Technical Potential of Methods for Methane Extraction From Geopressured-Geothermal Fluids at High Temperature and Pressure

Abstract

Summary Retaining pressure for direct-drive injection disposal of geopressured fluids is particularly cost-effective when combined with recovery of residual dissolved methane. We examine the technical feasibility of several chemical and mechanical techniques for extracting methane, and discuss specific chemical extractants, pros and cons of processes, and problems of implementation. Introduction Recent technical and economic assessments of methane (CH4) production from geopressured-geothermal reservoirs in the Texas and Louisiana gulf coast area have been made by Swanson and Osoba and Doscher et al. They conclude that to obtain a reasonable return on capital for CH4 recovery from reservoirs with properties shown in Table 1, the CH4 selling price must be in the range of $7.50 to $9.24/Mcf. Samuels also concludes that "unless the methane content and market value are sufficient to offset the cost of the production and reinjection wells, there is currently little incentive to develop this resource". It is clear that at the present value of methane (about $3.50/Mcf), commercialization of the geopressured-geothermal resource will not be expected soon. Well capital costs comprise the major share of expenses, but the operating and maintenance (O and M) costs for injection are also significant. Doscher et al. approximate the O and M costs for injection with the following formula. (1) where $/Mcf is the selling price of CH4 and p is the required injection pressure in 103 psi. Pump power costs are proportional to injection pressure and the value of CH4. The 2 cent constant accounts for pump maintenance costs. At 40 scf of CH4/bbl brine, the value of CH4/bbl brine is 0.04 ($/Mcf). Assuming that pressure required for injection into shallow aquifers (6,000 ft deep) is 1,000 psi, the ratio of injection O and M costs to the value of CH4 is given by (2) From Eq. 2, the injection O and M costs relative to the value of CH4 are between 17.5% for $10/Mcf gas and 26.8% for $3.50/Mcf gas, clearly a significant percentage. The O and M costs are predominantly pump power and maintenance costs, which rise steeply with required injection pressure and become prohibitively high for geopressured waters with low CH4 content. It is clear that measures that could reduce injection costs would contribute significantly to the economic viability of geopressured resource development. One potential option in the production of geopressured aquifers is to maintain sufficient pressure at the wellhead to reduce the injection-pump workload. For shallow injection horizons, direct injection without pumping may be feasible. If it becomes necessary to inject into the production reservoir for pressure maintenance and subsidence control, injection-pumping O and M costs could easily exceed the value of recoverable CH4. Again, by efficient use of excess wellhead pressures, the power requirements for injection pumping could be reduced substantially. However, the potential savings would be offset by the loss of CH4 still dissolved at the elevated pressures, as shown by the solubility curves in Fig. 1. Therefore, there is an incentive for extracting CH4 at high pressures and also at production fluid temperatures to permit recovery of the thermal-energy component from the CH4-depleted brine. There are several potential methods for extracting dissolved CH4 at produced-brine temperatures of 302 deg. F and at anticipated pressures of 1,000 to 1,500 psi for injection into 6,000-ft-deep aquifers. The chemical techniques include gas stripping, which is technically viable but may not be economical, and solvent extraction. JPT P. 504^