A Quantitative Technique for Hydrocarbon Show Evaluation while Drilling

Eli Tannenbaum, Klaus Zanker

Gas that evolves from the mud while drilling is the first indication of hydrocarbons in the subsurface. It also serves as an indicator of increasing pore pressure. Various approaches have been used to interpret the gas data in order to provide a show evaluation and determination of possible pay zones.

However, the current techniques for gas measurement are flawed mainly due to the problematic sampling and extraction methods applied. It is estimated that in show zones less than 30% of the gas which was incorporated into the well bore is measured. Further, this measured portion is not representative of the gas composition in the formation due to fractionation processes between individual hydrocarbons that take place during transport to the surface. These problems are severe obstacles to getting a reliable measurement and interpretation.

A new technique for quantitative mud-gas analysis is described. It carries out efficient sampling from the well bore and quantitative extraction of gas from the mud. An innovative gas chromatographic system performs a continuous analysis of hydrocarbons in the C₁-C₈ range plus some nonhydrocarbon gases (CO₂ and H₂S).

The continuous quantitative measurements provide valuable data on the distribution profiles versus depth of hydrocarbon and nonhydrocarbon gases. This enables better understanding of processes such as hydrocarbon generation, migration, and accumulation in sedimentary basins. Interpretation products for reliable show evaluation and pay-zone determination have been developed. Several case studies will be discussed.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.