Abstract
The U.S. Department of Energy/Morgantown Energy Technology Center (DOE/METC) awarded a contract in 1991 to Prime Energy Corporation (PEC) to demonstrate the benefit of using horizontal wells to recover gas from low permeability formations. The project area was located in the Chittim field of Maverick County, Texas. The Lower Glen Rose Formation in the Chittim field was a promising horizontal well candidate based on the heterogenous nature of the reservoir (suggested by large well-to-well variances in reserves) and the low percentage of economical vertical wells. Since there was substantial evidence of reservoir heterogeneity, it was unknown whether the selected wellsite would penetrate a reservoir with the desired properties for a horizontal well. Thus, an integrated team was formed to combine geologic analysis, seismic interpretation, reservoir engineering, reservoir simulation, and economic assessment to analyze the production potential and profitability of completing a horizontal well in the Lower Glen Rose formation.
A vertical well was drilled, and oriented core, geophysical well logs, and a repeat formation tester were used to determine the reservoir properties. A reservoir simulation was then conducted and an economic analysis of the forecasted recovery was used in a 48-hour decision process to: (1) kick-off and drill a horizontal well, (2) complete as a vertical well, or (3) plug and abandon. Although the analysis showed that the horizontal well would provide a very high rate of return, plans for a horizontal well were abandoned due to (1) a concern regarding the possible mobility of the gas-water contact and the associated risk of a limited reservoir and (2) absent a limited reservoir, the projected economics of the vertical well were very good making the incremental risks of horizontal drilling unattractive.
Subsequent to completion of the test well as a vertical producer, a 7-day well test was performed to investigate the reservoir away from the wellbore. Analysis of the well test showed evidence of reservoir heterogeneity and fracture porosity and permeability. However, the fracture-to-matrix permeability ratio of this system was small (3.5 to 1) compared with ratios typical of tight formations (> 1,000 to 1).
The integrated approach of using all of the available data and analysis techniques was vital in leading to the final decision not to drill a horizontal well. Reservoir simulation combined with economic analysis provided quick and inexpensive forecasts covering a wide range of possible reservoir properties. The analysis showed that the range of reservoir parameters conducive to drilling a horizontal well in a non-fractured reservoir is fairly small (though specific reservoirs should be analyzed case by case). Depending on the amount of data available for the reservoir, and the complexity and heterogeneity of the reservoir, a well test may provide better information on which to base the drilling decision than near wellbore measurements.
