Response of a geothermal reservoir consisting of multiple cracks to hydraulic stimulation

A model is presented for a geothermal reservoir consisting of multiple cracks and the characteristics of the response of such a reservoir has been clarified. The basic ideas of the model are as follows; (1) the reservoir consists of penny-shaped cracks developed along natural planes of weakness of a rock mass, (2) the stress intensity factor along the periphery of each crack is equal to the fracture toughness of the plane of weakness along which the penny-shaped crack is developed and (3) the fluid flow within the cracks is turbulent. It has been revealed that there are two basic cases depending on the injection flow rate and the fracture toughness of the planes of weakness. When the injection flow rate is low and fracture toughness is large, the wellbore pressure decreases with crack growth. And when the flow rate is high and the fracture toughness is small, the wellbore pressure increases with crack growth. The model has been applied to a set of field data of hydraulic stimulation conducted at a geothermal model field to clarify the growth process of the reservoir system, such as the change of morphology with respect to the flow rate. The results agree well with those estimated on the basis of AE data observed during hydraulic stimulation.