Copyright by Igor Shovkun 2018 The Dissertation Committee for Igor Shovkun certifies that this is the approved version of the following dissertation: Coupled chemo-mechanical processes in reservoir geomechanics Committee: David Nicolas Espinoza, Supervisor Mukul M. Sharma John T. Foster Matthew T. Balhoff Marc E. Hesse Coupled chemo-mechanical processes in reservoir geomechanics by Igor Shovkun DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF TEXAS AT AUSTIN May 2018 Dedicated to my wife Maria. Acknowledgments Prima facea, I would like to express the deepest appreciation to my adviser, Dr. Nicolas Espinoza, who guided me through the long road of my PhD program and gave me enough freedom to satisfy my scientific curiosity. Without his super- vision and persistent efforts, this dissertation would not have been possible. I would like to extend my deep appreciation to Professor Mukul M. Sharma. His efforts provided me with an extensive knowledge on hydraulic fracturing, as well as an opportunity to gain industry exposure via his Hydraulic Fracturing and Sand Control JIP. I would like to thank my committee members, Professor Matthew T. Balhoff and Professor Marc A. Hesse , whose work provided me with ideas for my research and served as a benchmark of excellence. I owe a great deal to Professor Foster who opened to me the frontiers of numerical methods. Finally, I take this opportunity to express the profound gratitude to my beloved wife, Maria, for being with me through thick and thin, believing in me, and being patient to my engineering eccentricity. v Coupled chemo-mechanical processes in reservoir geomechanics Igor Shovkun, Ph.D. The University of Texas at Austin, 2018 Supervisor: David Nicolas Espinoza Reservoir geomechanics investigates the implications of rock deformation, strain localization, and failure for completion and production of subsurface energy reservoirs. For example, effective hydraulic fracture placement and reservoir pres- sure management are among the most important applications for maximizing hydro- carbon production. The correct use of these applications requires understanding the interaction of fluid flow and rock deformations. In the past a considerable amount of effort has been made to describe the role of poroelastic and thermal effects in geomechanics. However, a number of chemical processes that commonly occur in reservoir engineering have been disregarded in reservoir geomechanics despite their significant effect on the mechanical behavior of rocks and, therefore, fluid flow. This dissertation focuses on the mechanical effects of two particular chem- ical processes: gas-desorption from organic-rich rocks and mineral dissolution in carbonate-rich formations. The methods employ a combination of laboratory stud- ies, field data analysis, and numerical simulations at various length scales. vi The following conclusions are the results of this work: (1) the introduced numerical model for fluid flow with effects of gas sorption and shear-failure-impaired permeability captures the complex permeability evolution during gas production in coal reservoirs; the simulation results also indicate the presence non-negligible sorption stresses in shale reservoirs, (2) mineral dissolution of mineralized frac- tures, similar to pore pressure depletion or thermal cooling/heating can increase stress anisotropy, which can reactivate critically-oriented natural fractures; in-situ stress chemical manipulation can be used advantageously to enlarge the stimulated reservoir volume, (3) semicircular bending experiments on acidized rock samples show that non-planar fractures follow high porosity regions and large pores, and that fracture toughness correlates well with local porosity. Numerical modeling based on the Phase-Field approach shows that a direct relationship between fracture toughness and porosity permits replicating fracture stress intensity at initiation and non-planar fracture propagation patterns observed in experiments, and (4) numer- ical simulations based on a novel reactive fluid flow model coupled with geome- chanics show that mineral dissolution (i) lower fracture breakdown pressure, (ii) can bridge a transition from a toughness-dominated regime to uncontrolled fracture propagation at constant injection pressures, and (iii) can increase fracture complex- ity by facilitating propagation of stalled fracture branches. The understanding of these chemo-mechanical coupled processes is critical for safe and effective injection of CO2 and reactive fluids in the subsurface, such as in hydraulic fracturing, deep geothermal energy, and carbon geological sequestra- tion applications. vii Table of Contents Acknowledgments v Abstract vi List of Tables xii List of Figures xiii Chapter 1. Introduction 1 1.1 Natural chemo-mechanical coupled processes in rocks . .1 1.2 Chemo-poro-mechanical processes in petroleum engineering appli- cations . .2 1.3 Outline of the dissertation . .4 Chapter 2. sorption 6 2.1 Introduction . .6 2.2 Model development . 