ABSTRACT:

Well productivity can be severely impacted by compaction-induced permeability loss associated with primary depletion. As horizontal well performance is dependent on horizontal (kh) and vertical (k v ) permeability we have developed a laboratory technique for synchronous measurement of both under triaxially compressive stresses. Geometric correction factors derived from numerical simulation have been utilized to enable measurement of kh from transverse flooding of cylindrical core plugs. As unconsolidated reservoirs are particularly prone to compaction-related problems, we have concentrated on evaluating the impact of stress path ratio on the development of permeability anisotropy in unconsolidated sand packs. The rate of productivity decline with increasing depletion appears relatively insensitive to stress path magnitude, but markedly dependent on sand grain textural characteristics. Observations are rationalized through consideration of plastic yielding controlled by elliptical caps.

1. INTRODUCTION

Permeability magnitude, spatial distribution and scaling relationships all exert first order control on fluid transport in geologic media [1] including hydrocarbon reservoir performance. Rock heterogeneity (spatial variability) and anisotropy (directional variability) exist at all scales as a direct consequence of sedimentary and tectonic processes, and as such strongly influence subsurface fluid migration. Vectorial variation in permeability is observed when rock fabric is present on a scale less than the scale of permeability measurement. On the core plug scale, this is most usually the result of mineral grain and/or pore space alignment combined with preferentially orientated microcracks associated with a prevailing stress field [2]. During primary production from hydrocarbon reservoirs, fluid pressure depletion can induce in situ effective stress variations including sizeable increases in stress anisotropy (induced shear stresses) depending on material properties, stress path, reservoir geometry and boundary conditions [3]. Depletion-driven amplification of stress anisotropy can in turn generate distortional strains that lead ultimately to reservoir compaction. Reservoir compaction continues to pose a considerable challenge, particularly for deepwater assets, in which high development costs make it essential to fully understand the physics associated with long-term reservoir production [4]. While compaction can augment reservoir performance via increased drive energy (distortional strains can induce pore collapse resulting in increased rock compressibility and pressure maintenance) it can also impede recovery by restricting flow to the wellbore (physical changes involving grain translation and fracturing can impair permeability). In this paper we explore the interrelationship between the degree of applied stress anisotropy and the resultant development of compaction-induced permeability anisotropy in initially isotropic unconsolidated sands. We restrict stress anisotropy to the vertical plane, and monitor the progressive evolution of vertical to horizontal permeability ratio, "kv/kh" (assuming no permeability variation with azimuth in the horizontal plane) associated with development of a transversely isotropic medium from an initially isotropic one. To this end, we quantify the potential for stress anisotropy to induce permeability anisotropy in initially isotropic matrix material, through progressive damage to the grain framework and thus distortion of the porous network.

2. BACKGROUND

Permeability distributions for reservoir simulation are usually derived from core analysis (supplemented by well-log and well-tes

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