Method for Producing Geothermal Energy and Minerals

The present invention relates to the production of geothermal energy and/or minerals from subterranean high temperature, high salinity water reservoirs.

High temperature, high salinity subterranean brine reservoirs, such as are located in the Imperial Valley of California, provide an important source of geothermal energy and/or minerals. Wells drilled in this area show that reservoirs at 6000 feet depth contain brines with up to 250,000 ppm salts content and 600.degree. F. temperature. The various soluble salts contained in these brines include sodium and potassium chloride, calcium, iron and magnesium salts. Also, other materials such as silica and ferrous compounds are often present in in the brines. Attempts to produce brine from these reservoirs for recovery of geothermal energy and minerals have been defeated by salt drop-out in the well bore, which occurs with reduction of pressure and temperature as the brines rise to the surface, rapidly plugging wells and surface equipment.

The present invention overcomes the problem of salt drop-out and permits recovery of energy and minerals from the geothermal brines.

The present invention is directed to a method for producing geothermal energy and/or minerals from subterranean high temperature and high salinity brine reservoirs in which fresh or low salinity water at ambient surface temperature is injected into the brine reservoir through a well; reservoir brine from the vicinity of the well bore is displaced; the injected water is heated by the reservoir heat and is then produced from the reservoir through the well. The volume of water to be injected in each injection-production cycle is selected on the basis of reservoir characteristics and overall project design. The well may be shut-in for a "soak" period following injection to permit time for the last water injected to be heated.

The sole drawing is a diagramatic illustration of the areal boundaries of a reservoir in accordance with one application of the invention.

In accordance with the present invention there is provided a method for recovering geothermal energy from subterranean high temperature and high salinity reservoirs, and, optionally, recovering minerals, by injecting fresh or low salinity water into the reservoir through a well bore drilled into the reservoir. The injected water is at ambient surface temperature. The injected water displaces reservoir brine from the vicinity of the well bore. In the reservoir the injected water is heated by contacting the hot reservoir rock, by heat conduction from overlying and underlying formations, and by mixing with the reservoir brine. After injection of a volume of water selected on the basis of reservoir thickness, porosity, and permeability and other factors in the design of the operation, water from the well is produced. The reservoir may normally be shut-in for a period of time, a "soak" period, to allow time for the last water injected to be heated. The shut-in period is preferred but not necessary.

When placed on production the well produces previously injected water which has been heated in the reservoir. Depending on operating pressures and temperatures a fraction of the water produced may be in the vapor phase (steam).

After a period of production and before producing all of the injected water the well will commence producing reservoir brine mixed with the injected water. The brine fraction is low initially and increases with time until it reaches a selected level at which time the production phase is terminated. The fraction of injection water produced before brine production commences and the rapidity of brine fraction buildup is dependent on reservoir mixing characteristics which are, in turn, dependent on several factors including (a) reservoir thickness, (b) reservoir stratification, (c) fluid densities, (d) fluid viscosities and (e) injection and production rates. The produced brine fraction at which the injection-production cycle is terminated will usually be the fraction which yields a total well bore effluent salts concentration just below that at which salt precipitation or drop-out would become an appreciable operating problem.

Since not all of the injected water will be produced from the reservoir some water injected during previous cycles will remain around the well bore at the start of subsequent injection-production cycles. Thus, more water can be produced from the reservoir per barrel of injected water before reaching the limiting well effluent salt concentration as the number of cycles increases. In this manner a mechanism is provided for moving heat and in situ minerals into the well from reservoir distances beyond those penetrated by injection water during the initial injection phase. The uppermost limit of the salt content of the injected water is that at which no substantial amount of salt precipitates and drops out of the water later produced from the reservoir. Thus, the injected water has a salinity ranging from zero to a selected amount such that when the water is produced from the reservoir, substantially no precipitation of salt occurs in the well bore.

Injection of cold water reduces the temperature of the reservoir around the injection wells. That reservoir volume can be reheated by overinjecting in selected wells to displace reservoir heat back to the vicinity of cold wells.

An operating method for reheating "cold" wells may be accomplished as follows.

Referring to the drawing there is shown the areal boundaries of a high temperature, high salinity reservoir and nine wells drilled into the reservoir. Wells "B" are drilled initially and operated on an injection-production cyclic basis as described above. When temperatures of the produced water approach an unsatisfactory low level row "A" wells are drilled and operated on the injection-production cyclic basis with row "B" wells shut-in. When temperatures of the produced water from row "A" wells approach an unsatisfactory level row "C" wells are drilled and used for the cyclic injection-production operation. Injection is continued into the row "A" wells to displace heat to the vicinity of the "B" wells. Production is then continued from wells in rows "B" and "C" with sufficient water injection into those wells to allow production at permissible salt concentrations. By this technique a large fraction of the reservoir heat and initial in situ water is recovered. Such a "line drive" displacement plan is but one of a number of injection-production well configurations and sequencing which might be used.

It may be desirable in some reservoirs to selectively perforate only part of the reservoir thickness in order to achieve optimum reservoir mixing of the injected and in situ waters.

Fresh or low salinity water from a source external to the geothermal reservoir is required to initiate exploitation in accordance with this invention. By low salinity water is meant a salt concentration less than the salt concentration of the reservoir brine and below that concentration which would result in substantial salt precipitation from the produced water and thereby an appreciable operating problem. After production is underway the produced low salinity water and steam condensate can be injected into wells operating on a cyclic basis and water with high salinity can be injected for reservoir drive displacement. Thus, the volume of fresh or low salinity water from external sources and the volume of produced water to be disposed of is minimized.

Advantages of the present invention as compared with methods proposed and tried previously for producing the high temperature, high salinity reservoirs include the following:

1. The severe salt drop-out problem is eliminated making exploitation of high temperature, high salinity reservoirs possible.

2. A higher fraction of the reservoir's geothermal energy is recoverable using the displacement drive of the present invention than could be recovered using the proposed natural reservoir fluid-rock expansion drive.

3. By maintaining flowing production through injection into the reservoir the need for artificially lifting brine from wells is eliminated.

Various changes and modifications in the illustrative embodiments of the invention shown and described herein can be made without departing from the scope of the invention as defined in the appended claims.