The Economic Potential of Reinjection Into Geopressured Aquifers

Abstract

Summary This paper evaluates the technical potential, the energy balance, and the economics of reinjecting produced brines into geopressured/geothermal aquifers. This analysis of reinjecting brines shows that, depending on the hydraulic turbine technology, 50 to 90% of the power requirements of reinjection into the producing aquifer can be met by extracting the thermal and kinetic energy from the produced geopressured/geothermal brines. Moreover, the technically recoverable methane resource could be increased 20 times by reinjecting the brines into the producing formation, compared to using more conventional disposal options. In addition, the reinjection of brines would reduce the potential for environmental damage. Reinjecting the produced brines could enable geopressured aquifers to become economical in the future, particularly when the gas content of the produced bane is relatively large. In addition, the resource will become produced bane is relatively large. In addition, the resource will become more economical as real energy prices increase, since a reinjection-based system, to a large degree, will be independent of outside purchased power. However, because of high costs and risk, geopressured aquifers are not yet an economically competitive energy source. Introduction Geopressured aquifers have been estimated to contain an in-place resource of 1,800 Tcf [51 × 10 12 m3] of methane in Texas and 1,300 Tcf [37 × 10 12 m3] in Louisiana in the onshore sandstones. Another 2,600 Tcf [74 × 10 12 m3] is estimated in the offshore sandstones. Because of the large size of this unconventional gas resource, efforts have been directed toward establishing the amount of gas that is economically and technically recoverable and the alternative technologies for producing these high-pressure, high-temperature formations. This work has included efforts by the U.S. DOE to drill wells specifically for testing and producing geopressured formations (design wells) and to collect information on dry wells drilled in the search for oil or gas that have penetrated geopressured formations (wells of opportunity). Earlier studies on the economics of producing methane from geopressured brines have concluded that the resource is marginally economical because of high capital costs, unproved technology, and the small net energy gain associated with only capturing the dissolved methane. Further, since the reservoir energy is sufficient to produce only 1 to 4% of the water in place, a viable project requires giant reservoirs of 3 cu miles [12.5 km3] or more, containing more than 1 billion bbl [0.16 × 10 9 m3] of water in place. One way to increase the attractiveness of the geopressured aquifers is to use the associated thermal and kinetic energy to reinject the brines back into the producing aquifers. This total energy system would have the following benefits. 1. It would lead to a greater recovery of the resource. 2. Environmental effects, such as subsidence, would be minimized. 3. Reservoirs that are an order of magnitude smaller than the "giants"could become feasible resource targets. 4. The productive life of the geopressured reservoirs would be extended greatly. 5. The use of the thermal and kinetic energy would reduce the need to purchase outside power for reinjection. Fig. 1 is a line diagram of a total energy system. The produced geopressured brines initially are flowed through a hydraulic turbine. As the pressure is reduced, kinetic energy is converted into electric power, and the released methane is captured in a gas/water separator. The brines then flow through a heat exchange where the thermal energy is extracted before they are reinjected. This paper examines the use of such a total energy system and the technical and economic potential of reinjection into producing geopressured/geothermal aquifers. producing geopressured/geothermal aquifers. Sample Reservoirs The analysis is based on five actual geopressured aquifers (in Texas and Louisiana) on which data have been gathered. The reservoir parameters of these five sample reservoirs are shown in Table 1. These parameters are based on published values and personal communication. The geopressured waters were assumed fully saturated with methane, and the gas content was derived from published correlations, allowing for temperature, pressure, and salinity differences (Fig. 2). The geopressured reservoirs are all hydrocarbon-bearing reservoirs larger than normal, and have high temperatures, pressures, and salinities. The first three represent shallow geopressured, deep geopressured, and ultradeep geopressured reservoirs in Texas; the last two represent shallow geopressured and deep geopressured reservoirs in Louisiana. JPT p. 1164