A system and method for evaluating the process parameters of composite acid fracturing in a dense dolomite reservoir

Abstract

The application provides a compact dolomite reservoir composite acid fracturing process parameter evaluation system and method, which comprises a simulated fracture 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. The acid injection pipeline is connected to the acid storage container and the acid injection liquid inlet hole of the simulated fracture device at both ends. The acid discharge pipeline is connected to the acid injection liquid outlet hole of the simulated fracture device. The circulating water injection device comprises a water storage container, a water inlet pipeline and a circulating water discharge pipeline. The water inlet pipeline is connected to the water storage container and the water injection liquid inlet hole of the simulated fracture device at both ends. The circulating water discharge pipeline is connected to the water injection liquid outlet hole of the simulated fracture device and the water storage container at both ends. The application adopts reverse thinking to evaluate the composite acid fracturing process parameters such as the preflush / acid viscosity ratio, the pump injection displacement and the alternating series by taking the fractal dimension, the acid volume, the flow conductivity and the acid fluid loss as the targets, and is more specific and systematic.

Description

Technical Field

[0001] This application relates to the technical field of hydraulic fracturing of continental shale reservoirs, and more specifically, to an evaluation system and method for composite acid fracturing process parameters of tight dolomite reservoirs. Background Technology

[0002] Carbonate reservoirs typically exhibit well-developed natural fractures and caverns, exhibiting severe heterogeneity. The fracture-cavity system serves as the primary site for oil and gas accumulation and seepage. Due to the low pore-throat coordination, poor connectivity, and low matrix permeability, fractures are the main channels for oil and gas flow. Theory and practice have proven that acid fracturing is a key technology for enhancing the production of carbonate oil and gas reservoirs. Through acid fracturing, the acid solution dissolves the reservoir in a non-uniform manner, creating acid-etched fractures that retain a certain length and conductivity even after the fractures close. These fractures connect and link oil and gas flow channels with the reservoir space, achieving the goal of increased production and ensuring normal well commissioning and high, stable production. The Pingqiao area's tidal flat dolomite reservoir, with a gas-bearing area of ​​350 km² and a resource volume of 321 × 10⁸ m³, possesses significant exploration and development potential. However, due to its deep burial, severe heterogeneity, and high tightness, relying solely on a single acid fracturing process or acid solution system is insufficient to achieve ideal results. Therefore, a composite acid fracturing approach is considered for reservoir stimulation. Combining non-reactive high-viscosity liquids with acids of various properties creates multi-stage injection composite acid fracturing technology of different types and scales. The process is: pre-flush liquid + acid (multi-stage alternating injection) + displacement liquid, which is mainly applied to low-permeability and ultra-low-permeability carbonate reservoirs.

[0003] Chinese invention patent CN110397430B discloses a method for predicting the conductivity distribution of acid-fracturing fractures in carbonate rocks. This patent uses a two-dimensional PKN fracture propagation model to simulate the fracture propagation process during fracturing, predicting the conductivity distribution along the fracture length and optimizing the design of the carbonate rock pre-fracturing fluid. Its limitation is that it predicts the conductivity distribution solely through numerical simulation without experimental verification, leading to potential errors in the model's accuracy.

[0004] Chinese invention patent CN112502685B discloses a method for optimizing the alternating acid fracturing stages in carbonate reservoirs, considering thermal effects. This patent optimizes the alternating stages during acid fracturing based on acid etching and conductivity experiments. However, its shortcomings lie in the fact that relying solely on acid etching and conductivity experiments fails to fully verify the acid fracturing effect, and optimizing only the number of alternating stages does not comprehensively verify the acid fracturing parameters, thus offering limited guidance for acid fracturing design.

[0005] Chinese invention patent CN110630240B discloses a method for optimizing the discharge rate of multi-stage alternating acid fracturing in carbonate reservoirs. This patent optimizes the discharge rate parameters of multi-stage alternating acid fracturing based on the areal-to-volume ratio, thereby improving the fracturing effect. Its shortcoming lies in that it only optimizes the discharge rate through the areal-to-volume ratio, the maximum discharge rate the reservoir can withstand, and the pressure-bearing capacity of the surface equipment, without analyzing the impact of the discharge rate on the fluid in the actual formation, and without considering the influence of other construction parameters on the fracturing effect.

