Acid-etched Fracture Research Articles

Evaluation of Acid Retardation Based on Acid-Etched Fracture Morphology

Evaluation of Acid Retardation Based on Acid-Etched Fracture Morphology

Numerical investigation into the acid flow and reaction behavior in the tight, naturally fractured carbonate reservoir during acid fracturing

Tight naturally fractured carbonate reservoirs require acid fracturing to build the connectivity between the wellbore and natural fractures (NFs), where hydrocarbon is stored. The high leakoff nature of the NF complicates the acid flow and etching pattern, raising the difficulty in acid fracturing design and optimization. To explore the acid flow and reaction behavior in such reservoirs, an acid fracturing model accounting for the NF distribution is developed, which consists of a fracture surface characterization model, a fracture propagation model, and an acid-etching model. Based on the model, the effects of injection parameters and the NF properties on the effectiveness of acid fracturing are investigated. Then, strategies for acid fracturing the tight naturally fractured carbonate reservoirs are proposed. Results show that high pumping rates and retarded acids with low hydrogen ion (H+) diffusion coefficient is conducive to achieving a long acid penetration distance, while a low pumping rate and acids with a high H+ diffusion coefficient facilitates the NF etching. Therefore, a large stimulated area can be achieved by applying a multi-stage alternating injection of the crosslinked acid with a high pumping rate followed by the gelled acid with a low injection rate. NFs impact acid fracturing in two distinct ways: enhancing the non-uniform etching of the fracture surface and reducing the effective acid-etched fracture length through high leakoff. When NF density is high, leakoff control techniques should be employed; and when the NF inclination is high, non-uniform etching techniques should be used to generate acid-etched channels in flow barriers.

Effect of acid viscosity on conductivity during online acid-fracturing in ultra-deep carbonate reservoir

The influence of viscosity change in the same acid system on etched-fracture morphology and conductivity was often neglected in many studies that conducted multi-stages alternative acid-etching experiments using different acid systems. In order to address this knowledge gap, the present study investigated the impact of single injections of low viscosity and high viscosity acid, as well as an alternative stages of low and high viscosity acid, on the acid-etched fracture morphology and conductivity. While the online acid-fracturing technology was proposed based on experimental results firstly in ultra-deep wells. The results show that injecting acid with different viscosities is more effective than using a single viscosity acid-etching, as it improves fracture morphology and conductivity. Additionally, the stages of alternating acid have a significant impact on conductivity. Experimental studies reveal that employing three stages of alternating acid with different viscosities enhance the fracture conductivity in the research area. Based on these findings, the P6 well successfully is implemented online acid-fracturing in an ultra-deep well reaching 8000 m firstly in Sichuan Basin. Compared to adjacent wells, this technique exhibited lower friction resistance, enabling higher acid-injection rates.

Acid-Etched Fracture Conductivity with In Situ-Generated Acid in Ultra-Deep, High-Temperature Carbonate Reservoirs

Acid-Etched Fracture Conductivity with In Situ-Generated Acid in Ultra-Deep, High-Temperature Carbonate Reservoirs

Acid-etched fracture morphology and conductivity of acid fracturing in carbonate rock using novel isolation membrane acid

In recent years, low-viscosity isolation membrane acid (ISM acid) has emerged as a novel retardant acid for matrix acidizing and acid-fracturing in carbonate reservoir. Its ability to create long wormholes in carbonate rock has been extensively studied, while its potential to etch hydraulic fracture and generate conductivity has received less attention. To address the knowledge gap, a series of experiments were conducted in this study, including rheological test, acid-etching simulation experiments, surface laser scanning of acid-fractures, and conductivity tests. The differences in rheological characteristics between ISM acid, blank HCl acid, and gelled acid were compared across different lithology along with the morphology and conductivity of the etched fractures. The viscosity of ISM acid is found to be nearly identical to that of blank HCl, which is 1/26th that of gelled acid. However, the acid-rock reaction rate of ISM acid is only 46–55% compared to blank HCl acid. In dolomite formations, the rate approximates that of gelled acid but is lower in limestone formations, indicating a strong ability for low-viscosity retarding. The heterogeneity of ISM acid etching morphology approaches that of blank HCl acid but exhibits greater retard strength than gelled acid. Under medium to high closure pressure (10–30 MPa), ISM acid generates higher conductivity and better retention ability. This study provides certain guidance for the design of ISM acid fracturing in carbonate reservoir.

