Some practical applications of recent core analysis advances

Abstract

The Eromanga Basin is Australia's main onshore oil province, consisting of a large number of thin good permeability reservoirs. This basin overlies the Cooper Basin, Australia's main onshore gas province, consisting of many thicker but low permeability gas reservoirs. Both basins often contain stacked reservoirs. In order to identify improvements in core analysis which can be applied to the basins the Operator has conducted a series of experiments using newly or recently developed procedures. Following are some highlights. (1) Reservoirs which may have leaked: Some Eromanga basin reservoirs are believed to have leaked into overlying reservoirs. For the reservoir which has leaked it is not really valid to compare the log analysis to a drainage capillary pressure curve. Neither does a traditional laboratory imbibition capillary curve exactly describe the process. A new laboratory imbibition procedure was developed which appears to match the log analysis better than drainage data. (2) Low invasion coring: This has become an established procedure in Australia. Many of our reservoirs are well suited to it. In one case we obtained saturations by four different laboratory procedures and compared them to the log analysis, obtaining a somewhat large spread in the various laboratory results. (3) Transitional reservoirs: For waterflood and some relative permeability experiments the traditional laboratory procedure of using high capillary pressure ( P c) to achieve s wc then a high flood velocity (or capillary number N) to achieve s or has been shown to produce incorrect results across the Eromanga Basin and has been replaced by a low P c/low N method, yielding better agreement with log analysis. (4) Mud testing on cores for formation damage: Comparison of a particle size distribution from the mud with a pore size distribution from the reservoir has a lot of pitfalls. It does not follow that particles much smaller than a pore automatically have the opportunity to enter the pore. (5) Spreading experiments: Two suites of experiments were conducted involving waterflooding a core followed by gasflooding it. One suite was run with a positive spreading coefficient, and the other with a negative spreading coefficient. Oil recovery was the same in both cases.