Print this pageModeling of a Vuggy Carbonate Reservoir (McElroy Field, West Texas)
DEHGHANI, KAVEH, PAUL M. HARRIS, KELLY A. EDWARDS and WILLIAM T. DEES
The main reservoir zone of McElroy field, which produces approximately 17,000 BOPD from a mature waterflood, is an approximately 24-m thick interval of heterogeneous dolomites of the Permian Grayburg Formation. The reservoir is primarily peloidal dolograinstones/packstones with interparticle/intercrystalline porosities. But thin high porosity-permeability vuggy zones in the central portion of the field lessen the effectiveness of waterflood leaving a major portion of oil bypassed in the lower permeability matrix.
A four step methodology was developed to model and then successfully history match the primary and waterflood phases in a 15 well, 100 acre vuggy portion of the field. (1) A detailed one million cell geostatistical distribution of total porosity was: derived using the well log data from 67 wells. Permeability was assigned using a cloud transform of core data from 10 wells. (2) A geostatistical distribution of vuggy zones was assigned. Sonic (matrix) porosity calculated using the Wyllie Time-Average Equation was subtracted from transform total porosity to yield secondary porosity. Significant differences in matrix and total porosity were considered to represent vugular intervals. Two vuggy zone distribution cubes were generated using different vug correlation lengths. (3) Each vug distribution cube was superimposed on the main permeability cube generated in the first step by assigning an exceptionally high permeability value to the vuggy zones (e.g. 4 Darcy). (4) The generated detailed permeability models were scaled-up to 12,000 cell models for simulation studies.
The models developed considering vuggy zone distributions showed a far superior history match of primary and waterflood process than one scaled-up directly from step one. Good quality history match was seen even on an individual well basis. Modeling shows that primary recovery produced 14% of OOIP and waterflood added 12% incremental recovery.