Effect of Acid-Injection Mode on Conductivity for Acid-Fracturing Stimulation in Ultra-Deep Tight Carbonate Reservoirs

The conductivity of acid-etched fractures and the subsequent production response are influenced by the injection mode of the fracturing fluid and acid fluid during acid fracturing in a carbonate reservoir. However, there has been a lack of comprehensive and systematic experimental research on the impact of commonly used injection modes in oilfields on conductivity, which directly affects the optimal selection of acid-fracturing injection modes. To address this gap, the present study focuses on underground rock samples, acid systems, and fracturing fluid obtained from ultra-deep carbonate reservoirs in the Fuman Oilfield. Experimental investigations were conducted to examine the conductivity of hydraulic fractures etched by various types of acid fluids under five different injection modes: fracturing fluid + self-generating acid or cross-linked acid; fracturing fluid + self-generating acid + cross-linked acid. The findings demonstrate that the implementation of multi-stage alternating acid injection results in the formation of communication channels, vugular pore space, and natural micro-cracks, as well as grooves and fish-scales due to enhanced etching effects. The elevation change, amount of dissolved rock, and conductivity exhibited by rock plates are significantly higher in comparison to those achieved through the single-acid injection mode while maintaining superior conductivity. It is recommended for optimal conductivity and retention rate in the Fuman Oilfield to adopt two stages of alternating acid-fracturing injection mode. Field application demonstrated that two-stages of alternating acid-fracturing generate more pronounced production response than the adjacent wells.

Numerical characterization of acid fracture wall etching morphology and experimental investigation on sensitivity factors in carbonate reservoirs

Carbonate reservoirs are marked by their low porosity and permeability, naturally occurring fractures that develop in a stochastic fashion and exhibit significant heterogeneity. Deep acidification techniques are essential for the efficient development of such oil and gas reservoirs. The effective distance of acid etching (acid etch fracture length) and the fracture conductivity stand as the two most critical parameters for assessing the efficacy of acidizing. The primary objective of using this technique is to extend the effective distance of the acid etch and enhance the fracture conductivity. The "relay-style" full-length acid etched fracture testing method is employed, achieving an accurate match between the testing temperature, pressure, fracture length, and the actual acid fracturing conditions. It employs a large-scale rock plate acid etching experimental device, in conjunction with 3D laser scanning technology, and utilizes orthogonal experimental design methods to conduct detailed numerical characterization of the rock plate fracture surfaces before and after etching with various acid fluid types, acid injection amounts, and acid injection rates. It quantifies the change in the concentration of the residual acid fluid after the acid-rock reaction, calculating the etching volume of the acid fluid and the length of the acidizing fracture. The experimental results demonstrate that both the gelled acid and the clean diverting acid exhibit long acidification distances and strong inhomogeneous etching capabilities. The etch morphology of the clean diverting acid is dominated by pitting-type and stripe-like dissociation, exhibiting a wavy pattern. In contrast, the etch morphology of the gelled acid appears band-like and groove-like. As the acid injection amount increases, the etch length increases. Under on-site working conditions, etch lengths of 46.42 m and 41.63 m were recorded for 20% gelled acid and clean diverting acid, respectively, after 60 min acid injection time. The factors affecting the etch length sensitivity are in the following order: acid injection rate > acid type > etching time. The optimal acid fracturing parameters to achieve the maximum etch length are: The etch is performed at a pump rate of 600 mL/min for 90 min using gelled acid. Quantitative characterization of acid etch fracture morphology can optimize designing parameters of acid fracturing techniques, enhance the efficiency of their modification, and further enhance the development benefits of oil and gas carbonate rock reservoirs.

