Geological/Stochastic Mapping of Heterogeneity in a Carbonate Reservoir

Abstract

Summary Two major problems in estimating interwell porosity and permeabilitypatterns are estimating permeability patterns are estimating permeabilityprofiles in uncored wells permeability profiles in uncored wells andinterpolating wellbore information between wells. A technique combining coreanalysis, rock fabrics, and wireline logs was used to estimate permeabilityvalues at 1 -ft intervals permeability values at 1 -ft intervals in 32 wells inSection 15 of the Dune field. Stochastic geologic interpolation was generatedwith a geostatistical method known as conditional simulation. Simulation ofwaterflooding showed that the stochastic realizations with a low degree ofcontinuity gave the most realistic results. Infill drilling to achieve anaverage well spacing of 2.5 acres increased recovery by 27 to 32%. Introduction San Andres and Grayburg reservoirs in the Permian Basin originally contained35 Permian Basin originally contained 35 billion bbl of oil in place and haveproduced about 10 billion bbl in the last 40 to 50 years. Current productionoperations will produce an additional 5 billion bbl, The remaining 20 billionbbl is divided into residual (immobile) and mobile oil. Residual oil is definedhere as oil trapped in the reservoir by capillary and viscous forces; it is thevolume of oil remaining in a rock swept by waterflood. Mobile oil is the oilthat can be moved by waterflood or by improved connection to a wellbore throughinfill drilling or other techniques. A multidisciplinary, integrated reservoirstudy of the Dune Grayburg reservoir in Crane County, TX, located about 13million bbl of remaining mobile oil in Section 15 of University Block 30. Onezone, Zone MA, contains 9 million bbl, with most of the oil located within anarrow belt through the middle of the section. This paper describes the 3Dspatial distribution of porosity, permeability, and saturation in Section 15. Use of these data in a reservoir simulator allowed prediction of the effect ofinfill drilling on oil recovery. Current methods of estimating oil recoveryfrom waterflooding are based on either pay continuity or the influence ofvertical stratification. George and Stiles assumed that the percentage of theformation that can be flooded is approximately equal to the pay continuity, which is defined as the fraction of total pay of one well connected to another. This approach uses well-to-well correlations and a cutoff porosity to definepay continuity and, therefore, cannot adequately describe the permeabilityheterogeneity between wells. The Dykstra-Parsons method assumes a verticallystratified reservoir with no lateral discontinuities. Numerous field studiesand infill-drilling programs, however, have demonstrated the significance oflateral discontinuities in reservoirs. This study uses geostatistical methodsconditioned by wellbore data to generate realistically complex interwellpatterns of heterogeneity that are critical to oil recovery. Dune Field Dune field is located in Crane County, TX, on the east side of the CentralBasin Platform (Fig. 1) and produces from the Platform (Fig. 1) and producesfrom the Grayburg formation of early Guadalupian (Upper Permian) Age. The fieldproduces from a Permian) Age. The field produces from a 200-ft-thick pay zoneat an average depth of 3,300 ft. Well spacing varies from 10 to 20 acres. Section 15 is currently drilled on 10-acre spacing. Water injection in Section15 was initiated in Well 1530 in 1976. Since 1980, the number of wellsconverted to injectors has steadily increased to 27 injectors in 1987. Thereservoir interval is divided into zones on the basis of quartz silt markersthat can be identified from gamma ray logs. In Section 15, the uppermostreservoir zone is Zone CZ, which is composed of pellet grainstone andpackstone. The next two zones, in descending order, the Zones BC and AB, whichconsist of fusulinid wackestone/ packstone. The lowest producing zone, the zonepackstone. The lowest producing zone, the zone of primary interest in thisstudy, is Zone MA, which is composed of a crinoidal packstone/grainstone faciesgrading laterally packstone/grainstone facies grading laterally into acrinoidal wackestone. Reservoir Characterization Reservoir characterization is the description of the 3D spatialrelationships of petrophysical parameters for the purpose of petrophysicalparameters for the purpose of improving recovery from oil and gas reservoirs. The petrophysical parameters of interest in the Dune study are porosity, permeability, and oil saturation. The characterization is conducted by firstdetermining the petrophysical values in the wellbores and then petrophysicalvalues in the wellbores and then interpolating these values betweenwellbores. Petrophysical Data From Borehole Petrophysical Data From BoreholeInformation. Porosity data were obtained from cores and wireline logs for theDune reservoir-characterization study. Saturation data were obtained from loganalysis; no capillary pressure curves were available. Permeability data wereobtained from core Permeability data were obtained from core material andrelated to wireline-log response by use of the rock-fabric method; no pressuredata were available. pressure data were available. JPT P. 1298