[0006] Conventional acid fracturing technology faces limitations such as rapid acid-rock reaction rates, limited acid penetration depth, and rapid decline in the conductivity of acid-etched fractures. It also encounters challenges like rapid post-fracturing decline and difficulty in maintaining stable production, resulting in underutilization of reserves and significantly impacting the economic development of similar reservoirs. The aforementioned optimization and evaluation of construction parameters for existing technologies do not incorporate analysis of fluid loss within the actual formation and fail to fully consider the limitations of optimizing only a single parameter during acid fracturing. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide an evaluation system and method for composite acid fracturing process parameters of tight dolomite reservoirs, which addresses the above-mentioned problems. It adopts reverse thinking and evaluates composite acid fracturing process parameters such as fractal dimension, acid volume, conductivity, and acid filtration loss as targets.

[0008] The embodiments of this application are implemented as follows:

[0009] This application provides an evaluation system for composite acid fracturing process parameters in tight dolomite reservoirs, including a simulated fracture device, an acid injection device, and a circulating water injection device. The simulated fracture device includes a fractured rock plate, a front frame, and a rear cover plate. The fractured 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 fractured rock plate and the front frame, and a rear slit is formed between the rear end face and the rear cover plate. Acid injection inlet holes and water injection inlet holes are respectively provided on both sides of the front frame corresponding to the positions of the front slits. Acid injection inlet holes and water injection inlet holes are respectively provided on both sides of the rear cover plate corresponding to the positions of the rear slits. The device is equipped with an acid injection and drainage port and a water injection and drainage port. The acid injection device includes an acid storage container, an acid injection pipeline, and an acid drainage pipeline. The two ends of the acid injection pipeline are connected to the acid storage container and the acid injection inlet port of the simulated crack device. The acid drainage pipeline is connected to the acid injection and drainage port 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 port of the simulated crack device. The two ends of the circulating drainage pipeline are connected to the water injection and drainage port of the simulated crack device and the water storage container.

[0010] In some alternative implementations, 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 implementations, 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 alternative implementations, 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.

[0013] In some optional implementations, the acid injection valve, acid discharge valve, and drain valve are all multi-port valves, and each channel of the multi-port valve is connected to the corresponding acid injection inlet, acid injection outlet, and water injection outlet.

[0014] In some optional implementations, the acid injection backpressure valve, water injection backpressure valve, acid discharge backpressure valve, and drainage backpressure valve are respectively connected to a backpressure multi-way valve, which is connected to a backpressure manual pump; the acid discharge backpressure valve and the drainage backpressure valve are respectively connected to an acid discharge backpressure gauge and a drainage backpressure gauge.

[0015] An evaluation method for a composite acid fracturing process parameter evaluation system for tight dolomite reservoirs, characterized by comprising the following steps:

[0016] Step a, Fracturing acid fingering experiment: Prepare fracturing fluid and acid of corresponding viscosities according to the experimental design requirements, store them in the corresponding acid storage containers, and perform water injection pressure tests in the forward and backward slits respectively to check the sealing. After the check is completed, inject the pre-filled fluid into the forward slit until the fracture is full, and then inject the acid. Record the fingering evolution pattern and related experimental parameters in the simulated fracture. Adjust the viscosity ratio of fracturing fluid and acid, pump flow rate and alternation stage, design different experimental schemes, repeat the above experiments, and evaluate and optimize the process parameters.

[0017] Step b, acid etching fracture conductivity test: Collect rock samples to prepare rock plates for the American Petroleum Institute standard conductivity chamber. Use a laser scanning device to obtain the roughness of the rock plate before acid etching. Prepare fracturing fluid and acid of corresponding viscosity according to the optimized parameters of the acid fingering experiment in the fracture. Inject the acid into the rock plate to obtain the acid-etched rock plate. Use a laser scanning device to obtain the surface roughness of the acid-etched fracture. Use a conductivity testing device to test the conductivity of the rock plate after etching.

[0018] Step c, acid filtration test in natural fractures: collect rock samples to prepare standard cores, construct artificial fractures, apply confining pressure to displace acid into the cores to simulate on-site construction, and collect relevant pressure and flow data to conduct acid displacement experiments;

[0019] Step d, rock slab strength damage experiment based on continuous strength test: collect rock samples to prepare rock slabs for the American Petroleum Institute standard flow chamber, perform scratch tests at multiple different positions on the X-axis of the rock slab, measure the original surface strength of the rock slab, and take the average value of multiple points as the standard. After acid etching of the rock slab, perform multiple scratch tests at flat positions of the rock slab, and take the average value as the surface strength of the rock slab after acid etching.

