Fluid Recharge Research Articles

Numerical studies of the heat and mass transport in the Cerro Prieto Geothermal Field, Mexico

Numerical simulation techniques are employed in studies of the natural flow of heat and mass through the Cerro Prieto reservoir, Mexico and of the effects of exploitation on the field's behavior. The reservoir model is a two‐dimensional vertical east to west‐southwest cross section, which is based on a recent hydrogeologic model of this geothermal system. The numerical code MULKOM is used in the simulation studies. The steady state pressure and temperature distributions are computed and compared against observed preproduction pressures and temperatures; a reasonable match is obtained. A natural hot water recharge rate of about 1×10−2 kg/s per meter of field length (measured in a north‐south direction) is obtained. The model is then used to simulate the behavior of the field during the 1973–1978 production period. The response of the model to fluid extraction agrees to what has been observed in the field or postulated by other authors. There is a decrease in temperatures and pressures in the produced region. No extensive two‐phase zone develops in the reservoir because of the strong fluid recharge. Most of the fluid recharging the system comes from colder regions located above and west of the produced reservoir.

Effect of device reliability on memory reliability : Jonathan A. Humphry. IEEE Trans. Reliab.R-29 (5) 416 (December 1980)

A consistent, agreed-upon terminology is prerequisite for geothermal resource assessment. Accordingly, we propose a logical, sequential subdivision of the “geothermal resource base”, accepting its definition as all the thermal energy in the earth's crust under a given area, measured from mean annual temperature. That part of the resource base which is shallow enough to be tapped by production drilling is termed the “accessible resource base”, and it in turn is divided into “useful” and “residual” components. The useful component (i.e. the thermal energy that could reasonably be extracted at costs competitive with other forms of energy at some specified future time) is termed the “geothermal resource”. This in turn is divided into “economic” and “subeconomic” components, based on conditions existing at the time of assessment.In the format of a McKelvey diagram, this logic defines the vertical axis (degree of economic feasibility). The horizontal axis (degree of geologic assurance) contains “identified” and “undiscovered” components. “Reserve” is then designated as the identified economic resource. All categories should be expressed in units of thermal energy, with resource and reserve figures calculated at wellhead, prior to the inevitable large losses inherent in any practical thermal use or in conversion to electricity.Methods for assessing geothermal resources can be grouped into 4 classes: (a) surface thermal flux, (b) volume, (c) planar fracture and (d) magmatic heat budget. The volume method appears to be most useful because (1) it is applicable to virtually any geologic environment, (2) the required parameters can in Sprinciple be measured or estimated, (3) the inevitable errors are in part compensated and (4) the major uncertainties (recoverability and resupply) are amenable to resolution in the foreseeable future.The major weakness in all the methods rests in the estimation of how much of the accessible resource base can be extracted at some time in the future. In a manner similar to mineral and fuel assessment, this recoverability is expressed as a “recovery factor”. For an ideally permeable hot-water system, the recovery factor may be as much as 50% and seems to be independent of temperature. It must decrease as effective porosity (φe) decreases, but the relation between the two is little more than a guess. On the other hand, for favorable systems like Larderello that produce steam by a mechanism of intergranular vaporization, the recovery factor is probably around 15–20%, decreasing to zero at an effective porosity of zero. According to the anlysis of Bodvarsson (1974), it increases with decreasing reservoir temperature, and as pointed out by Nathenson (1975a) is limited at low temperatures by the need to have sufficient reservoir pressure for extraction and use.The extent to which a geothermal reservoir can be resupplied with heat during “industrial” times of 10–100 yr can be evaluated using simple analytical models. The results, combined with gravity and levelling data of Hunt (1977) for Wairakei and Isherwood (1977) for The Geysers, confirm earlier conclusions by Ramey (1970) and Nathenson (1975a) that resupply to reservoirs producing only steam can be neglected, and the conclusion of Nathenson (1975a) that it may be significant for hot-water systems of high natural discharge.Major subjects that demand continuing investigation include: 1.1. Determination of recovery factors as functions of temperature and effective porosity, particularly for hot-water systems.2.2. Evaluation of fluid recharge and heat resupply by repetitive gravity, levelling and underground temperature surveys in producing geothermal fields.3.3. Analysis of the extent to which a recovery factor can be enhanced by stimulation and by use of confined circulation loops.

