Summary

The current state of shale gas reservoir dynamics demands understanding long-term production, and existing models that address important parameters like fracture half-length, permeability, and stimulated shale volume (SSV) assume constant permeability. However, observed steeply declining rates may involve pressure-dependent permeability (PDP).

This study accounts for PDP in two potential media: the shale formation and the created hydraulic fractures. Sensitivity studies comparing expected long-term rate and pressure production behavior show that PDP in the shale formation is distinct from PDP in created hydraulic fractures, and both are distinct from behavior without PDP. Successive buildups spaced apart in time reveal PDP behavior, but they do not necessarily indicate whether the permeability changes are in the shale formation or in the hydraulic fractures. The combined buildup and rate-normalized pressure data presentation displays both short-term and long-term transient trends that distinguish the source of the PDP behavior.

Pressure and rate field data demonstrate evidence of PDP in Horn River and Haynesville shale, but not in Fayetteville shale. Successive buildup transients in Horn River shale show evidence of permeability increase while in Haynesville shale they show significant permeability loss. In the Haynesville shale case, the impact of PDP is effectively reduced recovery from the stimulated shale volume defined by productive fracture half-lengths.

Introduction

A number of authors have discussed pressure (or stress) dependent permeability (PDP). Vairogs et al. (1971) showed dramatic permeability reduction under increasing net confining pressure as measured in cores with initial permeability values ranging from approximately 40 microdarcies to about 200 md. They mention that the magnitude of the stress dependence depends on presence or absence of microfractures in the rock and the direction stress is applied. Raghavan et al. (1972) extended the real gas pseudopotential introduced by Aronofsky and Jenkins (1956) and further developed by Al-Husseini et al (1966) to provide a general solution for a model that allows for pressure dependent permeability, porosity, and fluid properties. Pedrosa (1986) considered exponential dependence for porosity and permeability, and termed the exponent governing permeability dependence the permeability modulus. He offered an approximate analytical solution and showed sensitivity analyses of the behavior. Yilmaz et al (1991) considered exponential dependence for porosity, fluid density, and permeability, and termed the exponent governing permeability dependence the permeability compliance. They discussed practical reservoir engineering strategies for cases when the magnitude of the permeability compliance is comparable or greater than the magnitude of the exponential factors for porosity and fluid density.

URTeC 1573545

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