[0020] In some optional implementations, the artificial fracture construction in step c includes the following: the core is vertically split open along the diameter direction of the core using artificial fracture cutting technology, and artificial grooves are etched along the length direction in the center of the split half of the core with a cutting knife in accordance with a pre-designed fracture network pattern to simulate the fracture network in the carbonate reservoir. The depth and width of the artificially carved grooves are controlled within 1 mm. The two halves of the core are merged and the sides of the core are wrapped with tape for fixation.

[0021] In some alternative implementations, the rock slab described in step d is coated with copper gasket adhesive around its perimeter before the scratch test, air-dried at room temperature for 24 hours, and then saturated with simulated formation water under vacuum for 6 hours.

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

[0023] The beneficial effects of this application are as follows: This application provides an evaluation system and method for composite acid fracturing process parameters in tight dolomite reservoirs. The evaluation targets fractal dimension, acid volume, conductivity, and acid filtration loss, making the evaluation method more specific and systematic. Building upon traditional acid fracturing process parameter evaluation methods that only assess composite acid fracturing process parameters such as the pre-fracturing fluid / acid viscosity ratio, pumping rate, and alternation stages through in-fracture acid fingering experiments and acid etching fracture conductivity experiments, this application adds a natural fracture acid filtration loss experiment to comprehensively consider the influence of on-site composite acid fracturing process parameters on acid flow reaction and filtration loss within a single fracture. Furthermore, it adds a rock slab strength damage experiment based on continuous strength testing to comprehensively consider the influence of on-site composite acid fracturing process parameters on the surface strength of the rock slab. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the physical device for the acid fingering experiment in the crevices according to an embodiment of this application;

[0026] Figure 2 This is a schematic diagram of the acid fingering experiment in the suture according to an embodiment of this application;

[0027] Figure 3 This is a schematic diagram of the analysis results of the acid fingering experiment in the suture according to an embodiment of this application;

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

[0029] Figure 5 This is a schematic diagram of the rock slab after acid etching, according to an embodiment of this application.

[0030] Figure 6 This is a schematic diagram of the experimental analysis results of the conductivity of acid-etched cracks according to an embodiment of this application;

[0031] Figure 7 This is a schematic diagram of artificial fractures in the rock core according to an embodiment of this application;

[0032] Figure 8 This is a schematic diagram of the core sample after the displacement acidification experiment according to an embodiment of this application;

[0033] Figure 9 This is a box plot illustrating the effect of alternating stages on the compressive strength of rock slabs according to an embodiment of this application. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0035] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0036] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

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

[0038] Furthermore, terms such as "horizontal," "vertical," and "sag" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "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 expressly specified and limited, the terms "set up," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0040] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0041] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0042] This invention provides an evaluation system and method for composite acid fracturing process parameters in tight dolomite reservoirs. It adopts a reverse thinking approach, using fractal dimension, acid volume, conductivity, and acid filtration loss as targets to evaluate composite acid fracturing process parameters such as the pre-fracturing fluid / acid viscosity ratio, pump injection rate, and number of alternating stages.

[0043] The embodiments of this application are implemented as follows:

[0044] like Figure 1 As shown in the embodiment of this application, an evaluation system for composite acid fracturing process parameters of tight dolomite reservoirs is provided, including a simulated fracture device 1, an acid injection device, and a circulating water injection device. The simulated fracture device includes a fractured rock plate, a front frame, and a rear cover plate. The fractured 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 fractured rock plate and the front frame, and a rear slit is formed between the rear end face and the rear cover plate. Acid injection inlet holes and water injection inlet holes are respectively provided on both sides of the front frame corresponding to the positions of the front slits. The rear cover plate is provided on both sides corresponding to the positions of the rear slits. The fracture is equipped with acid injection and drainage holes and water injection and drainage holes respectively. The acid injection device includes an acid storage container 2, an acid injection pipeline 3 and an acid drainage pipeline 4. The two ends of the acid injection pipeline are connected to the acid storage container and the acid injection inlet of the simulated fracture device, and the acid drainage pipeline is connected to the acid injection and drainage holes of the simulated fracture device. The circulating water injection device includes a water storage container 5, a water inlet pipeline 6 and a circulating drainage pipeline 7. The two ends of the water inlet pipeline are connected to the water storage container and the water injection inlet of the simulated fracture device, and the two ends of the circulating drainage pipeline are connected to the water injection and drainage holes of the simulated fracture device and the water storage container.

