Hydraulic Fracturing in Limestone: A Case Study of Two EGS Projects in the Netherlands

Abstract

Geothermal projects in the Netherlands are developing steadily. As the country is in a low to medium enthalpy environment, all realized projects so far are using geothermal water for heating purposes. Recently, interest in deep geothermal energy for electricity production has increased. Two locations are currently undergoing a detailed study to explore the possibilities of heat extraction from a 5 to 7 km deep limestone formation. These depths represent minimum temperatures of respectively 160 to 220 ºC. This paper addresses the locations of Hoogeveen and Renkum as a case study to highlight the reservoir characteristics and the potential for deep geothermal energy. The challenge is to engineer a hydraulic fracturing treatment in a relatively poorly understood deep limestone formation. In the Netherlands the Early Carboniferous Limestone Group is represented by the Zeeland Formation (in literature also referred to as Kolenkalk). The formation is only found in a few wells within the Netherlands. As the depth affects the primary permeability, the highest values are in the order of microdarcies. From log measurements it is concluded that the formation is characterized by a high Young’s modulus and a high Poisson’s ratio. Natural fractures are believed to be present, though additional research is required to characterize the fracture network. From the limestone’s low permeability it can be concluded that a conventional hydrothermal circulation system is not feasible and therefore the focus is on reservoir stimulation. The chal-lenge is to establish high enough flow rates for a commercially attractive project without having the risk of a thermal breakthrough. The concept is to drill a horizontal well and generate a number of transverse, long fractures (>1000 m). By monitoring their orientation and length, the goal is to penetrate them with the second well to establish a circulation system. The second challenge is to design a hydraulic fracturing treatment when a natural fracture network is present. Sophisticated fracture modeling software (MFrac developed by Meyer and Associates) is used to model fracture dimensions and productivity given a certain set of input rock and fluid loss properties. First, a sensitivity analysis is conducted using a reliable set of input parameters from one of the three wells. The first model simulations show that several scenarios are possible. A conventional high conductivity fracture cannot be applied. It will not generate enough length due to the high-viscosity fracturing fluids. A proppant-less or low concentration proppant waterfrac creates sufficient length but lacks fracture conductivity. Moreover, waterfracs rely on shear stress to establish misalignment to prevent them from closing completely. This process is hard to model or control. A hybrid treatment using separate flushes of low and high viscosity fluids is investigated. Acid injection after the fracture treatment might create the extra fracture face roughness that is needed, in case shear stress is absent. This possibility is also evaluated.