Horizontal Permeability Anisotropy Characterization by Pressure Transient Testing and Geological Data

Abstract

SPE Members Abstract The anisotropy of horizontal permeability plays an important role in fluid flow distributions in reservoirs. However, it is rarely detected in the early stage of a reservoir study. Water break-through during production is usually the first evidence of such anisotropy. A technique to investigate the horizontal permeability has been developed, based on pressure transient analysis and geological data. Anisotropy has been detected and quantified using a pressure transient analysis and a knowledge of the reservoir geometry derived from geological analysis. The results obtained are subsequently confirmed by petrophysical measurements on oriented cores. The use of these results has been of primary importance in matching the pressure history of the "full-field" numerical simulation. The origin of the anisotropy was identified by a sedimentological study, based on core analysis and FMS interpretation. The anisotropy is related to the internal structure of the sandbodies (foresets), which were deposited in a shoreface environment. The azimuth of the paleo-coastline corresponds to the direction of the maximum value of permeability derived from the well-test analysis and the core measurements. Introduction Horizontal permeability anisotropies (variations with direction in the horizontal plane) occur most commonly in naturally fractured reservoirs when a family of fractures generates a preferential fluid flow direction. In matrix reservoirs, the vectoral parameter's horizontal component is generally assumed constant irrespective of the azimuth. This simplification is tempting because the horizontal anisotropy is difficult to recognize and measure. The anisotropy is often overlooked for two reasons :*the order of magnitude of the horizontal permeability anisotropy in these reservoirs is usually a lot smallerthan the value one finds in naturally fractured reservoirs and, consequently, its detection and estimation with dynamic data remains elusive;*furthermore, its characterization is conventionally based on petrophysical data, which are not of the same scale as the dynamic data. The derived values are, therefore, not necessarily representative of the reservoir's heterogeneities. However, full scale horizontal anisotropy linked to the mode of deposition becomes important in some depositional environments such as channel deposits (turdidite or river channel), in cross-bedded sands or in sandbodies with foresets (in both cases, the permeability is nearly always higher parallel to the bedding than normal to it). Kuchuk and Brigham showed that in the presence of an anisotropic horizontal permeability the production rate decreases with increasing anisotropy. An estimation of the anisotropy can be made by using data from several wells, observing steady-state flow into the first well and recording at the same time the pressure in three shut in wells, or analysing the effects of a flow-rate change on nearby wells. P. 39^