[0045] In some alternative implementations, 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 alternative implementations, the acid injection pipeline is sequentially equipped with an acid injection constant flow pump 8, an acid injection back pressure valve 9, and an acid injection valve 10, and the acid discharge pipeline is sequentially equipped with an acid discharge valve 11 and an acid discharge back pressure valve 12. The water inlet pipeline is sequentially equipped with a water injection constant flow pump 13 and a water injection back pressure valve 14, and the circulating drainage pipeline is sequentially equipped with a drain valve 15 and a drain back pressure valve 16.

[0047] In some optional implementations, the acid injection valve, acid discharge valve, and drain valve are all multi-port valves, and each channel of the multi-port valve is connected to the corresponding acid injection inlet, acid injection outlet, and water injection outlet.

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

[0049] The evaluation of the composite acid fracturing process parameters for tight dolomite reservoirs using the above-mentioned evaluation system includes the following steps:

[0050] Step a, Fracturing acid fingering experiment: Prepare fracturing fluid and acid of appropriate viscosity according to the experimental design requirements, store them in the corresponding acid storage containers, and perform water injection pressure tests in the forward and backward slits respectively to check the sealing. After the check is completed, inject pre-filled fluid into the forward slit until the fracture is full, and then inject acid. Record the fingering evolution pattern and related experimental parameters in the simulated fracture. Adjust the viscosity ratio of fracturing fluid and acid, pump flow rate and alternation stage, design different experimental schemes, repeat the above experiments, and evaluate and optimize the process parameters.

[0051] The relevant experimental parameters mentioned above include: fractal dimension, the percentage of the fingering region area, and the volume of acid used.

[0052] Step b, Experiment on the conductivity of acid-etched cracks:

[0053] Rock samples were collected to prepare rock slabs for the American Petroleum Institute standard flow chamber. The roughness of the rock slab before acid etching was obtained using a laser scanning device. Based on the optimized parameters from the acid fingering experiment in the fracture, fracturing fluid and acid of corresponding viscosities were prepared. The acid was injected into the rock slab to obtain the acid-etched rock slab. The surface roughness of the acid-etched fracture was obtained using a laser scanning device. The flow conductivity of the rock slab after etching was tested using a flow conductivity testing device.

[0054] The aforementioned laser scanning device employs a three-dimensional quad-eye scanner, and the scanned images are processed by programming to obtain the joint roughness coefficient (JRC).

[0055] Step c, acid filtration test in natural fractures: Collect rock samples to prepare standard cores, construct artificial fractures, and control the depth and width of the artificially carved grooves to within 1 mm. Apply confining pressure to the core to displace acid and simulate on-site construction, and collect relevant pressure and flow data to conduct acid displacement experiments.

[0056] Images of the core after acidizing were obtained through acid filtration experiments in natural fractures. The dissolution of acid in the main fracture and natural fractures was compared to evaluate the influence of on-site composite acid process parameters on acid flow reaction and filtration loss in a single fracture.

[0057] Step d, rock slab strength damage experiment based on continuous strength test: collect rock samples to prepare rock slabs for the American Petroleum Institute standard flow chamber, perform scratch tests at multiple different positions on the X-axis of the rock slab, measure the original surface strength of the rock slab, and take the average value of multiple points as the standard. After acid etching of the rock slab, perform multiple scratch tests at flat positions of the rock slab, and take the average value as the surface strength of the rock slab after acid etching.

[0058] The influence of composite acid fracturing process parameters on the surface strength of rock slabs was tested through a rock slab strength damage experiment based on continuous strength testing, which also helped to evaluate the impact of composite acid fracturing process parameters on conductivity. To prevent acid loss between the rock slab and the API inner tank, Permatex super copper gasket adhesive was applied around the rock slabs in the continuous strength testing rock slab strength damage experiment, and then air-dried at room temperature for 24 hours. Before acid etching, all rock slabs in the continuous strength testing rock slab strength damage experiment were vacuum-saturated with simulated formation water (2wt% KCl) for six hours.

