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ABSTRACT: Shaliness correction to improve accuracy of interpretation on Neutron Capture log in shaly sand reservoirs

Turseno, Itung1, Bambang Ismanto1, and Cholid Mas2
(1) VICO Indonesia, Jakarta, Indonesia
(2) Schlumberger GeoQuest, Indonesia

Thermal neutron capture cross section (sigma), a Pulsed Neutron Capture log has been used widely to identify the fluid movement in mature gas fields, in which the swept zones and the gas-liquid contact can be determined. Measurements of sigma are affected by the porosity, type of liquid in the pores, salinity of formation water and by type of rock matrix. In deltaic environment, the rock matrix contains shale as a part of the rock. As sigma of shale is much higher than sigma of gas, this existence of shale makes the interpretation becomes uncertain. To improve the interpretation in shaly sand, a tool-independent shaliness correction of sigma log reading was performed and has been proved in the interpretation accuracy of shaly sand gas-bearing reservoirs. An univariant equation of sigma (S) and volume of shale (Vsh) was applied to eliminate the shale factor in sigma reading. This paper will discuss the actual field history of cased hole logging experience, which also includes the utilization of corrected log in assisting selection of completion target. The conventional log interpretation provides good answers in clean sand reservoir. However in shaly sandstone, there is a need to modify the interpretation because of the direct effect of shaliness on the sigma reading. The shalier the sand the higher the sigma reading. Establishment of this correction algorithm can significantly improve the accuracy of interpretation in shaly sand reservoir, avoiding probability of bypassing good shaly sand gas-bearing reservoirs. The method has been applied successfully in Nilam field, where deltaic environments of ancient Mahakam River tends to have significant amount of shale as part of rock matrix. This method successfully identified "potential by-passed hydrocarbon" due to high sigma reading. As a result of this analysis, zones thought to be wet or swept produced significant quantities of gas.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia