ABSTRACT
A fully three dimensional multiphase (liquid water with dissolved methane ¿ methane gas) computer program has been developed for solving the system of equations which govern mass and heat flow in a geopressured geothermal reservoir. The computer model includes the effects of the four major reservoir drive mechanisms (water compressibility, reservoir rock compaction, evolution of methane gas, and influx of water from shale dehydration). A preliminary series of calculations was made in which the input data for the computer program were varied to assess the sensitivity of the reservoir performance to (1) the presence of gaseous methane in the pores, (2) relative permeability, (3) sediment compaction, and (4) reinjection of the waste fluids. The principal effect of the free gas, sediment compaction and reinjection is to increase the producing life of the reservoir. Effects of relative permeability depend upon the amount of gas saturation in the pore fluid.
INTRODUCTION
Geopressured strata underlie a band along the Gulf Coast of the United States about 750 miles long from the Rio Grande to the Mississippi estuaries which extends about 50-100 miles inland and (in all likelihood) a similar distance offshore. The zone in which geopressures are most commonly found begins at a depth of about 3 km (10,000 feet) and extends downward perhaps 15 km (50,000 feet) to the base of Cenozoic deposits. These strata contain undercompacted clays and sandstones, with interstitial fluids bearing the bulk of the total overburden pressure. The fluid pressure is generally well in excess of hydrostatic. Further, these waters are at elevated temperatures, owing to the low thermal conductivity and high heat capacity of the geopressured sediment. Finally, the high pressure, high temperature pore water is generally believed to be saturated with dissolved natural gas (principally methane). When a geopressured stratum is penetrated and the fluid is allowed to flow, a complicated and interrelated sequence of phenomena is set into motion. First, of course, pore fluids near the wellbore begin to flow toward the bore surface and up the well, causing a pressure drop which propagates outward into the reservoir. This pressure gradient sets more and more water into motion so that after a fairly short time water is moving radially inward toward the wellbore throughout the system. As the reservoir pressure begins to drop, the undercompacted rock matrix must assume more and more of the overburden load. The resulting deformation of the rock matrix reduces the total pore volume in the reservoir,. which tends to maintain the reservoir pressure and drive additional fluid out of the system. At the same time, however, the decrease in rock porosity causes a decline in permeability which retards the fluid flow. Furthermore, as the pressure drops, the interstitial water (which was initially saturated with methane in solution) becomes supersaturated, so that tiny methane bubbles appear in the pores. The appearance of this high pressure gas tends to maintain reservoir pressure and augment the flow. Finally, as the pressure drops, water of hydration is driven out of the interbedded clay inclusions which augments the flow by, in effect, providing a fluid "source" within the reservoir.
