Simulation of Hot Water Injection from Geothermal Resources to Coal Seam Gas Reservoirs
Engineering Honours Degree, 2012
University of Adelaide
The whole idea behind the project is to come out with a technique to reduce or to eliminate the dewatering stage so that the coal seam gas (CSG) can be produced at higher wellhead pressure. One method that has been thought of is thermal treatment on CSG reservoir. In the initial problem statement given, the real difficulty of the method is that there is currently no reservoir simulator that can simulate thermal recovery of CSG to study its feasibility. Therefore this thesis will build upon previous studies on the method employed to investigate the feasibility of the thermal recovery technique of CSG production.
This thesis will begin with a brief introduction on CSG basic production mechanism as well as a general idea on how thermal recovery of CSG works. Then there will be an introduction of geothermal systems and the reasons why it is ideally feasible to couple the geothermal system with the CSG reservoir in order to enhance CSG production. Later, the methodologies employed to investigate the feasibility of the method are outlined. First, the introduction of a basic model to represent coal matrix with equations to represent gas flow in the model. After that, the details of reservoir simulation are outlined, with a focus on the wellbore modeling which is the main part of this study.
In this study, the source/sink model was used due to its simplicity of requiring only one variable, pressure as an input to the diffusivity equations to model fluid flow. However, source/sink modeling cannot calculate heat loss in the wellbore and consequentially the temperature of the water at surface. Therefore another built-in function, the semi-analytical model (SAM) is employed to calculate heat loss in the wellbore. The heat loss is calculated based on enthalpy of the fluid. There are three elements that influence fluid’s enthalpy; water rate, casing radius and casing thermal conductivity that will be investigated through simulations. Finally, the STARS‟s simulation results will be used for another simulation package, GEM to model CSG production.
Simulations’ results show that the high water rate, low casing thermal conductivity and smaller casing radius will be likely to retain the temperature similar to its source. However, with current technology, to have a temperature equal to its original when the water is produced to the surface is almost impossible due to heat loss during water production. Nevertheless, the gas production will still show significant increase in recovery factor which is about 26 % after undergoing the thermal treatment.