Abstract

This paper investigates the gas production source of the Standard Draw Field in the Greater Green River Basin (GGRB) in Southern Wyoming in Townships 17, 18, 19, and 20; Ranges 92 and 93. The study was initiated to eliminate the speculation that the sole producer of all the gas in the Standard Draw Gas Field is the Almond Bar Sand. Detailed analysis of the OGIP and future production of the Almond were examined by three methods; the volumetric method, material balance equation, and decline curve analysis. The volumetric analysis yielded an OGIP of 1245 Bscf. The material balance analysis yielded an OGIP of 624 Bscf. Decline curve analysis yielded an EUR of 870 Bscf using an economic abandonment rate of 27000 Mscf. Consequently all the gas being produced can come from the Almond Bar Sand.

Introduction

The Standard Draw Gas field located in the Greater Green River Basin (GGRB) between Rock Springs and Rawlins produces from the Almond bar sand in the Upper Cretaceous Mesaverde group. The sand has been very productive and it has been postulated that the sand is connected by fractures to lower sands and hence is draining much more than the bar sand. The current project attempted to analyze the total field data by volumetrics, material balance and decline curves to examine the hypothesis on a broader scale.

Over millions of years gas has migrated to the top of the Mesaverde group where it has been trapped by threshold displacement capillary pressure. The source of gas located through out the Mesaverde group was deposited from a shell/barrier island system. It is assumed that most of the gas has migrated up to the point where it cannot overcome the capillary pressure. In tight sands the capillary pressure is greater than the gas migration forces. Therefore to push the water through the pore spaces in a tight sand reservoir the viscous forces must be greater than the capillary forces. In the late 1970s natural gas began being produced from the Almond formation which is near the top of the Mesaverde group. The upper and lower Almond sand has a productive depth range between 3500 to 12500 feet. The Almond varies in thickness between 0 and 800 feet.

These pockets of gas, which have been called "sweet spots," were discovered in the Almond Formation of the Wamsutter Arch in the Washakie Basin area in T17 R93, T18 R93, T19 R93, T20 R93. The Almond outcrops along Rock Springs in the West and Rawlins uplifts to the East of Wamsutter Arch. The deepest point of the Almond is 13000 feet in the Washakie Basin. On the Wamsutter Arch, the productive zones are between 8500 to 10000 feet. The thickness of the Almond Bar Sand associated with the Wamsutter Arch varies from 0 to approximately 30 feet and has an extended area of 162 square miles.

Production data was gathered within the 162 sq. mi area to determine the size of the reservoir. Data was compiled from logs obtained from the Oil and Gas Commission of Wyoming and Wyoming Geological Survey. The log data was used to determine the thickness and the porosity of the formation. Log data was also compiled from Petroleum Information (P.I.) publication. The P.I. data was used to determine the decline of production. This information gave the decline rate of the reservoir. Another form of data compiled was bottom-hole pressures and compressibility factors. A graph of P/Z versus the cumulative production was created. This was extrapolated to zero pressure to obtain the initial gas in place of the reservoir.

Control Volume Determination

The cumulative production data from the wells located in the 162 sq. mi. study area was employed to located the area of highest gas production.

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