Abstract

Since decades proppants have been used to enhance hydraulic pathways for the exploitation of hydrocarbon reservoirs. These technologies may also be adapted for geothermal applications, but geothermal reservoirs rocks strongly differ from those typically targeted by hydrocarbon industries. Therefore, the mechanical interaction between conventional proppants and geothermal reservoir rocks and their effect on hydraulic properties need to be investigated in more detail to maximize the success of such method adaption. In principle, two mechanical requirements must be fulfilled: (1) proppants must withstand the stresses that are present in a reservoir, and (2) proppants must not penetrate the rock formation of the reservoir, because this would reduce the fracture conductivity. Fine mobilized rock- or proppant particles can also clog the fracture. For the mechanical suitability of proppants, uniaxial compressive strength tests and long-term creep tests were carried out in the laboratory. The test setup consists of a proppant monolayer placed between two cylindrical rock samples that were axially loaded. We determined Young’s moduli and creep rates for sample stacks with various proppants under different stress conditions. The laboratory tests are part of the ZoKrateS Project, which aims at showing the feasibility of enhancing fractured carbonate rock mass by proppant placement for geothermal applications.

Introduction

Due to climate change and neutral climate balance, new developed strategies for the use of renewable energies are needed. Not only the established methods such as wind energy or solar energy should be applied, but also alternatives should be considered.

The collaborative research project ZoKrateS follows an alternative approach for the exploitation of deep geothermal reservoirs. The hydraulic efficiency of proppants, which have not yet been used in deep geothermal systems, is to be demonstrated. So far, proppants have been used in the hydrocarbon industry to stabilize hydraulically stimulated fractures and to increase the permeability of the geological target horizon [1]. To develop a petrothermal system, proppants, possibly in combination with zoned acidizing, could also be introduced into natural fracture zones of deep-seated carbonates. We present results of laboratory experiments conducted to evaluate the technical feasibility of such a pilot project. The experiments aimed at answering the following questions:

• Are the proppants capable of withstanding the in-situ stress, or do the proppants fail under stress application in contact with the carbonate formation?

• Are the proppants capable of keeping the fractures open, or are proppants pushed into the carbonate formation beyond a tolerable level for the relevant loading?

• Will rock fragments be mobilized due to fracture surface damage associated with the proppant compression, resulting in solids discharge that may clog the existing fracture or damage engineering facilities?

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