The Hydro-Thermal Influence of Fault Attributes on the Geothermal Reservoir Behaviour

Abstract

The fluid flow in a reservoir largely depends on faults, as faults can act like conduits or barriers. The influence of three fault parameters (permeability, thickness and angle) on the temperature and pressure behaviour of a geothermal reservoir, has been investigated by modelling a faulted reservoir in 2D in MATLAB, based on the Finite Element Method. The reservoir is assumed to be sandstone with a permeability of 40*10-14 m2. The fault has been modelled with a permeability ranging from 10-11 m2 to 10-17 m2, a thickness ranging from 10 cm to 150 m and an angle ranging from 0 to 90 degrees. While varying one parameter, the other two stayed fixed. The base values for the fault permeability, thickness and angle were 10-14 m2, 20 m and 0 degrees respectively. Both a finite and infinite fault have been modelled in the reservoir. Decreasing the permeability led to a higher production temperature, especially at a finite fault, where the fluid is able to bypass the fault and thus sweep the warmth of a larger part of the reservoir. At a fault permeability of 10-14 m2 the fluid has seemed to have extracted the maximum amount of heat in the reservoir, further decrease in permeability did not lead to a higher production temperature. At an infinite fault there is only a small increase in temperature at a fault permeability of 10-16 m2. The impedance at a finite fault is not much affected, as the fluid is able to flow around the fault. The impedance at an infinite fault, however, rapidly builds up with decreasing permeability. A thickness of 20 m, at the base permeability value of 10-14 m2, results in the highest production temperature. Larger thicknesses cause larger amounts of unrecoverable area and consequently less heat can be extracted. At a finite fault the impedance climbs only to a value of 4 MPa at a thickness of 150 m, but at an infinite fault the impedance already reaches 10 MPa at a thickness of 80 m. The fault has its largest influence at a perpendicular position to the flow. At a parallel position, the results are the same as a reservoir without fault, as the fluid will not have to dodge a lot to reach the production well. All three fault parameters have a significant influence on the temperature and pressure behaviour of the geothermal reservoir. They are all able to enlarge and eliminate the influence of the fault. A finite fault can cause a favourable higher production temperature without a large impedance, as the fluid is able to bypass the fault. An infinite fault has less effect on the temperature and causes the impedance to build up rapidly, which requires an unrealistic high pumping power and consequently an unprofitable geothermal system.