[0059] Example 1

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

[0061] S1, acid fingering test inside the crevices

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

[0063] 2) Connect the corresponding instruments (meters) and accessories with the appropriate pipelines. In particular, when assembling the simulated crack, make sure that the gaskets are placed flat. When assembling the back cover, the sealing bolts should be screwed in slowly.

[0064] 3) Start the constant flow pump and pump clean water into the rear slit for pressure testing. 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, release the back pressure, drain the water, and return to the previous step.

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

[0066] 5) After closing the acid injection valve, inject pre-filled liquid into the forward slit. When injecting pre-filled liquid, open all vent holes to remove as much air as possible from the slit, so that the pre-filled liquid can quickly and evenly fill the crack.

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

[0068] 7) Based on the fracturing fluid of the corresponding viscosity and the acid of the corresponding concentration used in the field construction, adjust the pre-flush fluid / acid viscosity ratio, pump flow rate and alternating stage design experimental scheme, repeat steps 1) to 6) above, and evaluate and optimize the process parameters.

[0069] Specifically, the on-site composite acid fusing process parameters are: acidic guar gum crosslinking liquid (50 mPa·s) + 0.8% gelling agent in gelling acid (15 mPa·s), with a discharge rate of 6 m³ / s. 3 / min. Complete the intra-suture acid fingering experiment according to the above procedure, including the start of fingering, fingering growth, and the fingering leading edge reaching the boundary. Figure 2 As shown in Table 1, the area ratio of the finger-injected region, fractal dimension, and acid volume are as follows.

[0070] Table 1. Parameters of the acid fingering experiment within the suture.

[0071]

[0072] By adjusting the viscosity ratio of the pre-fluid / acid, the pump flow rate, and the number of alternating stages, the experimental design scheme was used to evaluate and optimize the process parameters of the composite acid solution in the field. The experimental scheme is shown in Table 2.

[0073] Table 2 Experimental Scheme for Acid Fingering in the Crevice

[0074]

[0075]

[0076] The results of the acid fingering experiment within the suture were compared with the dimensional dimension, the area ratio of the fingering region, and the volume of acid used. Figure 3 As shown, the fractal dimension increases with increasing viscosity ratio and displacement, while decreasing with increasing order. In Experiment 5, the fractal dimension is large when the fingering leading edge reaches the edge, the fingering region area is small, the acid dosage is small, and the fingering effect is obvious.

[0077] S2, Experiment on the conductivity of acid-etched cracks

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

[0079] 2) Based on the requirements of the experimental design, the composite acid fracturing process parameters obtained by the evaluation and optimization of the acid fingering experiment in the fracture were compared with the experimental scheme. Fracturing fluid and acid of corresponding viscosity were prepared and then stored in the corresponding storage containers.

[0080] 3) The prepared acid solution is pumped from the storage tank into the API flow chamber using an acid-resistant pump. When the acid solution flows on the crack wall, it reacts with the rock surface, which approximately simulates the chemical reaction process between the acid solution and the formation rock during on-site acid pressure, and obtains the acid-etched rock slab.

[0081] 4) The surface roughness of the acid-etched cracks is obtained using a laser scanning device;

[0082] 5) The conductivity of the etched rock slab was tested using a conductivity testing device;

[0083] Specifically, the on-site composite acid fusing process parameters are: acidic guar gum crosslinking liquid (50 mPa·s) + 0.8% gelling agent in gelling acid (15 mPa·s), with a discharge rate of 6 m³ / s. 3 / min. Following the above procedure, an acid-etched rock slab was obtained. The surface roughness of the acid-etched cracks was obtained using a laser scanning device. The conductivity of the slab after etching was tested using a conductivity testing device. The roughness before and after acid etching is shown in the figure. Figure 4 , Figure 5 As shown.

[0084] The experimental design, which adjusts the pre-fluid / acid viscosity ratio, pump flow rate, and alternating stages, was used to evaluate and optimize the on-site composite acid process parameters. The experimental design is shown in Table 2 above, and its flowability analysis is as follows. Figure 6 As shown in the figure, the curves generally show a trend of decreasing conductivity as the closing pressure increases. The conductivity values ​​in the flat section under high closing pressure are mostly below 10 D·cm, with a few exceeding 15 D·cm. Among them, rock samples from Experiments 5 and 7 maintain better conductivity due to stronger surface etching and groove support. At 48.3 MPa, Experiment 7 exhibits the highest conductivity at 31.5 D·cm, while the conductivity of other rock samples is below 10 D·cm.

