Use of Mohr Diagrams to Predict Fracturing in a Potential Geothermal Reservoir

D.C.P Peacock,David J Sanderson,Bernd Leiss

doi:10.3390/geosciences11120501

Abstract

Inferences have to be made about likely structures and their effects on fluid flow in a geothermal reservoir at the pre-drilling stage. Simple mechanical modelling, using reasonable ranges of values for rock properties, stresses and fluid pressures, is used here to predict the range of possible structures that are likely to exist in the sub-surface and that may be generated during stimulation of a potential geothermal reservoir. In particular, Mohr diagrams are used to show under what fluid pressures and stresses different types and orientations of fractures are likely to be reactivated or generated. The approach enables the effects of parameters to be modelled individually, and for the types and orientations of fractures to be considered. This modelling is useful for helping geoscientists consider, model, and predict the ranges of mechanical properties of rock, stresses, fluid pressures, and the resultant fractures that are likely to occur in the sub-surface. Here, the modelling is applied to folded and thrusted greywackes and slates, which are planned to be developed as an Enhanced Geothermal System beneath Göttingen.

Highlights

The methods presented here have general applicability to making predictions about fractures in the sub-surface, but here we focus on several key questions about the proposed geothermal reservoir at Göttingen: 1
We consider the possible effects of stimulation by increasing fluid pressure on the folded, cleaved and veined Devonian and Carboniferous greywackes and slates, based on the modelling discussed in Section 5 and using a simple schematic figure for these rocks (Figure 7a)
This paper shows how simple mechanical modelling, using Mohr diagrams and reasonable ranges of values for rock properties, stresses, and fluid pressures, can be used to predict fracturing in a potential geothermal reservoir

Summary

Introduction

The reservoir rocks are the Devonian and Carboniferous metasedimentary sequence thought to occur at a depth of ~1.5 km, beneath a cover of Permian and Mesozoic sedimentary rocks [4]. The Triassic Bunter Sandstone is being considered as a potential medium-deep geothermal reservoir Two wells have yet penetrated the metasedimentary rocks in the Göttingen area, these not extending far below the base Zechstein, and so limited well data are available. Two seismic lines were shot in 2015, these being 10 km and 11 km long and to about 5 km deep These seismic lines allow interpretation of the post-Carboniferous sedimentary rocks in the north-south striking Leinetal Graben [3], but do not enable reliable interpretation of the sub-Zechstein rocks.

Objectives

Discussion

Conclusion