Experimental investigation of the impacts of supercritical carbon dioxide on acid-etching fracture morphology and conductivity in tight carbonate reservoir

Carbon dioxide (CO2) as energized fluid has been used for acid-fracturing in tight carbonate reservoir. CO2 is often pumped into the wellbore in liquid phase with acid systems, and transited into supercritical state under the high-temperature formation. While the effect of supercritical CO2 (SC-CO2) on acid-etched fracture morphology and conductivity have rarely been investigated. The acid-etching and conductivity experiments were conducted on the outcrops of the Ma5 sub-member of the Ordo Basin in China. The mixed phase reaction simulation device was developed firstly. These experiments involved the mixing of SC-CO2 with various fluids, including formation water and viscoelastic surfactant (VES) acid, as well as different SC-CO2 concentrations with VES acid. The results show that 100 % SC-CO2 is easy to activate natural fractures without dissolution. After 20%SC-CO2 mixed with formation water, the dissolution ability of the mixed solution and conductivity is only 11.8 % and 1 ∼ 2 % of VES acid respectively. Roughness of the morphology created by the mixture of SC-CO2 with VES acid, exhibits a reduction compared to VES acid-etching, accompanied by the presence of wormholes. Additionally, there is a decrease in conductivity and increase in its retention rate. With the increase of SC-CO2 concentration, the roughness of acid-etching fracture decrease and the injection pressure curve becomes more stable. While the reaction rate is also retarded nearly linearly as the SC-CO2 concentration increases. Especially, when the SC-CO2 concentration is up to 40 %, the acid-rock reaction rate reaches only 55 % of that of VES acid. The conductivity of mixed-acid-etched fracture exhibits a segmented characteristic. During the low-medium closure pressure stage, the acid-etched fracture by VES acid generates high conductivity, specifically when it is below 34.5 MPa. While the higher the SC-CO2 concentration in the mixed acid is, the greater the conductivity and retention rate are at pressures above 34.5 MPa during high closure pressure stage. The conductivity of SC-CO2 acid-fracturing fracture depends on the non-uniform etching of the acid solution. The increase in SC-CO2 concentration of the mixed acid not only enhances conductivity and retention rate under high closure pressure, but also reduces the acid volume required for acid-fracturing.

Experimental evaluation and optimization of improved conductivity of gelling acid etched fractures in deep, low-permeability reservoirs

Stimulation technology is the key to efficient development of deep low-permeability carbonate reservoirs. For such reservoirs, where the porosity and permeability are low, or where the reservoir is far from the borehole. An efficient production increase can only be achieved by using a deep percolation gelling acid fracture technique, maximizing the length of the acid etched fracture to expand the percolation area, and improving the conductivity of the acid etched fracture. A quantitative experimental study of the conductivity of acid etch fracture based on an acid rock tiling instrument, a 3D laser scanner, and an acid etch fracture conductivity evaluation device identified three typical supporting morphologies of gelling acid etch fracture walls: spot, line, and sheet. A systematic quantitative description and evaluation of the etch morphology of rocks etched by acid has been performed. The fracture morphology of acid etchings has been quantitatively evaluated under various experimental conditions such as fracture width, temperature, acid fluid consumption, displacement, and different acid fracture processes and their laws affecting fracture conductivity. The conductivity of acid corrosion fracture increases with the decrease of fracture width, and decreases with the increase of closure stress. The greater the amount of acid used, the higher the fracture conductivity; With the increase of flow Reynolds number, the conductivity of acid corrosion fracture shows a trend of increasing first and then decreasing. The fracture conductivity was compared and analyzed for different property placement conditions under the acid sand carrying technique, and the main effect of the acid etch fracture conductivity was identified. Optimization of high conductivity and deep penetration gelling acid fracturing techniques can help guide the design of acid fracturing and improve stimulation and recovery effects, thus achieving the goal of efficient development of deep, low permeability carbonate reservoirs to increase reserves and production.