[0085] Therefore, based on the fractal dimension, acid volume, and conductivity of the fracture, and through acid fingering experiments and fracture etching conductivity experiments, composite acid fracturing process parameters such as the pre-flush fluid / acid viscosity ratio, pump flow rate, and alternating stages were optimized and evaluated. The final optimal result was Experiment 5. Experiment 5: Acidic guar gum fracturing fluid (50 mPa·s) + 0.8% gelling agent gelling acid (15 mPa·s), flow rate 9 m³ / s. 3 / min.

[0086] S3, Acid Filtration Test in Natural Cracks

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

[0088] 2) The core is cut vertically along its diameter using artificial slit cutting technology. The cut surface of the core is flat and smooth, and the core can close well under certain confining pressure when it is reassembled.

[0089] 3) Use a carving knife to carve artificial grooves along the length of the cut rock core in the pre-designed fracture network pattern 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 network of cracks on one half of the core profile and seal the other half with tape. Finally, wrap and fix the sides of the core with pressure.

[0091] 5) Simulate on-site construction by displacing acid into the core at a constant inlet flow rate according to the on-site construction parameters, and collect relevant pressure and flow data to conduct displacement acidification experiments.

[0092] Specifically, after optimization and evaluation of processes S1 and S2, the composite acid fracturing process parameters are: acidic guar gum fracturing fluid (50 mPa·s) + 0.8% gelling agent in gelling acid (15 mPa·s), with a displacement of 9 m³ / s. 3 / min. Based on the acid filtration study of the S3 natural fracture network, the core sample was perforated. The perforated fractures were of uniform depth and moderate thickness. The perforation of the core sample was as follows: Figure 7 As shown. The reaction after acid injection into the core is as follows. Figure 8 As shown, the slower reaction rate of gelling acid can effectively slow down the rate of acid dissolution of natural cracks, inhibit the formation of acid-etched pits, and effectively increase the extension distance of the main acid-pumped cracks.

[0093] S4. Rock slab strength damage experiment based on continuous strength testing

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

[0095] 2) Processing rock slabs: Since the diamond cutting tip will scrape the rock surface downwards by about 0.2mm each time it is tested, the initial scraping thickness of the rock slab needs to be reserved.

[0096] 3) Scraping before acid etching: Before acid etching, scratch the rock slab at 1 / 4, 2 / 4 and 3 / 4 of the X-axis and take the average value of the three positions as the original surface strength of the rock slab before acid etching.

[0097] 4) Scraping after acid etching: Due to the presence of acid etching grooves on the surface of the rock slab, the rock contacted by the diamond cutting head may differ significantly from the size of the cutting head. Therefore, multiple scraping tests are conducted on a relatively flat target location (generally 2 to 3 times), and the average test result is taken as the surface strength after acid etching.

[0098] Specifically, after optimization and evaluation of processes S1 and S2, the composite acid fracturing process parameters are: acidic guar gum fracturing fluid (50 mPa·s) + 0.8% gelling agent in gelling acid (15 mPa·s), with a displacement of 9 m³ / s. 3 / min. Based on the S4 rock slab strength damage test based on continuous strength testing, taking the effect of alternating stages on rock slab strength as an example, the box plot of its effect on the compressive strength of the rock slab is as follows. Figure 9 As shown in the figure. The analysis shows that the number of injection stages has little effect on the strength of the rock slab. The average strength decreased by 12% to 17% after acid etching, and the softening coefficient was similar, indicating that increasing the number of injection stages has little effect on the strength of the rock slab.

