Abstract:

This paper examines the impact of mechanical interactions between hydraulic on their shape and geometry and growth patterns. The impact of rock anisotropy and natural fractures on the evolution of a fracture network is also examined. Both 2D and three dimensional simulations using DD-based methods and finite element modeling are presented.

Introduction

Development of unconventional geothermal and petroleum resources requires reservoir stimulation to access a large volume of rock with a network of fractures that provide both surface area and fluid flow pathways. In the case of petroleum source rock development, stimulation is often accomplished by multiple hydraulic fracturing of horizontal wells whereas geothermal reservoir stimulation relies upon injection induced shear on pre-existing fractures and their coalescence. Maximizing facture permeability generation and retention, and minimizing the risks of seismicity and other environmental impact is a highly desirable goal, so that stimulation modeling and analysis are called upon to guide the practice. The hydraulic fracturing process presents a complex mathematical problem that involves the mechanical interaction of propagating fracture(s) with the dynamics of the injection fluid. Specifically, fracture initiation, fracture propagation, fluid flow in a deformable fracture and fluid diffusion into the rock, heat transfer between the fluid and the rock, and hydraulic fracture interactions with the natural fractures need to be considered. Therefore, advanced modeling is necessary for improved understanding of the stimulation process and for design optimization. Such models can be used to predict fracture geometries, orientation, stimulated volume, and induced stresses in the reservoir (e.g., for MEQ interpretation and refrac analysis).

Hydraulic fracturing numerical models are usually developed based on a particular conceptual model. Early hydraulic fracturing numerical models (e.g., Carter et al., 2000; Ouyang et al., 1997; Yew, 1997; Lee et al., 1994; Clifton and Wong, 1991; Clifton and Abou-Sayed, 1981; 41; Vandamme and Curran, 1987; Cleary et al., 1985; Wiles and Curran, 1982; Abe at al., 1976) were developed for conventional reservoirs based on the concept of a tensile fracture propagating under fluid pressure (e.g., Barenblatt, 1962; Perkins and Kern, 1961; Geertsma and de Klerk,1969). However, field observations of micro seismicity during fracturing of geothermal reservoirs (e.g., Cornet, 1997; Roff et al., 1996; Fehler, 1990) and surface area estimations based on gas production in shale reservoirs (e.g., Ghassemi and Suarez-Rivera, 2012) show the existence of a complex fracture network, suggesting shear-slippage as a major mechanism of stimulation. Indeed, numerous hydraulic fracturing jobs have shown shear failure to be the main source of permeability enhancement, particularly where natural fractures are pervasive (Rutledge and Phillips, 2002; Mayerhofer et al. 1997; Chipperfield et al. 2007). However, analysis of pressure transients and rates often indicate the propagation of a major feature in the tensile mode consistent with the classical hydraulic fracturing concept. In this scenario, one could still envision leakoff from the major hydraulic fracture into the rock would cause slip on pre-existing or newly formed discontinuities and enhances the permeability in the vicinity of the main fracture via shear stimulation (e.g., (e.g., see Dusseault and McLennan, 2011). Different interpretations of field experiences have spurred different stimulations concepts and numerical models for analysis.

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