Origin of Large Variations in Gas-Oil Ratios at Horizontal Wells Landed in Upper Wolfcamp Reservoirs in the Delaware Basin Using Gas Isotope, SARA, and HRGC Data

Jennifer J. Adams* and Alan S. Kornacki; Stratum Reservoir.

Abstract

The wide range of initial GORs (3,200-22,000 scf/bbl) in wells landed in 3^rd  Bone Spring Sandstone or in Upper Wolfcamp pay zones at a wet gas field of the Delaware sub-basin exceed the natural variability of in-situ fluids due to local source rock maturity.  We used geochemical data measured on condensates and gas samples collected from ≈60 horizontal wells to determine if high GOR wells produce:  (1) a commingled mixture of different kinds of wet gas present in different Wolfcamp reservoirs related to hydraulic well stimulation; or (2) a mixture of rich wet gas generated by the kerogen in each pay zone, plus an additional charge of leaner wet gas (expelled locally by more mature Wolfcamp source rocks) and/or very mature dry gas (generated by much deeper Woodford or Barnett source-rock beds) that migrated vertically along deep faults that some horizontal wellbores intersect.  Production allocation results using HRGC data demonstrate that condensates with anomalously low or high pristane/phytane ratios and/or C₇ oil-generation temperatures (compared to the values of those parameters in condensates collected from most wells landed in the same pay zone) produce a significant amount of wet gas from shallower or from deeper intervals than the landing zone of each well.  However, the commingling of wet gas flowing from different pay zones does not explain the large GOR variations.  Instead, the geochemical data show that wells with unexpectedly high GORs produce an additional charge of very mature dry gas (demonstrated by the presence of methane enriched in isotopically-heavy C) and/or lean wet gas that was generated by Upper Wolfcamp C or D source-rock beds that migrated into shallower Upper Wolfcamp reservoirs over geological time.  These results, which were used to rank the effectiveness of frac barriers across the field, are partially explained by well placement relative to their proximity to major faults.    They also can be used to help adjust EURs and reserves estimates for each landing zone, make development decisions, and optimize completions.

BIO

Dr. Jennifer Adams received a B.Sc. (Honors Geology, 1998) from the Univ. of Waterloo, a M.Sc. (Hydrogeology) from the Univ. of Alberta (2001), and a Ph.D. in Petroleum Geochemistry from the Univ. of Calgary (2008). Jennifer worked on projects focusing on water chemistry, CO₂ & H₂S sequestration and basin fluid property prediction.  Jennifer co-founded an oil/gas geochemical service company, Gushor, in 2006 specializing in heavy oil fluid property measurement, production allocation and biodegradation studies. In 2009, she joined ConocoPhillips to support domestic and international exploration/production activities as a geochemist/basin modeler. Since 2016, Jennifer has worked with the OilTracers group of Stratum Reservoir (formerly Weatherford Labs) as a petroleum system analyst performing unconventional resource fluid evaluation, production allocation, water chemistry studies and exploration geochemistry projects.

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