Abstract

University Block 9 Field in Andrews County, Texas, was originally developed on 80-acre spacing in the early 1950’s. Previous field studies indicated that it was a trap-door structural closure from pre-Cambrian basement through Permian lower Wolfcamp sediments. The Devonian Thirtyone Formation was interpreted to be a reef that grew on the structural high. Utilizing formation micro-imaging logs, conventional cores, and 3D seismic, a new interpretation of the field’s reservoir architecture has evolved. Field production has increased by more than 2000 BOPD by applying this revised interpretation to a program of infill drilling and short-radius horizontal re-entry wells.

Nearly two dozen wells were logged with electrical imaging logs in an effort to better characterize the reservoir properties. The image log interpretations showed abundant vugs and numerous zones with high angle fractures and faults, in an otherwise low permeability, limestone matrix. A new method for differentiating vugs from primary porosity was developed by displaying core photographs at the same scale as the image log, and comparing the images to the vug index curves. Fractures identified in the images have 70-80 degree dip angles and vertical extents that caused them to act as conduits to deeper, water productive zones.

Core data showed that reservoir porosity was best preserved in shelf packstones and wackestones where the interparticle mud inhibited the precipitation of rim cements. Rescaled gamma ray curves were useful for differentiating the packstone/wackestone intervals from higher energy lithofacies. The distribution of the porous, low energy facies was identified by isopach mapping of high gamma ray intervals.

Three dimensional seismic was combined with the electrical image logs and core data in order to characterize the size and distribution of reservoir compartments. Seismic interpretations suggest that the structural history for University Block 9 Field is one of transpressional tectonism. A high angle fault, arcuate in shape, and varying in throw, defines the southern and western boundaries of the field. Numerous subordinate faults, antithetic to the field-bounding fault, further segment and compartmentalize the reservoir. There was no indication of an old, deep-seated structure.

Eight horizontal re-entries, each approximately 1000 feet long, have been drilled with the goal of reaching into untapped reservoir compartments. Two of these successful laterals were intentionally drilled through sealing faults into undrained reservoir compartments. Additionally, two of the laterals were drilled out of old, virtually depleted reservoir compartments and were successfully completed in new ones at rates exceeding 150 barrels of oil per day.

By incorporating these new technologies into planned infill wells, an old, poorly understood field has been rejuvenated. Porous intervals restricted by facies changes and faults can now be selectively developed by infill drill wells or inexpensive short radius lateral re-entries. High initial oil rates make both options economically attractive.