Uncertainty in Carbonate Reservoirs - Quantitative Assessment Using the Results of a Volumetric Look-Back Study from the Humma Marrat Reservoir, Partitioned Neutral Zone (PNZ), Saudi Arabia and Kuwait

Meddaugh, William ¹; Griest, Stewart ¹; Tang, Hong ¹; Barge, David ^2
1 Chevron Energy Technology Company, Houston, TX.
² Saudi Arabia Chevron, Houston, TX.

The Jurassic-age Marrat reservoir at Humma Field in the southern portion of the PNZ was discovered in 1998. A total of 11 wells have been drilled and completed in the limestone and dolomite reservoir through mid-2008. A historical look-back of volumetric calculations was done in 2005 and again, as part of this study, in mid-2008. The results of these look back studies allow a semi-quantitative assessment to be made of the uncertainty associated with the porosity and Sw as a function of the amount of data available as well as the date of analysis (e.g. prior to core data, after integration of first cored well plug data). The same type of statistical analysis was also done for structural uncertainty, a composite uncertainty that takes into account depthing (velocity model) and interpretation uncertainty, as well as the original oil-water contact (OOWC) uncertainty which is generally defined in this project by a lowest known oil and either a highest known water or lowest closing contour. Analysis of each of the individual contributors to original oil in place (OOIP) uncertainty suggests that the uncertainty range associated with a specific uncertainty source does not uniformly decrease as additional data is collected. Changing analytical workflows (e.g. parameter constraints, algorithm constants, or software platform) and/or data reprocessing may impact the range of uncertainty significantly even though the amount of data remains constant. Evaluation of the OOIP uncertainty using a consistent design of experiments based approach allowed the relative contribution of each of the uncertainty sources to be determined. Early in field development the OOWC and porosity uncertainties were the most important contributors to OOIP uncertainty whereas in later stages of development, the structure and water saturation uncertainties were most important. A normalized uncertainty index was defined and used to evaluate delineation efficiency during the look-back period.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009