11 2.2.1 Poromechanical model with sorption stress . 11 2.2.2 Fluid mass balance and flow . 13 2.2.3 Stress-dependent permeability model . 15 2.3 Numerical solution method . 17 2.3.1 Solution algorithm . 17 2.3.2 Finite Element implementation . 18 2.4 Validation . 18 2.4.1 Stresses in the vicinity of hydraulic fractures . 18 2.4.2 Gas flow . 19 2.4.3 Stress path . 20 2.5 Case studies . 20 2.5.1 San Juan coal basin . 20 viii 2.5.1.1 Stress-dependent permeability: fracture compress- ibility only . 23 2.5.1.2 Stress-dependent permeability: fracture compress- ibility, dilation and shear failure . 28 2.5.2 Barnett shale . 30 2.6 Discussion . 33 2.6.1 Parametric analysis of the model with fracture compressibil- ity only . 33 2.6.2 Parametric analysis of the model with shear failure . 38 2.6.3 Gas sorption stresses in shales . 40 2.7 Conclusions . 41 Chapter 3. dissolution 45 3.1 Introduction . 45 3.2 Analytical model of stress relaxation induced by localized mineral dissolution . 49 3.2.1 Dissolution of a single vein . 50 3.2.2 Critical amount of carbonates to attain shear reactivation . 53 3.3 Experimental evidence of stress relaxation . 55 3.3.1 Rock sample . 55 3.3.2 Procedure . 56 3.3.3 Vein dissolution results . 58 3.3.3.1 Constant deviatoric stress . 58 3.3.3.2 Constant total vertical stress . 59 3.4 Reservoir scale simulation . 60 3.4.1 Analogy of mineral dissolution with anisotropic thermal cool- ing ............................... 60 3.4.2 Case problem . 63 3.4.2.1 State of stresses during consecutive fracturing . 63 3.4.2.2 State of stresses during alternate fracturing and stress reorientation . 65 3.4.2.3 Reactivation of natural fractures due to acidizing . 68 3.5 Discussion . 70 3.5.1 Uniform dissolution versus etching . 70 ix 3.5.2 Reactive fluid flow modeling . 71 3.5.3 Impact of stress relaxation on permeability . 72 3.6 Conclusion . 73 Chapter 4. Fracture propagation in heterogeneous porous media: pore- scale implications of mineral dissolution 76 4.1 Introduction . 76 4.2 Experimental determination of fracturing behavior . 79 4.2.1 Methods . 80 4.2.1.1 Rock samples . 80 4.2.1.2 Acidizing procedure . 80 4.2.1.3 Semicircular bending experiments . 81 4.2.1.4 X-ray microtomography and image processing . 82 4.2.2 Experimental results . 83 4.3 Numerical modeling of fracture initiation and propagation . 86 4.3.1 Phase-field method . 86 4.3.2 Material properties: Toughness mapping . 87 4.3.3 Phase-field fracture simulation results . 88 4.3.3.1 Homogeneous samples: fracture toughness from peak load in SCB experiments . 88 4.3.3.2 Heterogeneous samples: fracture propagation . 92 4.4 Discussion . 97 4.4.1 The impact of the toughness-porosity relation on fracture propagation . 97 4.4.2 Fracture propagation with heterogeneous maps of Young’s modulus and fracture toughness . 99 4.4.3 Extension to reactive fluid flow models coupled with geome- chanics . 103 4.4.4 Comparison with previous work, other methods and fluid- driven modeling approaches . 105 4.4.5 Implications to natural and engineering geosystems . 107 4.5 Conclusions . 108 x Chapter 5. Propagation of toughness-dominated fluid-driven fractures in reactive porous media 110 5.1 Introduction . 110 5.2 A coupled phase-field model for fluid-driven fractures in reactive poroelastic media . 114 5.2.1 Mechanics formulation . 115 5.2.2 Fluid mass balance and flow . 116 5.2.3 Reactive flow modeling . 117 5.2.4 Toughness-porosity model . 118 5.2.5 Numerical solution method . 119 5.2.6 Validation . 120 5.3 Results . 124 5.3.1 Mineral dissolution-assisted fracture propagation . 125 5.3.2 Sensitivity analysis . 129 5.4 Discussion . 133 5.4.1 Extension to fully-coupled reactive flow including modeling of wormholes . 133 5.4.2 Experimental validation . 135 5.4.3 Applications for acid fracturing in tight formations . 136 5.5 Conclusions . 137 Chapter 6. Conclusions 139 Bibliography 143 xi List of Tables 2.1 Simulation parameters and initial conditions. (∗) Assumed, (∗∗) Best fit.................................... 25 2.2 Simulation parameters used in history matching of the data from San Juan coal using the model with shear failure. (∗) Best fit. 29 3.1 Typical in-situ stresses, shale properties and multi-stage hydraulic fracturing geometry for the Barnett Shale . 64 xii List of Figures 2.1 Typical values of Young’s moduli and maximum volumetric sorption- induced swelling strains in various organic shale and coal reser- voirs. 10 2.2 Validation of the numerical model. (a) Stresses profile along x-axis starting from the center of the fracture. (b) Stress profile along y- axis starting from the fracture tip. (c) Pressure profile as a function of distance from the wellbore. (d) Total horizontal stress path pre- dicted by plane strain and pseudo 3D linear poroelasticity (used in the study) models.