Effect of Acid Type on Acid-Etched Fracture Surface Morphology and Conductivity along the Fracture

Laboratory measurement of acid fracture conductivity is the most direct method for acid system selection during acid fracturing in carbonate reservoirs. However, the fracture in the laboratory test is too short compared with the in-situ fracture. Thus, the acid concentration and viscosity variation and their impacts on fracture morphology and conductivity along the fracture are seldom considered, leading to inaccurate estimation of conductivity and providing uncertainty in acid selection, especially for ultra-deep carbonate reservoirs with high temperatures. To fill this knowledge gap, a series of acid-etching experiments were performed utilizing the viscoelastic surfactant-based (VES) acid and gelled acid. Fresh acid with 20 wt % and spent acid with 15 wt % concentration were used to represent the acid at the fracture-wellbore interface and that in the inner fracture. A 3D laser scanner was applied to collect fracture morphology data, which were further used to build the 2D fracture-surface roughness model via the fractal theory. Based on the model, the distribution of acid-fracture width and its roughness were exhibited, while the conductivity was tested under different closure stress. Results indicate that in comparison with gelled acid, the fresh VES acid with low viscosity exhibits strong corrosion of rock and results in the heaviest dissolved-rock weight and the maximum acid-fracture surface roughness, but the situation is just the opposite for spent acid. Conductivity shows obvious segmentation characteristics because the acid fracture narrows with the increase of closure stress. Three contact modes of acid-fracture surface cause the loss of conductivity, including longitudinal strip-like, transverse strip-like, and longitudinal and transverse flaky modes. The VES acid is selected in the YSW block based on the experimental results. The field application proves that VES-acid hybrid fracturing could result in a large increase in well production. This study gives a new horizon to selecting reasonable acid systems by testing acid-fracture conductivity.

Evaluation of deep penetration of high-temperature sustained-release acid based on the reaction kinetics and conductivity of acid-etched fractures

Aiming at improving the acid fracturing effect of carbonate reservoirs at high temperatures, reaction kinetics and acid fracture conductivity of five high-temperature sustained-release acids were investigated. The experimental results show that viscous acid, gelled acid II, authigenic acid I, and gelled acid I have the highest rate constant, respectively. It was found that as the temperature increases, the reaction order of gelled acid I, viscous acid, and authigenic acid II decreases, while the reaction order of gelled acid II and authigenic acid I increase first and then decreases. Among the studied cases, the maximum reaction rate is 4.46 × 10 −5 mol/(L·s), which was achieved in gelled acid I. Variations of the reaction rate indicate that gelled acid I is in the “saturation state of the reaction’’. At a closing pressure of 40 MPa and a temperature of 150 °C, the acid corrosion fracture conductivity of gelled acid I is the largest, reaching 7.45 × 10 −2 μm 2 m, the fracture conductivity of authigenic acid I was higher than that of autogenic acid II, and the conductivity of the viscous acid was the lowest.

Acid System and Stimulation Efficiency of Multistage Acid Fracturing in Porous Carbonate Reservoirs

With little to no natural fracture development and the high calcite content in porous carbonate reservoirs, for multistage acid fracturing, different fluids are used to form a viscous fingering in the fracture, thus enhancing the degree of nonuniform etching. However, existing studies on multistage acid fracturing mainly focused on the combination of fracturing fluid and acid, which is less specific for porous carbonate rocks. Here, the rheological properties of five fluids, including guar-based fluid, cross-linked guar, gelled acid, cross-linked acid, and diverting acid, were studied at each temperature condition, and the viscosity relationship between each fluid was clarified. Based on the rheological properties, the differences between the seven liquid combinations on the etched morphology of the fracture walls were studied and analyzed. The conductivity of the seven acid-etched fractures under different closure stress was simulated. The experimental results showed that the viscosity relationships between the fluids at different temperatures were cross-linked guar > cross-linked acid > diverting acid (spent acid) > gelled acid > guar-based liquid > diverting acid (fresh acid). Because cross-linked acid has higher viscosity than gelled acid, it can form more obvious viscous fingering with a variety of liquids, which is more suitable for acid fracturing stimulation of porous carbonate reservoirs. In addition, the combination of cross-linked and diverting acids was screened out. The multistage alternate injection of this fluid combination could form tortuous and complex etching channels, and its acid-etching fracture conductivity was significantly higher than that of other fluid combinations at different closure stress. In this study, we optimized the fluid combination of porous carbonates and clarified the effect and mechanism of nonuniform etching to provide guidance for the fluid combination selection of multistage alternate acid fracturing process for porous carbonate reservoirs.

