Browsing by Subject "Acid fracturing"
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Item Mechanical and Hydraulic Behavior of Acid Fractures - Experimental Studies and Mathematical Modeling(1997-12) Gong, Ming; Hill, Daniel A.Acid fracturing is a well stimulation method commonly used in carbonate reservoirs. In the process, an HCl solution, sometimes viscosified or emulsified, is injected into the formation above the fracture pressure to create a fracture or to open existing natural fractures. Acid etches the fracture faces unevenly, leaving a conductive pathway for reservoir fluids to flow into the wellbore. The key to a successful acid fracturing is the achievement of acid penetration and the creation of sufficient fracture conductivity. Much research has been done to study the acid penetration in acid fracturing. However, the hydraulic conductivity created by acid etching is not well understood. There is an empirical correlation available to evaluate acid fracture conductivity, which was reported by Nierode and Kruk over 20 years ago. Acid etching is a stochastic process and the resulting hydraulic mechanisms of acid fractures are complex. The conductivity is affected by the aperture and contact area of the fracture under closure stress. The damage of the rock strength at the fracture surfaces by acid adds complexity to the prediction of hydraulic conductivity of acid fractures. The leakoff of acid into the formation through the fracture faces makes the situation even more complex. Acid contact time, acid leakoff, rock mechanical properties, and formation heterogeneity all affect the creation of hydraulic conductivity of an acid fracture. This work explores the mechanisms of hydraulic conductivity of acid fracture in two ways. The first is a systematic experimental study of the creation of acid fracture conductivity, including characterization of surface roughness created by acid etching, investigation of the damage of rock compressive strength by acidizing, and measurement of hydraulic conductivity under closure stress. To study the effect of rock mechanical properties on the creation of hydraulic conductivity of acid fracture, important mechanical properties of the rock sample have been carefully measured. In order to understand the damage of rock strength by acid, the microstructures at the grain scale of core samples have been examined. Experimental data have shown that longer acid contact results in rougher fracture surface and, in tum, higher hydraulic conductivity. The second focus of this work is the mathematical modeling of acid fracture conductivity. Several different theoretical models for fracture conductivity have been reviewed and examined. Based on our experimental results, a new fracture deformation model was derived with a consideration of both the surface roughness and the rock mechanical properties. The roughness of acid etched surfaces as well as the rock strength have been correlated to acidizing conditions. The fracture closure under stress is modeled with. the plastic deformation of asperities. Finally, a cubic law is used to calculate the fracture conductivity. The prediction of acid fracture conductivity using this model with appropriate parameters shows excellent agreement with experimental data.Item Numerical simulation of acid stimulation treatments in carbonate reservoirs(2021-08-13) Dong, Rencheng; Wheeler, Mary F. (Mary Fanett); Mohanty, Kishore K; DiCarlo, David A; Okuno, Ryosuke; Saaf, FredrikMatrix acidizing and acid fracturing are two main types of acid stimulation treatments that are extensively employed by industry in carbonate reservoirs to improve permeability and enhance production. Matrix acidizing involves injecting acid to dissolve minerals in order to create long highly conductive channels (wormholes) whereas acid fracturing is used to etch fracture surfaces and create fracture conductivity. Numerical modeling of acid stimulation treatments couples processes of fluid flow, reactive transport, and rock dissolution, which imposes great computational challenges. The purpose of this dissertation is to develop efficient and accurate numerical models for acidizing process and acid fracturing process respectively. In most of matrix acidizing simulations, acid transport is generally solved by a single-point upwinding (SPU) scheme based on finite volume method. Simulation results of wormhole growth may have large numerical errors due to grid orientation effect of SPU scheme. In this work, we apply adaptive enriched Galerkin (EG) methods for solving coupled flow and reactive transport equations of acidizing model. EG is constructed by enriching the standard continuous Galerkin (CG) finite element method with piecewise constant functions. Since EG is a higher-order method compared with standard finite volume method, EG reduces non-physical numerical errors caused by grid orientation effect. Wormhole growth usually exhibits fingering patterns, which requires very fine mesh to resolve. Instead of global mesh refinement, we apply adaptive mesh refinement technique to dynamically refine the mesh in the vicinity of wormhole interfaces and coarsen the mesh after dissolution fronts pass. The simulation runtime using adaptive mesh is only about 30% of the runtime using globally refined mesh in our numerical examples. The key to success in acid fracturing treatments is to achieve non-uniform acid etching on fracture surfaces. Carbonate reservoir heterogeneity such as heterogeneous mineral distribution can lead to non-uniform acid etching. In addition, the non-uniform acid etching can be enhanced by the viscous fingering mechanism. By injecting a low-viscosity acid into a high-viscosity polymer pad fluid, acid tends to form viscous fingers and etch fracture surfaces non-uniformly. Acid fracturing simulations rarely modeled the effect of acid viscous fingering. In this work, a 3D acid fracturing model is developed to simulate acid etching process with acid viscous fingering. Our acid fracturing model considers fluid flow inside the fracture, acid and polymer transport, and change of fracture