Permeability Evolution of Stressed Fractures Permeated by Reactive Fluids

Open Access
Author:
McGuire, Thomas Patrick
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 24, 2012
Committee Members:
  • Derek Elsworth, Dissertation Advisor
  • Chris Marone, Committee Member
  • Zuleima T Karpyn, Committee Member
  • Li Li, Committee Member
Keywords:
  • fracture permeability evolution
  • pressure solution
  • stress corrosion cracking
  • carbon dioxide sequestration
  • fracture strain
  • fracture dissolution
Abstract:
Understanding the dynamic response of stressed fractures during the flow of reactive fluids is an important contemporary research topic. Specifically, understanding the response of stressed fractures in carbonate is important for both energy (petroleum) production and sequestration of the products of energy use (carbon dioxide). We conduct tightly constrained experiments on artificial fractures with repeatable initial roughness and permeability to measure permeability response and rates of mineral dissolution during the flow of reactive fluids. These experiments are supplemented by detailed numerical modeling to better understand relevant mechanisms; namely pressure solution, stress corrosion cracking enhanced diffusion, precipitation, and free-face dissolution, that lead to permeability evolution of these fractures and to accurately quantify their relative rates. Experimental measurements are conducted on various lengths of 2.5 cm diameter cylindrical samples of tight, vuggy, Capitan Massive limestone. A single rough artificial fracture with repeatable initial permeability is created in the samples by making a single longitudinal saw cut prior to roughening the surface with either 60 grit (rough) or 150 grit (smooth) ceramic abrasive. The effective permeability-inferred hydraulic aperture of these fractures is monitored throughout each experiment by the mass balance of the percolating reactive fluid maintained at a fixed hydraulic gradient. Relative rates of pressure solution, stress corrosion cracking enhanced diffusion, precipitation, and free-face dissolution lead to either net fracture gaping or compaction. Dissolved mineral mass balance in the pore fluid provides an independent means of quantifying fracture closure or gaping, depending on the relative strength of each mechanism. To decipher these mixed effects numerical models are developed. Lumped parameter models of stress corrosion cracking enhanced diffusion and free-face dissolution provide adequate representation of short, low stress fracture aperture strain when coupled with experimentally measured fracture profiles and effluent concentrations of dissolved mineral. Finite element analysis of pressure solution, stress corrosion cracking enhanced diffusion, precipitation, and free-face dissolution also provide adequate agreement with both effluent concentrations of dissolved minerals and average rates of fracture aperture strain. These finite element models are based on previously quantified relative fracture flow areas, chemical kinetics, and other controling parameters. Results of these studies include development of a mass flux ratio to accurately determine the likely long-term evolution of fracture permeability in fractures under low ambient stress. In addition we show that pressure solution is a significant transient mechanism leading to permeability evolution, even in fractures subjected to low stress. We use the same experimental data and finite element model to present a method to quantify the probability of stable (compaction) and unstable (gaping) permeability evolution in fractures. Finally, agreement between experimental and simulation data allows strong assertions to be made regarding contrasting carbonate dissolution rates for free-face dissolution and pressure solution during flow of extremely under-saturated aqueous solutions of carbon dioxide. We measure a free-face dissolution rate that approaches theoretical pH-indicated limits during extremely short fracture residence times of extremely under-saturated fluids. This free-face dissolution rate rapidly declines to a rate simulated to be consistent with the proposed rates of pressure solution with high initial concentrations of dissolved minerals and long fracture residence times.