Mechanical zonation of rock properties and the development of fluid migration pathways: implications for enhanced geothermal systems in sedimentary-hosted geothermal reservoirs

Abstract

Oil and gas operations in sedimentary basins have revealed the occurrence of significant temperature anomalies 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 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 toughness, Ks) along the cored interval of a sedimentary formation with a known low-to-medium temperature geothermal anomaly in the Permian Basin, U.S. Our results reveal the presence of mechanical stratigraphy along the core, demonstrated by the alternation of distinct soft–hard (i.e.,less stiff-to-stiff) mechanical zone couplets composed of: (1) mechanically softer 0.17-m-thick Zone-A and 0.18-m-thick Zone-C with mean values of UCS = 110 MPa, ν = 0.25, Ks = 1.89 MPa·√m; and (2) mechanically harder 0.41-m-thick Zone-B and 0.15-m-thick Zone-D which show mean values of UCS = 166 MPa, ν = 0.22, and Ks = 2.87 MPa·√m. Although X-ray diffraction analyses of the samples suggest that the entire rock matrix is dominated by dolomite, the harder zones show an abundance of quartz cement (> 30%) and relatively lower phyllosilicate mineral content (< 2%) than the softer zones. Further, we observe that the mechanically harder zones have the greatest occurrences and thicknesses of hydrothermal alterations (anhydrite veins and nodules), indicating that the rock had experienced hydrothermal fluid circulation (basinal brines) in the past. We infer that the mechanical stratigraphy most likely influenced the spatial clustering of fractures that facilitated hydrothermal fluid migration in the past, and provides insight that is relevant for the exploitation of geothermal energy resources in sedimentary basins. We suggest that the harder zones or formation intervals with higher ratios of the hard zones relative to soft zones represent viable targets for hydraulic stimulation of a sedimentary-hosted geothermal reservoir, both for the emplacement of new fractures and the linkage of pre-existing fractures to allow efficient fluid circulation. Our findings in this study provide insight that is relevant for understanding the complexity of pre-existing mechanical heterogeneity in sedimentary-hosted geothermal reservoir targets in other places.

Pore morphology effect on elastic and fluid flow properties in Bakken formation using rock physics modeling

“…Previous field observations, experimental, and numerical studies have demonstrated that fractures propagating from a stiff interval towards a less-stiff one tend to propagate more efficiently and commonly penetrate the contacts (Laubach et al 2009;Gudmundsson et al 2010;Slatt and Abousleiman 2011;Ferrill et al 2014;Galvis et al 2018;Kolawole et al 2021b). These suggest that induced fractures from target lithofacies characterized by low aspect ratios, and stiffer rocks with higher bulk and shear modulus, will tend to generate a coherent and connected fracture network that can connect with other pre-existing natural fractures or reactivate faults.…”

“…This phenomenon is caused by the way these pores are shaped; cracks are weaker than intragranular pores, which are known as stiff pores, and as the pore spaces increase, this stiffness difference becomes more pronounced. Regarding the susceptibility of rocks to faulting and fracture, the seismic velocities (V p and V s values) are extremely important (Kolawole et al , 2021b. Therefore, fracturing and faulting are strongly associated with pores that have a low aspect ratio (0.01 and 0.05).…”

“…Unconventional reservoirs are a critical resource in this era of energy transition and sustainability due to their important contributions to energy production and geo-storage. Currently, there is an increased focus on CO2-enhanced oil recovery (CO 2 -EOR) (Welkenhuyse et al 2018;Wei et al 2021;Kashkooli et al 2022) and geological CO 2 storage (GCS) (Eshkalak and Aybar 2015;Senger et al 2015;Levine et al 2016;Azenkeng et al 2020;Kolawole et al 2021aKolawole et al , 2022Ozotta et al 2021a, b, c) in unconventional hydrocarbon reservoirs, and the extraction of hot fluid for energy through enhanced geothermal systems (Gholizadeh Doonechaly et al 2013;McClure and Horne 2014;Kolawole et al 2021b). These energy solutions can only be achieved through efficient stimulation to develop a complex fracture network and pore structure in the host rocks to extract heat and hydrocarbon, or for CO 2 storage.…”

Influence of Reservoir Heterogeneity on Simultaneous Geothermal Energy Extraction and CO2 Storage

“…Fluid migration pathways are pivotal in enhancing the efficiency and sustainability of geothermal energy extraction. Kolawole et al [44] elucidated the mechanical zonation of rock properties and the development of fluid migration pathways in sedimentary-hosted geothermal reservoirs. Fan et al [45] highlighted the depth of the Curie isotherm and the resistivity of sedimentary caprock as significant factors affecting a geothermal system's performance.…”