geometry due to mineral dissolution. A numerical simulator is developed to solve the acid fracturing model and compute the rough acid fracture geometry induced by non-uniform acid etching. We investigate the effects of viscous fingering, perforation design, and alternating injection of pad and acid fluids on the acid etching process. Our model is capable of simulating growth of acid-etched channels caused by acid viscous fingering. According to our simulation results, properly increasing the number of perforations can restrain the height of acid-etched channels and help sustain acid fracture conductivity under the reservoir closure stress. Compared with single-stage acid injection, multi-stage alternating injection of pad and acid fluids leads to narrower and longer acid-etched channels, which improves the effectiveness of acid fracturing treatments.Item Unpropped fractures in shale : surface topography, mechanical properties and hydraulic conductivity(2017-12) Wu, Weiwei, Ph. D.; Sharma, Mukul M.; Balhoff, Matthew T; Daigle, Hugh; Espinoza, Nicolas; Gale, Julia F.W.A large proportion of the hydraulic fractures created during a hydraulic fracturing treatment remain unpropped after hydraulic fracturing despite the significant quantities of proppant injected in the process. These fractures either have a fracture width smaller than the size of the proppants, or are too far away from the wellbore where proppant cannot reach. Their presence has been demonstrated and corroborated by multiple independent sources of evidence such as flowback, production and microseismic data. These unpropped fractures present a huge potential for production enhancement, since they possess a very large contact area with the reservoir. Unfortunately, this potential flow area is closed by the closure stress during production. Without the presence of proppants, unpropped fractures are expected to behave differently from propped fractures. In this study, fracture conductivities of unpropped fractures in shales are measured with preserved Eagle Ford and Utica shale cores to better understand their closure behavior, in particular those after exposure to fracturing fluids. The unpropped fractures exhibit fracture conductivities 2 to 4 orders of magnitude lower than those of propped fractures, and are more sensitive to closure stress. Plastic deformation is found to dominate the closure process, and strong hysteresis occurs in unpropped fracture conductivity with a 70-80% reduction after a loading-unloading cycle of closure stress. Exposure to water-based fracturing fluids reduces unpropped fracture conductivity by shale softening or fines production. Unpropped fracture conductivities also appear to be sensitive to shale mineralogy, which affects the shale mechanical properties and shale-fluid interaction. A numerical model is developed to simulate the closure of unpropped and natural fractures, and to compute their corresponding fracture conductivity. A conjugate gradient algorithm and fast Fourier transform technique are incorporated to dramatically enhance the computation efficiency. Plastic deformation and deformation interaction among asperities, ignored in some previous models, are considered and shown to play an important role in the closure process. The model is validated against analytical solutions and experiments, for both elastic-only and elastoplastic scenarios. The compliance of unpropped fractures is demonstrated to be sensitive to the roughness and hardness of fracture surfaces, while less affected by Young’s modulus. The new model is also capable of simulating closure of heterogeneous fracture surfaces. More plastic deformation and lower fracture conductivity is measured when surfaces with high clay content are used. Given the same mineralogy, the mineral distribution pattern shows a smaller impact on the closure behavior. The possibility of employing acid fracturing to stimulate unpropped fractures is also explored. The acid-etched topography of shale fracture surfaces is found to be dependent on both the content and distribution of the carbonate minerals. Shales with a high carbonate content (over 60 wt%) generally tend to develop rougher acid-etched surfaces. However, more carbonate content does not always necessarily lead to increased etched roughness. High etched roughness is more likely developed from a blocky, rather than scattered, distribution of carbonate minerals. A new experimental method, the “half-core approach”, is formulated to address the challenge caused by shale heterogeneity in experimentally evaluating and comparing fracture performance. The half-core approach splits one shale core into two half cores, which are then subjected to treatments of interest independently, followed by assemblage into individual full cores with a spacer for fracture conductivity measurement. The half-core approach is effective in creating a baseline with reduced sample variation among shales to improve evaluation of fracturing fluids. Similar mineralogy and mechanical properties are found between half-cores among preserved shale samples spanning a wide range of mineralogy from Barnett, Eagle Ford, Haynesville and Utica shales. By applying the half-core approach, acid fracturing is systematically benchmarked against brine with Eagle Ford shales categorized into low (below 40 wt%), medium (40-70 wt%) and high (over 70 wt%) carbonate content. Compared to brine exposure, non-uniform acid fracturing enhances unpropped fracture conductivities for shales for a wide range of carbonate contents, while uniform acid fracturing generally leads to lower fracture conductivities due to shale softening. The unetched zone in non-uniform etching reduces shale softening and creates a surface topography that enhances fracture flow. Channels are more likely to form in carbonate-rich shale (over 70 wt%). Development of channels substantially increases the unpropped fracture conductivity, and reduces the hysteresis of unpropped fracture conductivities to closure stress. The presence of carbonate veins is found to promote the development of non-uniform etching.