) The core is split vertically along the diameter direction of the core using artificial splitting technology. The cross section of the split core is flat and smooth, and the core can be well closed under a certain confining pressure when it is closed again;

[0089] 3) Use a carving knife to etch artificial grooves in the middle of the cut core along the length direction according to the pre-designed fracture network shape to simulate the fracture network in the carbonate reservoir. The depth and width of the artificially carved grooves should be controlled within 1 mm;

[0090] 4) Carve a fracture network on one half of the core section and seal the other half with tape, and finally wrap the side of the core to fix it and apply confining pressure;

[0091] 5) According to the on-site construction parameters, the acid is displaced into the core at a constant inlet displacement to simulate on-site construction, and relevant pressure and flow data are collected to conduct a displacement acidification experiment;

[0092] Specifically, after the optimization and evaluation of the S1 and S2 steps, the composite acid fracturing process parameters are: acidic guar gum fracturing fluid (50 mPa·s) + 0.8% gelling agent gelling acid (15 mPa·s), displacement 9 m 3 / min. According to the acid filtration study of S3 natural fracture network, the on-site core was processed by seam engraving. The seams carved were uniform and of moderate depth. Figure 7 The reaction after acid injection in the core is as shown in Figure 8 As shown, it can be found that the slower reaction rate of gelled acid can effectively slow down the rate of acid dissolution of natural cracks, inhibit the formation of acid-etched pores and cracks, and effectively increase the extension distance of the main acid-fracturing cracks.

[0093] S4. Rock plate strength damage experiment based on continuous strength test

[0094] 1) Collect rock samples to prepare API rock plates;

[0095] 2) Processing rock slabs: Since the diamond bit will scrape the rock surface about 0.2mm each time it is tested, the initial rock slab scraping needs to reserve the scraping thickness;

[0096] 3) Scrape before acid etching: Scrape the rock plate at 1 / 4, 2 / 4, and 3 / 4 of the X axis before acid etching, and take the average value of the three positions as the original surface strength of the rock plate before acid etching;

[0097] 4) Scraping after acid etching. Due to the presence of acid-etched grooves on the surface of the rock plate, the rock contacted by the diamond bit at the scraping position may be quite different from the size of the bit. Therefore, multiple scraping tests (usually 2 to 3 times) are selected at a relatively flat target position, and the average test result is taken as the surface strength after acid etching;

[0098] Specifically, after the optimization and evaluation of the S1 and S2 steps, the composite acid fracturing process parameters are: acidic guar gum fracturing fluid (50 mPa·s) + 0.8% gelling agent gelling acid (15 mPa·s), displacement 9 m 3 / min. According to the rock plate strength damage experiment based on continuous strength test of S4, taking the influence of alternating series on the strength of rock plate as an example, the box diagram of its influence on the compressive strength of rock plate is as follows Fig. 9 The analysis shows that the injection level has little effect on the strength of the rock plate. The average strength after acid etching decreases by 12% to 17%, and the softening coefficient is similar, indicating that the increase in the injection level has little effect on the strength of the rock plate.

Claims

1. An evaluation system for composite acid fracturing process parameters of dense dolomite reservoirs, comprising a simulated fracture device, an acid injection device and a circulating water injection device, wherein the simulated fracture device comprises a fracture rock plate, a front frame and a rear cover plate, wherein the fracture rock plate is clamped and fixed by the front frame and the rear cover plate, a front slit is formed between the front end face of the fracture rock plate and the front frame, and a rear slit is formed between the rear end face and the rear cover plate, an acid injection inlet hole and a water injection inlet hole are respectively arranged at positions corresponding to the front slit on both sides of the front frame, and an acid injection drainage hole and a water injection drainage hole are respectively arranged at positions corresponding to the rear slit on both sides of the rear cover plate, wherein: The acid injection device includes an acid storage container, an acid injection pipeline and an acid discharge pipeline, the two ends of the acid injection pipeline are connected to the acid storage container and the acid injection inlet hole of the simulated crack device, and the acid discharge pipeline is connected to the acid injection discharge hole of the simulated crack device; the circulating water injection device includes a water storage container, a water inlet pipeline and a circulating drainage pipeline, the two ends of the water inlet pipeline are connected to the water storage container and the water injection inlet hole of the simulated crack device, and the two ends of the circulating drainage pipeline are connected to the water injection discharge hole of the simulated crack device and the water storage container.

2. The evaluation system for composite acid fracturing process parameters of tight dolomite reservoirs according to claim 1, characterized in that: The water storage container is a constant temperature water bath, and the acid storage container is immersed in the constant temperature water bath.

