Modeling a vuggy carbonate reservoir, McElroy field, West Texas

The McElroy field produces approximately 17,000 BOPD (barrels of oil per da y) under a mature waterflood from the Permian Grayburg Formation. The main pay zone in the reservoir is primarily peloidal dolograinstones/packstones with interparticle/intercrystalline porosities. The central portion of the field is more heterogeneous because of thin high-porosity and high-permeabi lity vuggy zones. The occurrence of these zones is confirmed by core descri ption and measurements, porosity logs, tracer studies, and injectivity meas urements. These thin high-porosity and high-permeability vuggy zones dimini sh waterflood effectiveness ana leave millions of barrels of bypassed oil i n the lower permeability matrix. A method was developed to identify the vuggy zones on logs, create geostati stical models of porosity and permeability incorporating the vuggy zones, a nd characterize them in simulation models. The methodology involved the fol lowing: (1) developing a log trace to identify zones of high secondary poro sity, mainly vuggy porosity, in the area of the field that was modeled, (2) creating a detailed geostatistical model (1 million cells) of total porosi ty using well-log data, (3) creating a geostatistical permeability model ba sed on total porosity, (4) creating a separate detailed geostatistical mode l of secondary porosity, and (5) superimposing exceptionally high permeabil ity in areas of the permeability model defined by high secondary porosities . The detailed permeability models were scaled-up to 12,000-cell models for s imulation studies. The models incorporating vuggy permeability distribution s showed a far superior history match of primary and waterflood processes t han did models that did not incorporate vuggy permeability; these models al so showed good-quality history matches for individual wells. Successful his tory matching of the simulation models validates our method and indicates t hat core data underestimate the permeability of vuggy zones due to sampling and measurement issues.