Journal of Conference Abstracts

Carbonate Pore Facies as Predictors of Reservoir Recovery Efficiency

Wayne Ahr (ahr@tamu.edu)

Department of Geology and Geophysics, Texas A&M University, College Station, Texas, 77843-3115, USA

Primary oil production recovers only about 1/3 of original oil in place (OOIP). The remaining 2/3 is yet to be extracted from most fields around the world. Gaining additional recovery requires infill drilling and/or costly injection of various fluids, steam, detergents, or other substances - to dislodge stubborn oil and move it from injection wells, through favorable reservoir zones, and ultimately to production wells. Carbonate reservoirs pose special problems because their pore systems include 1 or more of 3 end-member genetic pore types - those of depositional, diagenetic, or fracture origin. Optimum combinations of porosity and permeability in reservoirs correspond to zones that offer least resistance to fluid movement; these zones may be termed "flow units." As flow units may include any or all of 3 genetically different categories, they may be defined on the basis of structural, depositional, diagenetic, or hybrid genetic characteristics. Our study focuses on identification and analysis of pore types within flow units - or "pore facies." In addition to defining flow units, reservoir recovery efficiency is probably the most important geological factor that determines the economic outcome of field development. With this in mind, we have studied the Permian carbonate reservoir at Happy Spraberry Field, Texas. It is about 100 feet thick and consists of three main depositional facies: oolitic-peloidal grainstones, oolitic-peloidal packstones, and skeletal rudstones, bindstones, and floatstones associated with isolated biohermal buildups. Shallow burial diagenesis modified depositional porosity, particularly on paleo-highs with attendant unconformity-related dissolution. Principal pore types (pore facies) include grain-moldic, vuggy, solution-enhanced intergranular, solution-enhanced intercrystalline and cement-reduced intercrystalline categories. Flow units correspond closely with depositional facies suggesting both fabric and facies-selective diagenesis. Fourteen flow units were initially defined by ranking combined values of core porosity and permeability and linking the values with measured pore characteristics from thin sections. Primary production has been mainly from specific zones within the oolitic-peloidal grainstone depositional facies. Flow units in the rudstone-bindstones-floatstone facies are interpreted to contain bypassed oil. Mercury injection capillary pressure characteristics of samples from the principal flow units are now being studied to determine the distribution of pore throat sizes within each flow unit in order to establish a more precise petrophysical definition of "pore facies." Ultimately, the goal of this research is to establish mercury withdrawal efficiency (injected volume - withdrawn volume / injected volume ...quantity x 100) as a predictor of reservoir recovery efficiency from within a hierarchy of flow units and pore facies, provided reservoir fluid composition and drive mechanism are known.

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