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For the change in unpropped fracture conductivity after fracturing in tight sandstone gas reservoirs, an experimental method for unpropped fracture conductivity evaluation with fracture wall simulation was established to investigate the damage mechanism of conductivity in terms of the microscopic morphology, roughness, strength and other aspects of the fracture wall, and to clarify the variation law of fracture conductivity. The study shows that after the fracture was exposed to water, the wall clay was hydrated and compacted under stress, and the average height of the wall was decreased by 8. 5%; meanwhile, the fracture wall was softened and the average hardness decreased by 34. 3%. The more frequent the change in production nozzle size, the higher the conductivity of the unpropped frac- ture under high stress; the fracture conductivity of the third well opening was 91. 7%-98. 5% lower than that of the first well opening; the conductivity of misaligned fractures was 18. 1-140. 4 times that of non-misaligned fractures. With the formation water displacing fracturing fluid after fracturing, the conductivity of the final fracture was 3. 45 times that of the original fracture. In this paper, the conductivity damage mechanism in the production of tight gas reservoirs was defined, and the variation law of unpropped fracture conductivity under the action of different factors was clarified, which provides a basic theoretical basis for the protection of unpropped fractures in tight sandstone gas reservoirs. [ABSTRACT FROM AUTHOR]

Abstract Hydraulic fracture is widely performed to enhance oil and gas production in the development of unconventional reservoirs. This paper proposes a numerical model for the calculation of the unpropped fracture conductivity of shale. This numerical model consists of three basic models, namely, the numerical reconstruction model of fracture morphology, fracture deformation model under closed stress, and fluid flow model. The numerical model can be used to analyze the influencing factors of the unpropped fracture conductivity of shale quantitatively. The results demonstrate that the fracture misalignment of shale is a prerequisite for unpropped fractures with conductivity at high closed stress. Furthermore, the unpropped fracture conductivity of shale decreases exponentially with the increase in closed stress. Moreover, closed stress is the factor having the most significant influence on the unpropped fracture conductivity of shale, followed by roughness and fracture morphology. The negative effect of closed stress can be reduced by increasing the fracture morphology during the fracturing operation.

Hydraulic fracture is widely performed to enhance oil and gas production in the development of unconventional reservoirs. This paper proposes a numerical model for the calculation of the unpropped fracture conductivity of shale. This numerical model consists of three basic models, namely, the numerical reconstruction model of fracture morphology, fracture deformation model under closed stress, and fluid flow model. The numerical model can be used to analyze the influencing factors of the unpropped fracture conductivity of shale quantitatively. The results demonstrate that the fracture misalignment of shale is a prerequisite for unpropped fractures with conductivity at high closed stress. Furthermore, the unpropped fracture conductivity of shale decreases exponentially with the increase in closed stress. Moreover, closed stress is the factor having the most significant influence on the unpropped fracture conductivity of shale, followed by roughness and fracture morphology. The negative effect of closed stress can be reduced by increasing the fracture morphology during the fracturing operation. [ABSTRACT FROM AUTHOR]

Unpropped fractures constitute a major part of the fracture network in shale after hydraulic fracturing, and their closure process under in situ stress has a significant effect on the well production. In this study, a 3D simulation method based on core experiments and reverse engineering technology was established to simulate the unpropped fracture surface. A contact and deformation model of rock with a rough boundary and normal stress was constructed to investigate the closure process. The plastic characteristics of shale were taken into account through the embedment of the Drucker–Prager yield criterion. An unpropped fracture closure experiment was conducted for validating the model, and the influence of stress state, mechanical properties, fracture roughness and slippage degree on the closure process was analyzed. Numerical results demonstrate that an unpropped fracture with greater roughness and slippage degree possesses more flow regions and a larger residual width under a low stress state, which decreases rapidly as the normal stress rises because of the small contact area. Young's modulus is considered to have a positive effect on resisting fracture closure and maintaining effective flow regions under a high stress state. For the shale of the deep Longmaxi formation in south Sichuan, China, the simulation results indicate that the unpropped fractures always retain a residual width greater than 0.7 mm under a closure stress of 60 MPa, which is suitable for flow passages of oil and gas in shale. Methods for moderating the closure process were also analyzed. The numerical simulations provide insights on the unpropped fracture closure process under in situ stress, which will be helpful for fracturing design and production prediction of shale wells. • A 3D simulation method was established to simulate the unpropped fracture surface. • A contact and deformation model of rock with a rough boundary and normal stress was constructed. • An unpropped fracture with greater roughness and slippage degree possesses a larger residual width under a low stress. • Young's modulus has a positive effect on resisting the closure of unpropped fracture. [ABSTRACT FROM AUTHOR]

Wells with poorly propped fracture are often considered to be refracture candidates. However, neglecting the influence of unpropped segments in analyzing transient pressure data from a well can produce large errors in characterization of the orientation of the refracture. To better understand the refracture orientation, we derived a semi-analytical model for pressure-transient analysis of poorly propped fractured wells intercepting reoriented refracture in low-permeability gas reservoirs. Our initial fracture-flow model considered unpropped segments with lower conductivity than propped segments. The resulting solutions display underlying characteristics (notably on log-log plots); these characteristics will be especially useful to determine the refracture orientation. We observed that the transient pressure behavior during specific flow regimes will be distorted by poorly propped fracture. The paper discusses the effect of refracture orientation on transient pressure behavior, and further examines the pitfalls in the characterization of the orientation of poorly propped fracture wells. The results can provide a benchmark to estimate fracture parameters in field applications. In addition, we simulated a synthetic case to develop the relationship between refracture orientation and pressure responses during pseudo-boundary-dominated flow. This relationship provides a quantitative basis to directly estimate the refracture orientation by analyzing transient pressure data. • We present a model for analyzing transient behavior of poorly propped fractured wells with refracture reorientation. • The initial fracture is considered as partially propped fracture. • We provide a relationship to characterize the refracture orientation quantitatively. [ABSTRACT FROM AUTHOR]