The Influence of Stress Anisotropy on Horizontal Well Performance Predicted Via Special Core Analysis Under True Triaxial Conditions

Abstract

Abstract For optimisation of recovery, a thorough understanding of reservoir heterogeneity and anisotropy on as detailed a scale as possible is a prerequisite. Despite recent developments in probe permeametry enabling quantification of sedimentary heterogeneity down to the lamina-scale, no parallel advance has been evident in the field of core analysis with regard to improving the realism of the applied stress field. The conventional "triaxial" method can only achieve vertical stress anisotropy and is limited to axisymmetric uniform radial pressure. Thus it is incapable of adequately simulating the in situ rock stress field,. Accordingly, a new true triaxial cell and servo-control system has been developed which is capable of applying independent and unequal radial stresses to the curved surface of a cylindrical core plug. Pulse decay permeability experiments have been conducted on three reservoir sandstone analogues showing different degrees of geological heterogeneity. Permeabilities were measured under both true triaxial and conventional "triaxial" conditions, for the purposes of comparison. In terms of mean stresses, (v+ H+ h)/3, stress-sensitive permeability profiles were very much influenced by lithology. Complex interaction between stress anisotropy and rock fabric resulted in a non-systematic variation in permeability with mean stress for the laminated, heterolithic rock type. For the scenario of a horizontal wellbore drilled parallel to the minimum principal stress, ah, the measured permeabilities were used to calculate I3-factors (=kh/kv) for simple productivity evaluation. This sensitivity analysis indicated that well performance curves were also susceptible to the degree of stress anisotropy. The effect was evident in discrepancies of up to several hundred feet in calculated horizontal wellbore lengths required to achieve desired levels of productivity. Introduction With less than one-quarter of additions to the world's oil reserves Coming from new discoveries, over three-quarters is necessarily sourced through better management of existing reservoirs. Thus profitability is dependent on increasing recovery from already producing fields. Perhaps the single most important prerequisite for optimisation of recovery is a thorough understanding of reservoir "heterogeneity" (different properties at different locations) and "anisotropy" (different properties in different directions) on as detailed a scale as possible. Geological heterogeneity (sedimentary structures and tectonic discontinuities) governs the relative connectivity/compartmentalisation of the reservoir, sweep efficiencies and residual oil saturations, all critical factors in the determination of recovery. Such complex variability involves manifold geometries at a diffuse range of length scales from lamina (mm to cm) to bedding (cm to m) to formation (m to 100's m). Whilst 3-D seismic can distinguish sedimentological structure and fault geometries on an interwell scale, it still lacks the resolution to pick out fine detail. P. 923^