Allocating the Contribution of Oil from the Eagle Ford Formation, the Buda Formation, and the Austin Chalk to Commingled Production from Horizontal Wells in South Texas Using Geochemical Fingerprinting Technology

Baskin, David K.; McCaffrey, Mark; Kornacki, Alan

"Production allocation" involves determining the amount of oil or gas produced from each of several zones in a single well. This traditionally has been accomplished using mechanical logging methods. However, geochemical fingerprinting technology can be used to accurately (≈2% error) allocate commingled production from multiple pay zones at <5% of the cost of production logging.

Geochemical fingerprinting is possible when the composition of oil in each reservoir is different. In that case, when oils produced from discrete zones are commingled, subtle chemical differences in a produced oil sample can be used to assess the contribution from each pay zone. This technique uses high-resolution gas chromatography (GC) data obtained on each oil sample, and also requires the availability of samples of each "end-member" oil that contributes to the production stream. GC peak heights, which reflect the abundance of each compound, can be used to allocate commingled production using linear algebra methods.

We describe the first known application of geochemical fingerprinting technology to a shale-oil reservoir. We studied 18 oil samples produced from wells completed in the Austin Chalk, the Eagle Ford Formation, or the Buda Formation in two fields in Frio County and La Salle County, South Texas. Our goal was to determine if some of the oil produced from horizontal wells completed in the Eagle Ford Formation contains Buda oil because natural or induced fractures in the Eagle Ford extend into the underlying Buda reservoir (a fractured carbonate). This first required us to determine if: (1) oil fingerprinting can differentiate oils produced from Austin, Eagle Ford, and Buda reservoirs; (2) compositional differences can be used as natural tracers to evaluate vertical communication between Eagle Ford and Buda reservoirs; and (3) the amount (if any) of Buda contribution to "Eagle Ford" oil produced in one of the wells. Our results demonstrate that compositional differences do exist between Austin, Eagle Ford, and Buda oils. Using Buda and Eagle Ford end-member oil samples, we conclude that ≈9% of the oil produced from a horizontal well completed in the Eagle Ford Formation actually was produced from the underlying Buda Formation. Using the same end-member oil samples, we conclude that oil from a nearby horizontal well only produces oil from the Eagle Ford reservoir in which that well was drilled.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013