--> --> Reservoir quality controls on downdip natural gas production degradation; insights from the Canyon Creek Field, Sweetwater County, Wyoming

AAPG Rocky Mountain Section Meeting

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Reservoir quality controls on downdip natural gas production degradation; insights from the Canyon Creek Field, Sweetwater County, Wyoming

Abstract

The Canyon Creek Field is a Laramide-age four-way closure draped with gas-charged Almond Formation sandstones. Gas production from the Almond Formation sandstones does not conform to a simple accumulation model with uniform charge to a structural spill point. For economic field development to proceed, a more detailed understanding of the controls on this complex production behavior is required. Two cores were cut in the same parasequence (one updip, one downdip) to understand the reservoir quality controls on production variability. Initial production from the updip well had a cored zone IP of 300 mcfd based on production logs, while the downdip zone IP was 44 mcfd. Core analyses demonstrate that routine porosity and Dean Stark water saturations are indistinguishable from an updip to a downdip position. Grain size and facies are generally consistent between the two cores. Routine permeability data fall within the same range. Each well had a high and low porosity sample selected for porous plate capillary pressure tests. Each sample was drained to 400 PSI and the relative permeability to gas (Krg) was measured at the endpoint saturation of the test. For the high porosity samples, the updip well has a Krg that is three orders of magnitude higher than the downdip well. Average grain size in the updip sample is medium, while in the downdip sample the average grain size is fine. Thin section analysis demonstrates that the updip sample has a greater abundance of pores which are not occluded by diagenetic minerals, while the downdip sample has porosity that is mostly occluded by fibrous illite. Based on these data we demonstrate that the presence of diagenetic illite in the pore system has a strongly negative effect on Krg in the Canyon Creek Field, but negligible effect on other routine permeability measurements. We conclude that early gas charge into relatively high-quality reservoirs, with larger grain size and lower capillary entry pressure, prevented fibrous illite growth and preserved thin, higher permeability pathways in the reservoir, leading to improved flow rates. In the absence of early charge, reservoirs in downdip positions had significant fibrous illite growth which, when combined with the relatively high Almond Formation water saturation, led to sub-economic gas flow rates.