ABSTRACT: Production Data as an Indicator of Gas Reservoir Heterogeneity in the Vicksburg S Sandstones (Oligocene), McAllen Ranch Field, Hidalgo County, Texas

E. G. Wermund, R. P. Langford

To assess reservoir heterogeneity in low-permeability Vicksburg "S" sandstone reservoirs in McAllen Ranch gas field, production and pressure histories of 49 wells were analyzed; predominant well spacing is 80 acres. These histories were compared both fieldwide and in local areas defined by faults or facies. Production is through casing perforations, which commonly extend over 600 ft gross intervals within vertically stacked potential reservoir sandstones.

The S reservoir comprises five sand-rich intervals that together have produced 249 bcf of gas since 1965. Cumulative production per well

ranges from 39 bcf for 24 years to 0.8 bcf in 11 years. Average cumulative production is 6.3 bcf per well. To date, the largest average monthly production for a well is over 360 Mmcf; the mean for average monthly production (based on publicly available data) for all wells is 30.5 Mmcf. There is poor correlation among gross thicknesses of perforated intervals and cumulative production (R = 0.024). Well-head shut-in pressures range from 12,500 psi on completion to 1300 psi at abandonment. Maps of cumulative production and normalized BHP/Z show good production/pressure correlations. The slopes of regression curves for (1) monthly production decline histories and (2) periodic tests of daily production of adjacent well pairs show lithic influence from adjacent well completions or refractur ng, even at relatively close well spacing. Monthly production in a newly completed or refractured adjacent well commonly exceeds the last production rate of a nearby older well.

Production and pressure histories in adjacent well pairs suggest that limited communication occurs between S sandstone reservoirs of paired wells. This limited communication may be a consequence of limited drainage radius due to low permeability and of stratigraphic/diagenetic heterogeneity within the reservoir. However, differences in hydraulic fracturing efficiency and improvements in completion practice over time must also be considered in making historical assessments of production data.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990