3. The evaluation system for composite acid fracturing process parameters of tight dolomite reservoirs according to claim 2, characterized in that: The acid injection pipeline is provided with an acid injection constant flow pump, an acid injection back pressure valve and an acid injection valve in sequence, and the acid discharge pipeline is provided with an acid discharge valve and an acid discharge back pressure valve in sequence.

4. The evaluation system for composite acid fracturing process parameters of tight dolomite reservoirs according to claim 3, characterized in that: The water inlet pipeline is provided with a water injection constant flow pump and a water injection back pressure valve in sequence, and the circulating drainage pipeline is provided with a drainage valve and a drainage back pressure valve in sequence.

5. The evaluation system for composite acid fracturing process parameters of tight dolomite reservoirs according to claim 4, characterized in that: The acid injection valve, acid discharge valve and water drain valve are all multi-way valves, and each channel of the multi-way valve is connected to the corresponding acid injection inlet hole, acid injection discharge hole and water injection discharge hole in a one-to-one correspondence.

6. The evaluation system for composite acid fracturing process parameters of tight dolomite reservoirs according to claim 4 or 5, characterized in that: The acid injection back pressure valve, water injection back pressure valve, acid discharge back pressure valve and water discharge back pressure valve are respectively connected to the back pressure multi-way valve, and the back pressure multi-way valve is connected to the back pressure manual pump; the acid discharge back pressure valve and the water discharge back pressure valve are respectively connected to the acid discharge back pressure gauge and the water discharge back pressure gauge.

7. An evaluation method for an evaluation system for composite acid fracturing process parameters of a tight dolomite reservoir according to claim 6, characterized in that: The steps include: Step a, acid fingering experiment in the fracture: according to the experimental design requirements, a fracturing fluid of corresponding viscosity and an acid of corresponding viscosity are prepared, stored in a corresponding acid storage container, and water injection pressure test is performed in the front slit and the rear slit respectively to check the sealing condition. After the inspection, a pre-fluid is injected into the front slit until the fracture is full, and then the acid is injected. The fingering evolution morphology and related experimental parameters in the simulated fracture are recorded, the viscosity ratio of the fracturing fluid and the acid, the pumping displacement and the alternating series are adjusted, different experimental schemes are designed, and the above experiments are repeated to evaluate and optimize the process parameters; Step b, acid-etched fracture conductivity experiment: rock samples are collected to prepare rock plates for use in the standard diversion chamber of the American Petroleum Institute, the roughness of the rock plates before acid etching is obtained using a laser scanning device, a fracturing fluid of corresponding viscosity and an acid of corresponding viscosity are prepared according to the parameters optimized after the acid fingering experiment in the fracture, the acid is injected into the rock plates to obtain acid-etched rock plates, the roughness of the acid-etched fracture surface is obtained using a laser scanning device, and the conductivity of the rock plates after etching is tested using a conductivity testing device; Step c, natural fracture acid loss experiment: rock samples are collected to prepare standard cores, artificial fractures are constructed, and confining pressure is applied to displace acid into the cores to simulate on-site construction, and relevant pressure and flow data are collected to conduct displacement acidification experiments; Step d, rock plate strength damage experiment based on continuous strength test: collect rock samples to prepare rock plates for American Petroleum Institute standard diversion chamber, perform scratching tests on the rock plates at multiple different positions on the X-axis, measure the original surface strength of the rock plates, and take the average value of multiple locations as the standard. After the rock plates are acid-etched, perform multiple scratching tests on the flat position of the rock plates, and take the average value as the surface strength of the rock plates after acid etching.

8. The method for evaluating the process parameters of composite acid fracturing of tight dolomite reservoirs according to claim 8, characterized in that: The artificial fracture structure described in step c includes the following contents: the core is vertically split along the diameter direction of the core using the artificial fracture splitting technology, and an artificial groove is etched in the middle of the split half of the core along the length direction according to the pre-designed fracture network shape with a carving knife to simulate the fracture network in the carbonate reservoir. The depth and width of the artificially carved groove are controlled within 1 mm, and the two halves of the core are combined and fixed by wrapping the side of the core with tape.

9. The method for evaluating the process parameters of the composite acid fracturing of a tight dolomite reservoir according to claim 8, characterized in that: The rock plate described in step d is coated with super copper gasket glue on all sides before the scraping test, air-dried at room temperature for 24 hours, and vacuum-saturated with simulated formation water for 6 hours.

10. The method for evaluating process parameters of composite acid fracturing of tight dolomite reservoirs according to claim 8, characterized in that: The relevant experimental parameters described in step a include fractal dimension, area ratio of the finger-entry region, and acid volume.

Images