document

This paper (SPE 50995) was revised for publication from paper SPE 38368, first presented at the 1997 SPE Rocky Mountain Regional Meeting, Casper, Wyoming, 18-21 May. Original manuscript received for review 18 June 1997. Revised manuscript received 27 April 1998. Paper peer approved 1 June 1998.

Summary

The Coal Gulch-Echo Springs-Standard Draw field complex is one of the largest commercial gas accumulations in the Rocky Mountain region with over 1 Tcf of gas of recoverable reserves. Gas is produced from both the Upper Almond barrier bar and shoreline sandstones at the top of the Mesaverde Group (Upper Cretaceous) and from underlying Main Almond fluvial and marginal marine sandstones.

Some recently published models suggest that although the bulk of the produced gas in the fields is from the Upper Almond bar interval, simple volumetric calculations can only account for about 50% of the estimated ultimate recovery from this reservoir. These models imply that the depleting Upper Almond reservoir might be actively recharged by gas leakage from deeper Main Almond sandstones, with contributions from the deeper reservoirs of up to 10 to 30 Bcf of gas per well. This is in stark contrast to typical Main Almond-only producers outside the field area, which have mean reserves of less than 1 Bcf of gas and rarely produce more than 2 Bcf of gas per well. The implication is that the Upper Almond bar sand acts as a gas flow conduit, and its presence is required for efficient drainage of the Main Almond.

We determined the gas in place (GIP) for all field wells drilled before 1993. The GIP within the Upper Almond reservoir only was determined by detailed openhole log analysis and volumetric mapping to be 1,050 Bcf of gas. Total reserves from all producing intervals (Upper Almond and Main Almond combined) are estimated by decline curve analysis to be 1,003 Bcf of gas. The Main Almond lenticular reservoirs contribution to total production is then assumed to be statistically similar to Main Almond-only producers outside the field area, giving an estimated total contribution from Main Almond completions of 96 Bcf of gas; therefore, the recovery factor from the Upper Almond alone is estimated to be (1003 - 96)/1050=86%. We conclude that field volumetrics do not support a disproportionate contribute of Main Almond gas to the total field production, nor does the volumetric analysis support the active reservoir recharge hypothesis.

P. 328