A Technique for Obtaining In-Situ Saturations of Underpressured Reservoirs

Abstract

Summary Pressure coring offers a method for obtaining in-situ reservoir saturations. However, because of the requirement of pressure balance during coring, it so far has been limited to use in reservoirs with pressure gradients greater than 0.25 psi/ft (5.7 kPa/m). This paper describes techniques used to obtain the first known successful pressure cores taken using a foam mud system, thereby extending the useful range of pressure coring to underpressured reservoirs. Stable foam is a compressible non-Newtonian fluid that requires special design considerations when used in conjunction with pressure coring. Careful well design is necessary to ensure that bottomhole pressure (BHP) during drilling and coring operations does not fall below reservoir pressure. This can occur easily if foam degradation and nonlinear pressure gradients are not considered. A complete technique for using foam to pressure core, including well design, field implementation, and core handling, is presented in this paper. This technique includes a wellbore design, a pressure analysis method, a method of selecting optimal foam design, a description of logistics, an empirical calibration test, and a description of pressure coring operations and core handling. Introduction The Wellman Unit is approximately 12 miles (19 km) southwest of Brownfield, TX and was discovered in the early 1950's. It is a reefal Wolfcamp feature on the northwest edge of the horseshoe atoll. The reservoir is fairly small in areal extent, covering only 1,320 productive acres (5 × 106 m2) but has a gross thickness at Ne center of the feature of nearly 800 ft (240 m). Because of this large thickness, the reservoir is estimated to have contained 100 million to 175 million bbl (16 × 106 to 27 × 10-6 m3) OOIP. It has a moderately active water drive, and from discovery until unitization, in Dec. 1978, was produced under primary depletion methods with an allowable of approximately 7,300 B/D (1160 m3/d) oil. After unitization in Dec. 1978, our company became operator and initiated secondary recovery pressure-maintenance operations in July 1979. Pressure response was almost immediate, and the allowable subsequently was raised to 9,300 B/D (1475 m3/d) oil. Original reservoir pressure was 4,100 psi (28 300 kPa) at 9,300 ft (2835 m), but under primary depletion the pressure had declined to 1,050 psi (7240 kPa). Since this was below the original bubble-point pressure (estimated to be approximately 1,300 psi (8950 kPa), a small secondary gas cap was formed at the crest of the structure. After successful implementation of secondary recovery operations, we concluded that tertiary recovery evaluations should begin. This tertiary recovery study hinged on accurate residual oil saturation values both for the secondary gas cap and for below the water/oil contact. In April 1980, we began engineering design to develop a method of obtaining these required residual oil saturations. Discussions with tertiary recovery consultants indicated that pressure coring would be the most accurate method for obtaining residual saturations and in-situ GOR's at the Wellman Unit. This included the assumption that we could find a practical method to obtain the pressure cores. The major problem stemmed from the fact that pressure balance is an absolute requirement for successful pressure coring. Too large an overbalance during coring operations will result in flushing of the core with mud filtrate, thereby altering the core saturations and properties. Underbalance during coring operations will result in an exuding of core fluids and alter the in-situ saturations. JPT P. 2701^