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Natural fractures are oftentimes not orientated in the direction of in-situ principal stresses. When developing naturally fractured reservoirs, the complex fracture network is affected by the shear stresses acting on the fracture surfaces. In some rare cases, the main fracture may not align with the principal stresses. Under such circumstances, shear stresses and displacements during fracture closure will impact the overall conductivity, especially for acid fracturing treatment, where flow paths are supported by the roughness of unevenly acid-etched fracture walls.
In this paper, we present a new approach to model acid fracture conductivity by allowing combined normal and shear loading on fracture walls. The closure of rough surfaces is modeled as elastic deformation of asperities. The model considers the mechanics of asperity contacts and statistical descriptions of interface roughness and asperity contact orientation for shear slippage between asperities. A numerical simulation algorithm is implemented to evaluate the fracture displacement under complex loading conditions. The overall conductivity is then obtained from numerical simulation based on deformed fracture apertures. The model was validated with the experimental observation under normal closure condition. A parametric study was carried out to evaluate the rock mechanical properties and fracture geometries on the conductivity behavior of acid fracturing.
Modeling results show that the interface closure behavior can be significantly affected by the asperity contact orientations. If slipping among asperities is considered, larger deformations during fracture closure are observed from simulation comparing to results from conventional fracture closure models based on summit-to-summit asperity contact. Sensitivity analysis indicates that the overall conductivity of a fracture depends on mechanical properties of the rock, distribution of asperity contact orientation, and shear loading conditions.
The new approach proposed in this paper provides access to acid fracture conductivity estimation under various in-situ stress conditions and fracture orientations. This model can be helpful in evaluating fracture treatment in reservoirs with natural fractures or complex subsurface structures.
Lai, Jie (Southwest Petroleum University) | Guo, Jianchun (Southwest Petroleum University) | Wu, Kaidi (Southwest Petroleum University) | Chen, Chi (Southwest Petroleum University) | Wang, Kun (Southwest Petroleum University) | Wang, Shibin (Southwest Petroleum University) | Lu, Cong (Southwest Petroleum University) | Zhao, Xuepei (Southwest Petroleum University) | An, Huan (The Fourth Exploit Factory Huabei Oilfield Company)
ABSTRACT: Fracture surface features and proppant are the two essential factors controlling hydraulic fracture conductivity. In this work, we measured the effect of fracture surface and proppant parameters on fracture conductivity based on duplicated rough fracture surfaces. To simulate the real hydraulic fracture, large number of artificially split rock samples were collected from shale outcrops. Rough fracture surface was scanned and carving technique was utilized to duplicate rough fracture surfaces on flat rock samples. After that, unpropped and propped fracture conductivity was tested. Experimental results indicated that shale samples perpendicular to bedding exhibited higher strength and rougher fracture surface, resulting in higher unpropped fracture conductivity than samples parallel to bedding. There was not a strong positive relationship between displacement and unpropped conductivity. Partial-monolayer proppant placement could bring high conductivity under low closure stress, while multilayer proppant placement sustained enough conductivity under high closure stress. Optimal proppant parameters were changed with fracture surface roughness and closure stress. Taking initial roughness of fracture surface into account, experimental results can allow us to understand the collation of rough fracture surface and proppant, and help field engineers to make rational choices for the selection of proppant parameters in shale gas fracturing.
In shale gas fracturing, fracture conductivity depends not only on proppant parameters, but also on fracture surface features. There is a complex fracture network after shale fracturing, composed of unpropped and propped fracture. It is of high significance to figure out how to satisfy the respective fracture conductivity requirement in the fracture network. Unpropped fracture conductivity has been routinely tested using artificially split rock samples. However, testing propped fracture conductivity between smooth-surface fractures has been the standard method to optimize proppant parameters for years. In reality, fracture surface would not be smooth in real condition and the effect of initial roughness of fracture surface should be emphasized.
