Abstract

Numerical simulations were used to study three methods for modeling hydraulic fracturing in low permeability formations: simple geometry (SG), mixed-mechanism stimulation (MMS), and primary fracturing with shear stimulation leakoff (PFSSL). Simulations with SG have a small number of hydraulic fractures and do not explicitly represent natural fractures. The PFSSL conceptual model assumes one or a few large hydraulic fractures at each stage, with fluid leakoff into explicitly represented natural fractures. The MMS conceptual model involves a larger number of hydraulic fractures and includes the effect of hydraulic fracture termination against natural fractures and other planes of weakness. The simulations in our study were performed using a discrete fracture network simulator that efficiently couples fluid flow with the stresses induced by fracture deformation. Our investigation focused on conceptual model of stimulation affects parameters observed at the field scale: fracture network length, the impact of induced stresses, stimulation heterogeneity, and trends in initial shut-in pressure (ISIP) along the well. Compared to models with SG, it was found that PFSSL or MMS models yielded results more consistent with common observations from field data. The SG simulations exaggerated the effect of induced stresses, lacked the variability and heterogeneity commonly observed in field data, predicted very low net pressure, and had excessively long fractures unless relatively high matrix permeability was assumed.

1. INTRODUCTION

Conventional hydraulic fracturing models assume simple fracture geometry, such as a single planar fracture per stage [1]. However, a variety of observations suggest that fracturing creates complex geometries [2-7], especially in unconventional resources [8, 9, 10].

A variety of numerical and analytical tools are being used to describe hydraulic fracturing in unconventional formations. Most assume that one or a few large planar fractures form per stage. Some of these models use discrete fracture networks to explicitly model the leakoff of fluid from the primary fracture(s) into surrounding natural fractures, which experience stimulation in response to the increase in fluid pressure [11, 12]. We refer to this type of model as "primary fracturing with shear stimulation leakoff" (PFSSL). Other approaches use one or a few hydraulic fractures per stage and do not explicitly represent the stimulated natural fractures [13-17]. We refer to these "simple geometry" (SG) models. A third class of model incorporates the effect of hydraulic fracture termination against natural fractures. In these models, large, continuous hydraulic fractures are unable to propagate through the formation because they sometimes truncate against natural fractures or other planes of weakness. Instead of forming one or a few dominant hydraulic fractures, stimulation creates a more distributed and branching network that includes both new and preexisting fractures [18-22]. We refer to these models as "mixed-mechanism stimulation."

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