Preliminary studies of brine reinjection at the Cerro Prieto geothermal field

At the present time, Units 1 and 2 of the Cerro Prieto power plant generate 75 MW of power. For this purpose, about 2200 t/h of fluids are produced. Of this, 700 t/h of steam are sent to the plant and 1500 t/h of spent brine are discarded into an evaporation pond. The thermal energy extracted is estimated to be on the order of 723 × 10 9 cal/h, of which about 260 × 10 9 cal/h is lost with the discarded water to the atmosphere. If, instead of disposing this water, it were possible to recirculate it by sending it back into the reservoir, most of the energy presently dissipated into the atmosphere could be returned underground. Reinjection would also lengthen the productive life of the reservoir because it would constitute an additional source of fluid recharge. Furthermore, reinjection of separated brines would solve the problem of surface disposal. About 1500 t/h of brine are presently sent to the evaporation pond, and the problems will become more acute when units 3 and 4, each with a 37.5-MW capacity, go on line in mid-1979. At present, a number of studies are being conducted to evaluate alternative methods of injection. The methods being considered are: cold or hot injection with open, closed or mixed systems. For each of these systems, laboratory tests will be carried out using columns packed with different grain-sized sands. The sands used are from alluvial fans of the Cucapa range. The purpose of these tests is to establish the scale-forming tendencies of the water when injected under different conditions (Figs. 1–3. Later, similar tests will be made on models built with natural sandstones. Because of its favorable characteristics, well M-6 (see Fig. 1) might be used in a pilot injection field test. The feasibility of reinjecting into two different levels, at 300 m, and between 527 and 741 m depth, is being evaluated. Other possible wells for reinjection could be M-3 and M-7.

Methods for regional assessment of geothermal resources

A consistent, agreed-upon terminology is prerequisite for geothermal resource assessment. Accordingly, we propose a logical, sequential subdivision of the “geothermal resource base”, accepting its definition as all the thermal energy in the earth's crust under a given area, measured from mean annual temperature. That part of the resource base which is shallow enough to be tapped by production drilling is termed the “accessible resource base”, and it in turn is divided into “useful” and “residual” components. The useful component (i.e. the thermal energy that could reasonably be extracted at costs competitive with other forms of energy at some specified future time) is termed the “geothermal resource”. This in turn is divided into “economic” and “subeconomic” components, based on conditions existing at the time of assessment. In the format of a McKelvey diagram, this logic defines the vertical axis (degree of economic feasibility). The horizontal axis (degree of geologic assurance) contains “identified” and “undiscovered” components. “Reserve” is then designated as the identified economic resource. All categories should be expressed in units of thermal energy, with resource and reserve figures calculated at wellhead, prior to the inevitable large losses inherent in any practical thermal use or in conversion to electricity. Methods for assessing geothermal resources can be grouped into 4 classes: (a) surface thermal flux, (b) volume, (c) planar fracture and (d) magmatic heat budget. The volume method appears to be most useful because (1) it is applicable to virtually any geologic environment, (2) the required parameters can in Sprinciple be measured or estimated, (3) the inevitable errors are in part compensated and (4) the major uncertainties (recoverability and resupply) are amenable to resolution in the foreseeable future. The major weakness in all the methods rests in the estimation of how much of the accessible resource base can be extracted at some time in the future. In a manner similar to mineral and fuel assessment, this recoverability is expressed as a “recovery factor”. For an ideally permeable hot-water system, the recovery factor may be as much as 50% and seems to be independent of temperature. It must decrease as effective porosity ( φ e ) decreases, but the relation between the two is little more than a guess. On the other hand, for favorable systems like Larderello that produce steam by a mechanism of intergranular vaporization, the recovery factor is probably around 15–20%, decreasing to zero at an effective porosity of zero. According to the anlysis of Bodvarsson (1974), it increases with decreasing reservoir temperature, and as pointed out by Nathenson (1975 a) is limited at low temperatures by the need to have sufficient reservoir pressure for extraction and use. The extent to which a geothermal reservoir can be resupplied with heat during “industrial” times of 10–100 yr can be evaluated using simple analytical models. The results, combined with gravity and levelling data of Hunt (1977) for Wairakei and Isherwood (1977) for The Geysers, confirm earlier conclusions by Ramey (1970) and Nathenson (1975 a) that resupply to reservoirs producing only steam can be neglected, and the conclusion of Nathenson (1975 a) that it may be significant for hot-water systems of high natural discharge. Major subjects that demand continuing investigation include: 1. 1. Determination of recovery factors as functions of temperature and effective porosity, particularly for hot-water systems. 2. 2. Evaluation of fluid recharge and heat resupply by repetitive gravity, levelling and underground temperature surveys in producing geothermal fields. 3. 3. Analysis of the extent to which a recovery factor can be enhanced by stimulation and by use of confined circulation loops.

