Reservoir Modeling Predicts Effect of Cold-Water Injection on Geothermal PTA

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 212615, “Reservoir Modeling To Predict the Effect of Cold-Water Injection in Geothermal Pressure Transient Analysis,” by Purnayan Mitra, SPE, University of Petroleum and Energy Studies, and Nihal Mounir Darraj, SPE, Imperial College London. The paper has not been peer reviewed. Geothermal reservoirs are one of the cleanest renewable sources of energy poised to address the global energy challenge. A major issue in the exploitation of geothermal reservoirs, however, is to find best-fit analytical methods for pressure transient analysis (PTA). This is because the assumptions made to predict PTA in hydrocarbon reservoirs are not satisfied by geothermal reservoirs. In the complete paper, the effect of cold-water injection on PTA of geothermal reservoirs is studied by varying the temperature of the injected cold water from room temperature to reservoir temperature. A major method of extracting heat energy from the Earth is the injection of water. Cold water is injected deep into geothermal reservoirs at a depth of 2–4.5 km. In this environment, cold water is essentially heated by the hot granite rock. Hydraulic fracturing is used to produce a large crack within the geothermal reservoir. Two boreholes intercept the crack. These boreholes are used for passing the cold fluid stream and the hot stream, respectively. In many scenarios, the gradient within the geothermal reservoir is so strong that a dry stream is produced. The greater the temperature difference between the injected fluid and the interior of the Earth, the greater the heat transfer. Therefore, it is always desirable to inject cold water inside geothermal reservoirs to maximize heat transfer and extract more heat. The steam coming out from the reservoir after heat transfer is used to run turbines to generate electricity. The steam also is used for a variety of other purposes. However, instances exist in which the temperature gradient is not as high, and it is difficult to produce a sufficiently heated dry stream. In such cases, an organic Rankine cycle is used for heating the steam on the surface. In such a case, the hot water or steam mixture is passed through a heat exchanger for heating the fluid to a desired temperature. When cold water is injected into the reservoir, a need exists to analyze the pressure transience throughout the reservoir. Different formations affect PTA in different ways. PTA across the geothermal reservoir currently is performed using the empirical correlations available for hydrocarbon reservoirs. Although the method is not 100% effective because of differences in reservoir parameters, PTA provides an idea about reservoir conditions. To reduce imperfection, it is often preferred to use reservoir parameters rather than injectate properties. In the complete paper, the authors study the effect of injected water on geothermal reservoirs while varying temperature from 14 to 312°C.