Claims

1. An evaluation method for a composite acid fracturing process parameter evaluation system for tight dolomite reservoirs, wherein the evaluation system... The system includes a simulated fracture device, an acid injection device, and a circulating water injection device. The simulated fracture device comprises a fractured rock slab, a front frame, and a rear cover plate. The fractured rock slab is fixed by the front frame and the rear cover plate. A front slit is formed between the front end face of the fractured rock slab and the front frame, and a rear slit is formed between the rear end face and the rear cover plate. Acid injection inlet holes and water injection inlet holes are respectively provided on both sides of the front frame corresponding to the positions of the front slits. Acid drainage holes and water drainage holes are respectively provided on both sides of the rear cover plate corresponding to the positions of the rear slits. The acid injection device includes an acid storage container, an acid injection pipeline, and an acid drainage 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 fracture device, and the acid drainage pipeline is connected to the acid injection drainage hole of the simulated fracture 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 fracture device, and the two ends of the circulating drainage pipeline are connected to the water injection drainage hole of the simulated fracture device and the water storage container. The system is characterized in that… The evaluation method using the above evaluation system includes the following steps: Step a, Fracturing fluid fingering experiment: Prepare fracturing fluid and phase of appropriate viscosity according to the experimental design requirements. Acid of appropriate viscosity is stored in the corresponding acid storage container. Water is injected into the front and rear slits for pressure testing to check the sealing. After the test, pre-filled fluid is injected into the front slit until the fracture is full, and then acid is injected. The fingering evolution morphology and related experimental parameters in the simulated fracture are recorded. The viscosity ratio of fracturing fluid and acid, pump flow rate and alternation stage are adjusted to design different experimental schemes. The above experiments are repeated to evaluate and optimize the process parameters. Step b, Acid-etched fracture conductivity test: Collect rock samples to prepare American Petroleum Institute standard conductivity. For indoor use, the roughness of the rock slab before acid etching is obtained using a laser scanning device. Based on the optimized parameters of the acid fingering experiment in the fracture, fracturing fluid and acid of corresponding viscosity are prepared. The acid is injected into the rock slab to obtain acid-etched rock slab. The surface roughness of the acid-etched fracture is obtained using a laser scanning device. The conductivity of the rock slab after etching is tested using a conductivity testing device. Step c, acid filtration test in natural fractures: Collect rock samples to prepare standard cores and perform artificial fracture analysis. The structure was constructed, and the acid was displaced into the core under confining pressure to simulate on-site construction. Relevant pressure and flow data were collected to conduct displacement acidification experiments. Step d, Rock slab strength damage experiment based on continuous strength testing: collect rock samples and prepare US rock samples. The slabs used in the standard flow chamber of the Petroleum Society were scratched at multiple different positions on the X-axis to measure the original surface strength of the slabs. The average value of multiple locations was taken as the standard. After acid etching, the slabs were scratched multiple times at flat locations, and the average value was taken as the surface strength of the slabs after acid etching.

2. The evaluation method of the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 1, characterized in that, The artificial fracture structure described in step c includes the following: the core is vertically split along its diameter using artificial fracture cutting technology, and artificial grooves are etched along the length of one half of the core in the center of the split half with a sculptor in accordance with a pre-designed fracture network pattern to simulate the fracture network in the carbonate reservoir. The depth and width of the artificially etched grooves are controlled within 1 mm. The two halves of the core are then joined together and the sides of the core are wrapped with tape for fixation.

3. The evaluation method for the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 2, characterized in that, Before the scratch test, the rock slab described in step d was coated with copper gasket adhesive around its perimeter, air-dried at room temperature for 24 hours, and then saturated with simulated formation water under vacuum for 6 hours.

4. The evaluation method for the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 3, characterized in that, The relevant experimental parameters mentioned in step a include fractal dimension, the area ratio of the fingering region, and the volume of acid used.

5. The evaluation method for the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 1 or 4, 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.

6. The evaluation method for the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 5, characterized in that, The acid injection pipeline is equipped 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 equipped with an acid discharge valve and an acid discharge back pressure valve in sequence.

7. The evaluation method for the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 6, characterized in that, The water inlet pipeline is equipped with a water injection constant flow pump and a water injection back pressure valve in sequence, and the circulating drainage pipeline is equipped with a drainage valve and a drainage back pressure valve in sequence.

8. The evaluation method for the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 7, characterized in that, The acid injection valve, acid discharge valve, and drain valve are all multi-port valves, and each channel of the multi-port valve is connected to the corresponding acid injection inlet, acid injection outlet, and water injection outlet.

9. The evaluation method for the composite acid fracturing process parameter evaluation system for tight dolomite reservoirs according to claim 8, characterized in that, The acid injection backpressure valve, water injection backpressure valve, acid discharge backpressure valve, and drainage backpressure valve are respectively connected to the backpressure multi-way valve, which is connected to the backpressure manual pump; the acid discharge backpressure valve and the drainage backpressure valve are respectively connected to the acid discharge backpressure gauge and the drainage backpressure gauge.

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