Recovery of Energy From Fracture-Stimulated Geothermal Reservoirs

Energy recovery by in-place boiling was studied with a laboratory to model, of a fracture-stimulated geothermal reservoir that can be heated to 500 deg. F and to a pressure of 800 psig. An analytical mode to evaluate rock-energy extraction was developed and agreement between predicted results and experimental results was satisfactory. Introduction Geothermal resources used for electric power generation are hydrothermal systems that exist either as vapor- or liquid-dominated reservoirs. Although the latter type is believed to be more abundant, the extent of hydrothermal resources is considered small compared with hot, igneous rock resources that. do not spontaneously produce steam or hot water because of a lack of permeability, porosity, and fluid storage. Economic development of geothermal resources will require advanced technologies for enhanced recovery of energy from hydrothermal reservoirs and the introduction of artificial circulation systems for extracting energy from hot, igneous rock. Several fracture-stimulation techniques have been proposed, such as explosive fracturing, hydraulic fracturing, and thermal stress cracking. Smith et al. described a large-scale hydraulic fracturing experiment in hot, igneous rock where a closed circulation loop of surface water under high pressure is used to extract the thermal energy by convective heat transfer. A variation of that extraction scheme using multiple fractures was proposed by Grinparten and Witherspoon . Stimulation of geothermal reservoir by nuclear explosive fracturing was proposed by Carlson and Kennedy. Nuclear explosive stimulation of natural gas reservoirs has been demonstrated successfully on an experimental basis . Explosive stimulation of hydrothermal systems was discussed by Ramey et al. They showed that the total energy in a hydrothermal system consists of tie thermal energy stored in the rock and the thermal energy stored in the geothermal fluids. The ratio of the magnitudes of these two components depends principally on the rock porosity. Ramey et (11. also demonstrated that the amount of energy extractable from a hydrothermal system can be enhanced by operating the system nonisothermally (for example, by pressure reduction) so that boiling (flashing) takes place within the rock formation. Nonisothermal heat-transfer processes in fractured rock systems have not been studied experimentally. However, several laboratory experiments on nonisothermal flow in porous media have been conducted. The studies described in this paper were designed to explore(1) conditions for optimum energy extraction, (2) rock heat-transfer characteristics, (3) moving flash fronts, (4) fluid-withdrawal reservoir pressure behavior. (5) effects of cool and hot fluid recharge, and (6) cyclic production/recharge operation of fracture-stimulated production/recharge operation of fracture-stimulated systems. The experiments were carried out in a laboratory model using rock loadings with characteristics that resemble the highly fractured region of a fracture-stimulated hydrothermal reservoir. Experiments were conducted with two rock loadings of different porosities and mean rock sizes at pressures and temperatures found in geothermal reservoirs. The extent of fluid recharge was limited so that energy addition by fluid recharge was less than the energy extracted from the rock. Preliminary evaluations of the results were reported by Hunsbedt et al. and detailed results were reported by Hunsbedt. This paper highlights these results. JPT P. 940