Lai, Jie (Southwest Petroleum University) | Guo, Jianchun (Southwest Petroleum University) | Chen, Chi (Southwest Petroleum University) | Wu, Kaidi (Southwest Petroleum University) | Ma, Huiyun (Petro China Southwest Oil & Gasfield Company) | Zhou, Changlin (Petro China Southwest Oil & Gasfield Company) | Wang, Shibin (Southwest Petroleum University) | Ren, Jichuan (Southwest Petroleum University) | Wang, Zhi (Southwest Petroleum University)
Abstract As the most commonly used technology to exploit tight dolomite reservoirs, acid fracturing usually begins with injecting pad fluid to create rough-surface fractures, followed by pumping acid to form non-uniform etching on fracture surfaces. Thus, the etching pattern and acid fracture conductivity depend largely on initial character of rough-surface fractures. In this work, experiments were conducted to examine the effects of initial roughness and mechanical property of fracture surface on acid fracture conductivity. Eight artificially split core samples were collected from tight dolomite outcrops and classified into three categories based on the surface topography and splitting force curve. Rough fracture surfaces were scanned utilizing the 3D laser scanner. Then, dynamic acid etching tests were conducted, varying the acid flow rate and acid-rock contact time. Besides, the roughness of fracture surfaces were measured utilizing the 3D laser scanner again. After that, acid fracture conductivity was determined. The effects of acid flow rate, acid-rock contact time, fracture surface topography and mechanical property on acid etching and acid fracture conductivity were discussed. The experimental results demonstrated that the initial fracture surface topography and acid flow rate jointly controlled the acid etching pattern and the resulting acid fracture surface topography. The orientation of the fractures distributed on the fracture surface had significant effects on the acid fracture conductivity. Dissolved mass increased with longer acid-rock contact time. Longer acid-rock contact time brought higher acid fracture conductivity under low closure stress, while shorter contact time sustained higher acid fracture conductivity under high closure stress. Higher maximum splitting force referred to higher mechanical property, and more breaking stages referred to more microfractures developed. Rock samples with higher maximum splitting force and only one breaking stage exhibited higher acid fracture conductivity. This paper provides a systematic method to study the effects of initial roughness and mechanical property of fracture surfaces on acid fracture conductivity. Compared with the results based on smooth-surface fracture, the experimental results based on rough-surface fracture can guide acid fracturing design and optimization in a more accurate way. Accordingly, a cost-effective stimulation outcome can be expected.
Closure of rough surfaces under normal closure stress is investigated in this study. Rough surface closure model presented in this paper is based on surface asperity deformation. The main components of deformation are asperity compression and half-space deformation. Mechanical interaction among asperities which is a consequence of half-space deformation is considered in the model and its impact on the closure behavior is analyzed. Asperities are assumed to be elastic-perfectly-plastic materials and therefore may experience inelastic deformation under closure stress. Modeling results indicate that a significant portion of closure takes place earlier on at low stress levels because there are fewer asperities in contact initially. Asperity inelastic deformation is found to influence rough surface closure with its degree of impact depending on surface profile. A mechanical interaction sensitivity analysis indicates that neglecting interaction among asperities may lead to erroneous results particularly in surfaces with closely spaced asperities. By conducting an analysis on the elastic properties of asperity and half-space we found that the normal stiffness is much more influenced by Young’s modulus of half-space rather than that of asperity.
Acid fracturing is a stimulation technique which is being used in carbonate reservoirs. This technique is considered as an alternative to the well-known propped hydraulic fracturing. Fractures tend to close due to the in-situ stresses acting normal to the plane of fracture. Fracture closure has detrimental effect on the conductivity and therefore, should be prevented. Proppant is widely used in the hydraulic fracturing process and this material serves to keep the fracture open against closure stress. However, the mechanism by which the fracture is being held open is essentially different in acid fracturing technique.
Acid fracturing is a complex process in which acid reacts with rock and removes some parts of it, resulting in two random rough surfaces. Asperities on these surfaces act as pillars to keep the fracture open. Fracture surfaces come into contact after the pump pressure is dissipated. The success of an acid fracturing job depends on how well the asperities withstand the closure stress. Increasing effective stress often reduces the fracture aperture and its conductivity.
Chen, Chi (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University) | Wang, Shouxin (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University) | Lu, Cong (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University) | Wang, Kun (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University) | Lai, Jie (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University) | Liu, Yuxuan (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University)
Abstract Hydraulic fracturing technology provides a guarantee for effective production increase and economic exploitation of shale gas wells reservoirs. Propped fractures formed in the formation after fracturing are the key channels for shale gas production. Accurate evaluation of local propped fracture conductivity is of great significance to the effective development of shale gas. Due to the complex lithology and well-developed bedding of shale, the fracture surface morphology after fracturing is rougher than that of sandstone. This roughness will affect the placement of the proppant in the fracture and thus affect the conductivity. At present, fracture conductivity tests in laboratories are generally based on the standard/modified API/ISO method, ignoring the influence of fracture surface roughness. The inability to obtain the rock samples with the same rough morphology to carry out conductivity testing has always been a predicament in the experimental study on propped fracture conductivity. Herein, we propose a new method to reproduce the original fracture surface, and conductivity test samples with uniform surface morphology, consistent mechanical properties were produced. Then, we have carried out experimental research on shale-propped fracture conductivity. The results show that the fracture surfaces produced by the new method are basically the same as the original fracture surfaces, which fully meet the requirements of the conductivity test. The propped fracture conductivity is affected by proppant properties and fracture surface, especially at low proppant concentration. And increasing proppant concentration will help increase the predictability of conductivity. Due to the influence of the roughness of the fracture surface, there may be an optimal proppant concentration under a certain closure pressure.