Abstract: Detecting Tubing String Communication by Biomarker Analysis of Oils

R. J. Hwang, R. J. Elsinger

Many wells in the Gulf Coast use multiple completions to coproduce from more than one zone. Over time, the plumbing in these wells can develop leaks that may not be detected by changes in the production log of a well. However, routine geochemical analyses of reservoir fluids can often provide an early indication of such problems and allow timely corrective action.

The oils produced from a dual completion well in the Main Pass 299 Field, offshore Louisiana, were periodically sampled and characterized by biomarker analysis and other geochemical techniques. All analyses show significant variation in composition among the oils produced over ~20 years from the long-string tubing (7800^prime sand) but only slight variation among the short-string tubing (7000^prime sand) oils produced over the same period. Compositional differences between the initially produced oils (in 1967) from the 7000^prime and 7800^prime sands reflect slight differences in the degree of oil biodegradation and thermal maturity. Subtle but persistent changes in thermal maturity of the long-string oils over production time indicate mixing of two oils with different maturities. Var ations in oil composition with production time are thus interpreted as mixing of oils from the two producing zones caused by leakage in the long-string tubing. A recent workover of the well confirmed the fracture in the long-string tubing.

This study demonstrates that periodic sampling and geochemical analysis of produced oils can be an effective and inexpensive tool for monitoring well performance. The multiple geochemical parameter approach used in the study reveals not only the tubing string communication problem but also geological processes responsible for compositional differences between the oils in the two producing zones, pertinent information for understanding reservoir filling history.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994