Development of gelled acid system in high-temperature carbonate reservoirs

Acid fracturing technology is an effective way to reconstruct carbonate reservoirs. However, considering the characteristics such as strong heterogeneity, deep burial, and high temperature, the conventional acid fracturing system cannot be used in the low-permeability area effectively. Aiming at resolving this problem, Tarim Oilfield was taken as the research object and a new high-temperature-resistant gelled acid system with modified xanthan gum was developed in the present study. Then the temperature resistance and acid resistance of the thickening agents were tested through experiments. Moreover, the conductivity of acid-etched fractures was studied. The acid flow in the fracture was simulated to study the deep penetration capacity of the acid system. The obtained results show that the gelled acid system has good temperature resistance, acid resistance, and shear resistance, and the viscosity of the acid system at 160 °C remains 21 mPa s. The acid pressure experiment with a developed gelled acid system shows that the acid pressure effect is significant at indoor high temperatures of 160 °C. At a confining pressure of 30 MPa, the acid-etched fracture has a conductivity of 9.39D•cm. It was found that as the experimental ambient temperature increases, the conductivity of acid etching fracture gradually increases and stabilizes at 9.5D•cm. Finally, a “pink-out” gradient reduction appears in the acid concentration of the fracture, and flows along the fracture to the deep part of the formation, and propagates the fracture. It is concluded that the proposed method can be applied to communicate with the reservoir to increase production and injection. This article is expected to provide a theoretical foundation for the exploration of high-temperature resistance xanthan gum in the field of acid compression.

Case Study of Successfully Staged Acid Fracturing on the Ultra-Deep Horizontal Well for the Qixia Fm HTHP Tight Carbonate Gas Reservoir in China

The lower Permian Qixia formation gas reservoir is characterized by ultra-deep buried, high temperature and high pressure (HTHP), low porosity and permeability, ultra-high closure pressure, and thin thickness in the western Sichuan Basin, China. Acid fracturing has been adopted as the main well stimulation technology. The average test production of the acid-fractured vertical well was 46 × 104 m3/d. The horizontal well with staged acid fracturing is considered a reliable measure to enhance gas production. However, the staged acid fracturing on the ultra-deep horizontal well has been challenged. The reaction rates of conventional, self-diverting, and gelled acid and the acid-etched fracture conductivity experiments under HTHP are measured to optimize the acid fracturing technique. The reaction rate of gelled acid with Qixia Fm limy dolomite at 150°C is 3.4–6.7 times faster than that with dolomite and similar to that with limestone. The gelled acid with better retardation and friction reduction performance is selected as the main acid to increase the effective acid-etched fracture length. The acid-etched fracture conductivity can be greatly enhanced by using gelled acid, increasing acid volume, and closed fracture acidizing (CFA) under a closure stress of 50 MPa. Moreover, the feasibility of staged acid fracturing for ultra-deep horizontal wells is discussed. The first stage is assembled by fracturing sleeve, spared ball drop sleeve, and circulating valve to ensure activation of the fracturing channel. The tested production is significantly enhanced with temporary plugging and open-hole packer and sliding sleeve staged acid fracturing.

Research and application of stimulation fluid technology for deep marine carbonate rocks

