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Abstract The interaction of hydraulic fractures with the pre-existing natural fractures may play a major role in increasing productivity from unconventional formations. When a hydraulic fracture meets a natural fracture, the hydraulic fracture can cross the natural fracture or be arrested. If the natural fracture is permeable, fracturing fluid can leak from the hydraulic fracture into the natural fracture causing elevation of pore pressure in the natural fracture and reducing the effective normal stress acting on the natural fracture, which could then lead to shear failure or slippage along the natural fracture plane. Shear-slip causes dilation, potentially increasing fracture conductivity and enhancing fluid flow deeper into the natural fracture. The conductivity of unpropped shear-induced fractures can play an important role in enhancing the productivity from ultralow-permeability formations like shale. In this paper, we first evaluate analytically the shear-slip condition and its propagation along a natural fracture under remote normal and shear stresses, when it is exposed to the fluid pressure in a hydraulic fracture. Analytical approximations under some limiting conditions are considered. A rigorous 2D numerical model based on coupling between fluid flow and rock deformation using displacement discontinuity method and fluid flow in the fracture is then described. The results of numerical simulations are presented to illustrate the effect of rock stress anisotropy, initial natural fracture conductivity, and fluid properties on the evolution of the fluid and slip fronts along the natural fracture and the associated permeability enhancement. 1. INTRODUCTION In the last decade, following the success of horizontal drilling and multistage fracturing in the Barnett Shale, exploration and drilling activities in shale gas and shale oil reservoirs have skyrocketed in the US and abroad. Economic production from these reservoirs depends greatly on the effectiveness of hydraulic fracturing stimulation treatment. Microseismic measurements and other evidence suggest that creation of complex fracture networks during fracturing treatments may be a common occurrence in many unconventional reservoirs [1-3]. The created complexity is strongly influenced by the preexisting natural fractures and in-situ stresses in the formation. To optimize the fracture and completion design to maximize the production from these reservoirs, engineers must have a good understanding of the fracturing process and be able to simulate it to obtain information such as the induced overall fracture length and height, propped versus unpropped fracture surface areas, proppant distribution and its conductivity, and potential enhanced permeability through stimulation of the natural fractures.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.95)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- North America > United States > Mississippi > Woodlands Field (0.89)
Abstract The majority of planar hydraulic fracture models use two distinct approaches. The first one, referred to as the planar 3D model, is more accurate but also very CPU intensive. The second one is referred to pseudo-3D (P3D) model, and separately considers the vertical growth and horizontal propagation of the fractures. This approach is less CPU intensive, but requires the fracture being initiated in the lower stress layer. In practice, this assumption is not always verified, and the fracture height growth can become unstable. This paper presents a new model as an enhancement of the P3D, which consists of multiple rows of elements vertically stacked and connected. For each row of elements, the assumption of the fracture front being in the lower stress layer is satisfied locally. The width profile and stress intensity factor at the top and bottom of the fracture depend on the stress profile and the pressure profile along the stack of elements. This model predicts the fracture height more accurately than the P3D model, and gives results close to the ones from the full planar 3D model. 1. INTRODUCTION The rapid development of shale resources in the past decade has brought a focus on the process of hydraulic fracturing. Shale reservoirs tend to be characterized by a complex 3D stress field and vertically heterogeneous mechanical properties, which have always been challenging for hydraulic fracturing modeling and particularly for properly predicting the shape of an induced fracture [1]. Most state-of-the-art planar fracture simulators use two distinct approaches. In the first one, referred to as the planar 3D model (PL3D), the fracture is assumed to be a plane and its entire footprint is discretized into elements. The equations governing fluid flow, elasticity, and mass balance are solved numerically, coupled with the fracture propagation rules. This approach is very accurate but also very CPU intensive [2]. This type of model is mostly used when a large portion of the fracture propagates outside of the zone where the fracture was initiated and significant amount of vertical flow is expected. The second approach is based on the cell-based pseudo-3D (P3D) model [3], which separately considers the vertical growth and horizontal propagation of the fractures. In this approach, the width profile and fracture height are calculated based solely on the local pressure and local vertical stress profile. This approach is less CPU intensive, but relies on several assumptions including the fracture being initiated and its leading front propagating in the lower stress layer compared to the neighboring layers above and below. If this is not the case, the fracture height growth can become unstable, since it is not directly correlated to the global fracture mass balance as in the PL3D model, and this can lead to significant inaccuracy in the predicted fracture height growth.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.45)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Shale gas (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.86)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Shale oil (0.74)