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

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.

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Mechanical Zonation of Rock Properties and the Development of Hydrothermal Fluid Circulation Pathways: Implications for Enhanced Geothermal Systems

10.21203/rs.3.rs-122204/v1 ◽

2020 ◽

Author(s):

Oladoyin Kolawole ◽

Ion Ispas ◽

Folarin Kolawole ◽

Christophe Germay ◽

John D McLennan

Keyword(s):

Mechanical Properties ◽

Hydrothermal Fluid ◽

Sedimentary Basins ◽

Geothermal Reservoir ◽

Rock Properties ◽

Initial Assessment ◽

Fluid Circulation ◽

Hydraulic Stimulation ◽

Stimulation Of

Abstract Oil and gas operations in sedimentary basins have revealed significant temperatures at depth, raising the possibility of major geothermal resource potential in the sedimentary sequences. The efficient development of such a resource may require enhancement by hydraulic stimulation. However, effective stimulation relies on an initial assessment of in-situ mechanical properties and a Discrete Fracture Network (DFN) model of the rock response. Here, we examine the distribution of mechanical properties (unconfined compressive strength, UCS ; ultrasonic velocity-derived Poisson ratio, ν; and, scratch-derived fracture toughness, K s ) along the cored interval of a sedimentary formation with a known geothermal anomaly in the Permian Basin, U.S. Our results reveal the mechanical heterogeneity of the rock, demonstrated by four distinct alternating mechanical zones, which include: (1.) mechanically weaker 0.17 m-thick Zone-A and 0.18 m-thick Zone-C with mean UCS = 110 MPa, ν = 0.25, K s = 1.89 MPa·√m; and (2.) mechanically stronger 0.41 m-thick Zone-B and 0.15 m-thick Zone-D which show mean UCS = 166 MPa, ν = 0.22, and K s = 2.87 MPa·√m. Although X-ray Diffraction analyses of the samples suggest that the entire rock matrix is dominated by dolomite, the stronger zones show a higher abundance of quartz (>30%) and relatively lower phyllosilicate mineral content (<2%) than the weaker zones. Further, we observe that the mechanically stronger zones have the greatest occurrences of hydrothermal alterations (anhydrite veins and nodules ), indicating that the cored interval had experienced hydrothermal fluid circulation in the past. We infer that the denser clustering of fractures in the stronger zones which facilitated the hydrothermal vein development was due to the influence of mechanical stratigraphy on the brittle deformation and alteration of the sedimentary-hosted hydrothermal reservoir. Thus, we suggest that the stronger zones represent viable targets for hydraulic stimulation of a geothermal reservoir, both for the emplacement of new fractures and the linkage of pre-existing fractures. Our findings in this study provide an analog for hydraulic stimulation of viable geothermal reservoir targets at higher in-situ temperatures and higher geothermal gradients.

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