Abstract

We conducted a number of experiments on cubic rock specimens with low matrix permeability to create and investigate multifrac systems for geothermal heat production on laboratory scale. Special focus was set on phenomena impeding the parallel propagation of multiple fractures like stress shadow and redistribution effects, rock heterogeneity and discontinuities. We subjected the specimens to a true-triaxial stress state using servo-controlled hydraulic pistons for external loading. High viscosity fluids were injected into parallel boreholes at controlled volume rates with metering pumps. Pressure transducers were connected to all boreholes to register injection pressure and pressure responses in neighbouring boreholes. We also recorded acoustic emissions during the experiment for spatial and temporal monitoring of fracture growth processes and the interaction between propagating fractures. After the creation of the hydraulic fractures, the hydraulic connection between the boreholes was tested by fluid circulation. As inferred from acoustic monitoring, radial fractures were propagated stable and technically controllable. Hydraulic connection between the boreholes was established and verified by hydraulic circulation.

Introduction

The production of geothermal heat from deep reservoirs with low permeability often requires the creation of artificial conductivity for sufficient fluid circulation rates. The multifrac concept, which is well established in the hydrocarbon industry, involves hydraulic fracturing of at least two parallel wellbores with horizontal orientation in a reservoir. The horizontal sections of these wellbores are drilled in the direction of the minor principal horizontal stress to enforce a radial orientation of the hydraulic fractures. Size of the fractures and distance of the parallel wellbores are designed to create a hydraulic connection between the wellbores for fluid circulation. Using e.g. multi-stage fracturing with multi-packer tools, several of these fractures can be created in parallel orientations along the horizontal sections of the wellbore. The objective is to create a large surface area in the reservoir rock to produce geothermal heat using fluid circulation.

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