The influence of reservoir heterogeneities on geothermal doublet performance

SUMMARY The current main problem with deep geothermal energy in the Netherlands is the uncertainty in terms of attainable flow rate and life time. The goal of this research is therefore modeling a geothermal doublet in a heterogeneous reservoir, using a numerical 3D model created in simulator TOUGH2. A base model is defined by choosing stratigraphy, rock properties and boundary conditions, approximately modeling the Slochteren sandstone in the North of the Netherlands, as well as process and simulation parameters. This model is used for two subsequent simulations; the first model creates the naturally occurring steady state situation while the second calculates 200 years of geothermal production. This model then serves as a base and reference for a number of experiments modeling different heterogeneities. In practice all reservoir properties are heterogeneous to some extent, but in this research only reservoir permeability is chosen as such. Five experiments are conducted, each investigating a particular low permeability structure placed as an obstacle in the reservoir: a horizontal layer, a wedge occupying a part of the reservoir, a slanted layer cutting through the reservoir, a fault partially penetrating the reservoir and a reservoir boundary in the neighborhood of the doublet. Overall the pressure drop over the doublet takes the same value as that of the base model or is higher; up to 7 times higher pressure drops are observed. The breakthrough time of the cold front becomes both higher and lower than in the base model, representing an improvement or regression in geothermal life time of the doublet. In each experiment several factors that describe the low permeability obstacle are varied. The main conclusion is that the distance between the obstacle and the wells has the largest effect on doublet performance. Surprisingly very little gain in geothermal lifetime can be expected by varying well perforations.