In view of the characteristics of deep marine carbonate reservoirs in the Sichuan Basin, such as a deep burial depth (5000–8120 m), high temperatures (150°C-180 °C), a large inclination toward wells, and large acid-rock reaction rate, a high-temperature gelled acid and a preflush containing a self-generating acid that matched with the high-temperature gelled acid were developed. The results of laboratory evaluation showed that the viscosity of the high-temperature gelled acid at 180 °C was 30 mPa·s, with a corrosion rate lower than 70 g/m2·h.The drag reduction and acid-rock reaction rates reached 64%–71% and 1.569 × 10−4 mol/cm2·s respectively, demonstrating obvious retarding performance. In addition, the viscosity of the preflush containing a self-generating acid (after shearing at 160 °C for 1 h)remained at 200 mPa·s. Compared with a full injection of the high-temperature gelled acid, the acid-etched fracture conductivity after an alternate injection of the preflush containing self-generating and high-temperature gelled acids increased by more than 50%. Field application showed that the high-temperature gelled acid played a vital role in connecting fractures where as the preflush containing self-generating acid demonstrated an outstanding fracture-formation ability. After a three-stage alternate injection (a preflush containing self-generating and gelled acids), a remarkable stimulation effect was achieved for increase natural gas production.

Research and Application of Segmented Acid Fracturing by Temporary Plugging in Ultradeep Carbonate Reservoirs.

The ultradeep carbonate reservoir in Sichuan Basin is characterized by deep burial depth, high temperature, and strong heterogeneity. In the early stage of production, the vertical well acid fracturing is the main reservoir stimulation method, and the horizontal well stimulation technology is not mature enough to release the production capacity of gas wells. Segmented acid fracturing of the ultradeep horizontal wells currently faces the following problems: the strong heterogeneity of reservoir leads to the difficulty of fine segmentation; the high reservoir temperature requires higher performance of working fluid; the reaction rate between acid and rock is fast and the action distance of acid is short, and there is low fracture conductivity under high closure stress. In view of the above problems, the fine segmented design method was studied, and the high-temperature-resistant authigenic acid and gelling acid systems were developed. The viscosity of authigenic acid is greater than 150 mPa s after shearing at 160 °C and 170 s–1 for 50 min, and the highest acid generation concentration is 4.05 mol/L. The gelling acid system has both the properties of high-temperature resistance and low friction resistance; not only canit meet the requirements of the retarding rate and the corrosion inhibition ability when the reservoir temperature is 160 °C but also the resistance reduction rate is up to more than 70%. By alternating injection of authigenic acid and gelling acid, the acid-etched fracture length and conductivity were, respectively, increased by 80% and 45%. The application of this technology in the horizontal well of the ultradeep carbonate reservoir in Sichuan Basin can increase the productivity by 3 times when compared with the vertical well acid fracturing, and a better stimulation effect has been achieved.

Simulation and evaluation for acid fracturing of carbonate reservoirs based on embedded discrete fracture model

Acid fracturing is an important means of reservoir stimulation, whose purpose is to form an incompletely closed acid-etched fracture as the flow channel for oil and gas during production. The length and conductivity of acid-etched fractures can be used to evaluate acid fracturing and directly impact production. To study their influence on the stimulation effect and final production, an acid fracturing coupling model including a fracture propagation model coupled with reservoir flow and temperature field models is established for the first time in this study based on an embedded discrete fracture model (EDFM), which can realize the coupling of fracture propagation and reservoir flow and simplify the solution of fracture and reservoir temperatures. The simulation results of the acid fracturing coupling model are introduced into the productivity model, which is also based on the EDFM to analyze and evaluate well productivity. The results show that: (1) the EDFM can easily couple fracture propagation and reservoir flow and can be used to rapidly solve the temperature fields in the fracture and reservoir successfully for the first time. (2) Reservoir flow impacts the propagation of fractures by increasing or decreasing the leak-off velocity of the working fluid. (3) Temperature diffusion is much weaker than pressure diffusion during acid fracturing and is limited near the acid fracture. The reaction between the acid and rock increases the local temperature around the acid fracture, and may even exceed the initial formation temperature. (4) Raising the injection rate reasonably enhances H+ diffusion, increases the effective length of acid-etched fractures, enlarges the drainage area of oil and gas, and benefits long-term well production.

