Abstract: Use of Gas Isotope Analyses for Reservoir Management

Martin Schoell, P. D. Jenden, M. A. Beeunas, D.D. Coleman

Reservoirs of oil and gas fields are often filled with multiple charges of compositionally different gases. Natural gases vary, therefore, in their composition and isotope concentrations of the hydrocarbons. Compositional and isotopic variations of gases are often caused by mixing of two endmembers of differing isotopic composition of their individual constituents. Isotopic properties in gases can be used to determine the mixing ratio of two endmembers. The variation of isotopic properties of gases within a continuous reservoir is generally small but can be significant between fault blocks of one reservoir or between unconnected but closely stacked reservoirs. The inter-reservoir variations can be utilized for reservoir management during development and production of oil and gas field as well as in gas storage operations. Examples from SW Germany, Italy, Kansas, New York State and the Gulf of Mexico will be used to demonstrate the following applications:

1. Reservoir Correlation: Stacked reservoirs in Spilamberto (N-Italy) and Ship Shoal 69 (Gulf of Mexico) show systematic changes in the isotopic and compositional properties of the gases due to mixing of bacterial and thermogenic gas. Each reservoir can be differentiated from the other but gases within a single reservoir are very similar. Isotope analyses of gases could be helpful in such cases in identifying new completions and correlating reservoir sands from well to well.

2. Reservoir Compartmentalization and Fault Block Mapping: The 7800^prime sand in the South Pass 78 field (Gulf of Mexico) shows up to 5 per mil differences in the carbon isotopic composition, of methane which are related compartmentalization into fault blocks. Such variations can be used by field geologist to better define faults.

3. Reservoir Allocation: Isotope analyses in commingled production could be used to allocate contributions from individual sands if isotopic differences exist in the contributing reservoirs. Two endmember scenarios and three endmember scenarios will be discussed.

4. Gas Storage: Gas isotopes have been successfully used for monitoring the filling process of gas storage reservoirs. The displacement of native gas with injection gas can be traced during injection of storage gas. Isotope mass balance may allow reconstruction of preferential filling pathways.

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