Numerical Simulation of Seismicity Induced by Hydraulic Fracturing in Naturally Fractured Reservoirs
Authors: Zhao X. P. , Young R.P. 
Published: June 2014

Abstract
The problem of the interaction between hydraulic and natural fractures is of great interest for energy resource industry because natural fractures can significantly influence the overall geometry and effectiveness of hydraulic fractures. Based on the tri-axial fracturing lab experiments presented in other publications and fluid stimulation in the field, a 2D discrete element model with fully dynamic and hydromechanical coupling is validated to simulate fluid injection into a reservoir containing a natural fracture by comparing modeling geometries of hydraulic fractures and induced seismicity with actual results in laboratory and field data.
At the lab scale, the numerical model simulated a series of fracturing experiments on rock blocks with pre-fractures with different orientation, and the model captured three interaction types (crossing, dilating, and arresting) between induced fractures and pre-fractures and also illustrated three types of crossing depending on the differential stresses and orientations of pre-fractures. Furthermore, seismic mechanisms obtained from the model confirmed that hydraulic fractures were arrested by shear slippage of the pre-fracture. In the field scale, the calibrated model simulated the stimulation conducted in the tight gas reservoir at Dowdy Ranch field, USA. The model produced the scope and orientation of induced fractures similar to results obtained from the actual recorded microseismicity, and a similar seismic magnitude range. Moreover, the model showed deformation and cracking occurring ahead of the fluid pressure front and hydraulic fractures were arrested by the dilation of the fault. At the same time, the leakage of large fluid volume through the fault area was qualitatively predicted by the 2D model. These confirmed that the effective half-length is shorter than the created fracture half-length deduced from microseismic locations, which is the case during the multistage fracturing treatment in the Bossier formation. In addition, from the modeling results, it was concluded that the horizontal principal stresses with a ratio no less than 2 may be enough to cross a natural fracture with a single hydraulic fracture. Therefore, the validated model can help examine in detail the micromechanism behind the failure, and the relationship between the induced seismicity and the fluid front through direct observation of the model.

Introduction
Hydraulic fracturing and seismic monitoring are established techniques to improve the production of hydrocarbons from unconventional oil and gas reservoirs (Pearson 1981; Maxwell and Urbancic 2001; Sharma et al. 2004; Le Calvez et al. 2006), enhance geothermal energy in hot dry rock (Sasaki 1998; Norio et al. 2008), and facilitate slurry waste re-injection operations(Warpinski et al. 1999). Due to ubiquitous natural fractures, the problem of the interaction between hydraulic and natural fractures is of great interest for the energy resource industry because natural fractures can significantly influence the overall geometry and effectiveness of hydraulic fractures.
A considerable amount of research has been carried out in the past few decades trying to understand the complexity and mechanics of hydraulic fractures in fractured reservoirs. Blanton (1986) conducted scaled laboratory experiments on naturally fractured Devonian shale and hydrostone under different angles of approach and states of stress. These experiments show that hydraulic fractures crossed pre-fractures only under high differential stress and high approaching angles, while at low differential stress and angles of approach the existing fracture opened, diverting the fracturing fluid and preventing the induced fracture from crossing, at least temporarily. Beugelsdijk et al. (2000) also performed laboratory experiments on Portland cement blocks to analyze complex hydraulic fracture geometry as a function of horizontal stress difference, stress regime, flow rate and discontinuity pattern. Many field work took in naturally fractured formations reveal that effects of natural fractures on fracture propagation are enhanced fluid leakoff, premature screenout, arrest of the fracture propagation, formation of multiple fractures, fracture offsets, high net pressures (Britt and Hager 1994; Vinod et al. 1997; Rodgerson 2000; Azeemuddin et al. 2002; Sharma et al. 2004).