Interaction Law between Natural Fractures-Vugs and Acid-Etched Fracture during Steering Acid Fracturing in Carbonate Reservoirs

Steering acid fracturing is a technique that improves the conductivity of carbonate reservoir. It is widely used in a carbonate reservoir. However, due to the lack of comparative experiments, the application of steering acid to improve the fracturing results is still unknown. Therefore, a series of true triaxial acid fracturing experiments were conducted to study steering acid fracturing in carbonate reservoir. The carbonate specimens used in the experiment were from the Qixia group and Dengsi Member in Sichuan, China. In this study, slick water, cross-linked gel, and self-generating acid were used as ahead fluid to cooperate with steering acid. Experimental results show that (1) the low-viscosity ahead fluid with steering acid can result in more complex fractures; (2) the complexity of fractures is influenced by natural fracture and the viscosity of the ahead fluid; and (3) based on the 3D scanning results of the fracture surface, different ahead fluids will lead to different corrosion results. This study provides useful suggestions on steering acid fracturing design and physical simulation experiments.

Large-Scale Acid Fracturing Based on a Large-Scale Conductivity Apparatus.

The conductivity of an acid-etched fracture is a necessary indicator for the stimulation of dolomite formation, which affects commercial development. The widely accepted test method involves the use of a small-scale conductivity cell for etching and measuring conductivity. However, the field production reflects that the actual conductivity does not match the measured one and is usually lower. Consequently, the existing studies had limitations and hence the stimulation mechanism needed to be explored further. To understand it more realistically, a novel large-scale apparatus was used in this study to test the conductivity of the acid-etched fracture. The use of this apparatus avoided the near-core excessive eroding and weak heterogeneity with continuous etching in a 1000 mm fracture. The results showed that the conductivity was indeed dissimilar to that in small-scale tests. The morphology of etched large-scale cores featured diversity and complexity, including deep and punctate channels, nonuniform pitting grooves with connected channels, and scale-shaped wavy grooves, which exactly demonstrated the multiple morphology under the influence of carbonate heterogeneity in real reservoirs. Moreover, the effect of increasing injection rate led to the unique etching morphology of scale-shaped wavy and pelviform grooves because of scouring flow and turbulence effects. The degree of surface roughness promoted nonuniform etching along the longitudinal and propagation direction, thus enhancing the conductivity of the whole fracture and confirming that the field treatment limited the pressure rather than the injection rate. The conductivity under different acid type, acid concentration, reaction temperature, and injection rate conditions was lower than that reported, confirming the experimental deviation in small-scale conductivity. The proposed large-scale apparatus test represented the acid-etched fracture conductivity more realistically, thus proving beneficial for the development of carbonate reservoirs.

Experimental study on surface morphology and relevant plugging behavior within acid-etched and unetched fractures

Plugging and diverting acid fracturing is an indispensable and promising technology that aims to enhance productivity for complex carbonate reservoirs. The key to success for this technology is the formation of effective plugged zones within preexisting fractures. However, because fracture pattern before and after acid-etching is quite different, the plugging behavior of fibers and particulates within acid-etched fractures is correspondingly distinct with respect to within smooth and hydraulic fractures. Till now, a number of researches have been studied on plugging experiments with various apparatuses. However, the plugging laws within acid-etched fracture are still unclear, and the experimental results are insufficient in revealing specific plugging laws, particularly with respect to fracture morphology. In this paper, we propose new parameters to analyze fracture morphology and introduce an integrated experimental method as well to study plugging behavior within fractures with different morphologies. Firstly, we obtain acid-etched fracture profiles with a 3D scanner and reproduce the rough physical-model fracture using 3D printing technology. Then, we perform a series of experiments to grasp plugging laws. Finally, in terms of fracture features analysis and experimental results, we further analyze the correlation between plugging behavior and surface morphology. The calculation results of digital parameters show that after acid-etching the roughness of the whole fracture increases due to acid-rock interaction, and channels are formed along the flow direction. Meanwhile experimental results show that flat fractures are relatively difficult to be plugged with respect to rough fractures because it has corresponding lower abilities to capture fibers and particulates. Pure fibers can independently plug 2 mm-wide fractures, as fracture width increased, the addition of particulates will improve the plugging efficiency. Additionally, fracture surface morphology has a huge influence on plugging behavior and dominate the possibility of plugging formation. Plugged zones generally are formed in the roughest areas of the fracture.