The energy potential of geopressured reservoirs: hydrogeologic factors

The energy potential of geothermal waters in the geopressured reservoirs of the Gulf Coast ultimately depends on the yield of wells tapping these reservoirs. An analysis is made to determine possible well yields in a geopressured reservoir in Hidalgo County, Texas. The reservoir lies beneath an area 16 kilometres (10 miles) wide and 48 kilometres (30 miles) long, with the long axis extending northeast-southwest parallel to and east of the McAllen fault. The average pressure-to-depth ratio in the reservoir is 17 kilonewtons per square metre per metre (0.75 pound per square inch per foot). The average temperature of the water is 135°C (275"F), and the average salinity is about 25,000 milligrams per litre. On the basis of solubility data, the average methane content is estimated to be 4.8 standard cubic metres per cubic metre (standard cubic feet per cubic foot). Based on an idealized model of the reservoir, the results of the analysis indicate that a single 0.23-metre (0.75-foot) diameter well at the center of the reservoir could sustain a flow rate of 0.31 cubic metre per second (11 cubic feet per second) for 20 years. The total production rate from the reservoir could be increased to 2.7 cubic metres per second (95 cubic feet per second) for the 20-year period by assuming that a minimum flow rate of 0.15 cubic metre per second (5.3 cubic feet per second) per well is satisfactory and by developing the reservoir with 18 wells at optimum spacing. Estimates of subsidence under the 18-well production scheme indicate that the average subsidence over the reservoir area would be about 1 metre (3 feet) at the end of the 20-year production period. In the immediate vicinity of a centrally located well, subsidence would be about 2 metres (6 feet). The thermal, mechanical, and methane energy contained in the waters produced with the 18-well development scheme is equivalent to 11.32 X 10{sup 17} joules (10.73 X 10{sup 14} British thermal units). The thermal and mechanical energy components of this total could be converted to 94 megawatts (estimated) of electrical power over the 20-year production period. In addition to this analysis of a geopressured reservoir, the sensitivity of well yields to hydrogeologic factors is examined. It is concluded that the most important hydrogeologic factor in the development of geopressured reservoirs is the transmissivity. The results of calculations made for this sensitivity determination are presented graphically and can be used to make quick estimates of the yield of wells tapping geopressured reservoirs. (13 figs., 2 tabs., 20 refs.