DEPLOYING A FIT-FOR-PURPOSE FORMATION EVALUATION PROGRAM TO DE-RISK TIGHT GAS SAND PLAYS

摘要

challenges for petrophysical analysis where uncertainties are often much larger when compared to the usual conventional reservoirs. These low porosity and permeability rocks are associated with complex mineralogical properties and require an integrated approach to better characterize the reservoir, starting from a fit-for-purpose logging suite, the acquisition of high quality core and the implementation of an optimised core analysis program. In this paper the authors discuss an approach used to de-risk a tight gas play, where they made particular use of NMR log and Dean-Stark analysis for porosity and water saturation model calibration. OBM drilling fluids could not be used due to environmental restrictions consequently Deuterium Oxide (commonly known as D2O) was used to dope the water based drilling mud, to discriminate between the volumes of mud filtrate and formation water within the formation. The formations intercepted by the boreholes proved to have similarities from a stratigraphic/geological point of view, but showed significant differences when seeking to define producible water volumes using NMR estimates of irreducible water saturation (Swi) and core derived water saturations. In Well#1 resistivity derived water saturations were low and very close to NMR irreducible water saturations suggesting little or no formation water should be produced. In contrast in Well#2, a much more conglomeratic rock, a discrepancy was observed between core derived water saturations and NMR irreducible water saturations suggesting that this formation was not a irreducible water saturation and would likely contain producible water. It was also noteworthy that there existed a salinity variation between the two wells, where in Well #2 for formation water was more “fresh-brackish” then in Well #1. Well #1 was subsequently fracture stimulated and produced a mixture of gas, condensate and water (assumed to be frac fluid - based on salinity measurement) and well#2 was put on hold based on high water saturation indication and complex conglomeratic rock matrix. Petrophysical analysis played a key role in enabling the business decision through the integration of advanced logging and core analysis.