Performance behavior of compressed liquid geothermal reservoirs under a constant energy production scheme

A reservoir model is a utility describing the change in reservoir pressure and temperature as a function of time or cumulative fluid production. Pressure and temperature are the main parameters to consider when modelling the geothermal reservoirs. Primary objectives of geothermal reservoir modeling are to estimate production capacity of the geothermal fields, the rate of reservoir pressure decline and the effects of fluid recharge and injection on field performance. There are mainly three methods of modeling available in the literature, which are decline curve analysis, numerical modeling and lumped parameter modeling. Lumped parameter modeling, also known as zero-dimensional modeling, is a commonly used method when there is less data available at the early life of the field. In this model, reservoirs and aquifers are defined as homogeneous tanks and their average properties are used. Pressure and temperature behaviors are obtained by solving mass and energy conservation equations. In the literature, many lumped parameter (tank) models have been proposed for isothermal and non-isothermal geothermal fields. However, most of these models are valid for constant flow rate production scenario. In this study, a new non-isothermal single tank model was developed. Pressure and temperature behaviors are obtained by solving mass and energy conservation equations. With the new model developed, the temperature and pressure behaviors of the reservoir can be predicted for both scenarios, namely constant flow rate production and constant energy production scenarios. The tank system used in the model to represent the geothermal system is a reservoir tank and a recharge source. In the model, the tank is assumed to be recharged from a constant pressure source from the outer edge of the tank. Natural recharge is considered by applying the Schilthuis steady state water influx model between the recharge source and the reservoir. The main objective of this study is to compare reservoir performances under constant-energy and constant-flow production scenarios and to discuss the effects of certain parameters on reservoir performance behavior under constant energy production scenario. From this study, it is concluded that for the assumed reservoir parameters, pressure and temperature at late time for constant energy production scenario stabilize at some value like in the constant flow rate production scenario. When flow rate production is considered under constant energy production scheme, it is observed that mass flow rate is dependent on temperature and and specific heat capacity of the water. While bulk volume affects pressure and temperature at early and middle times, it has no effect on the steady state pressure and temperature under both scenarios. Under constant energy production scheme, required energy value is observed to affect reservoir steady state pressure, however, it has no effect on the steady state temperature for constant energy production scheme. It is also observed that steady state pressure and temperature are not affected by the rock porosity. Since pressure and temperature at late time depend on the injection rate, they will stabilize at different values for each case of reinjection ratio. While recharge constant has no effect on the steady state temperature, it affects steady state pressure under constant energy production scheme.