THE ROLE OF PORE STRUCTURE IN PERMEABILITY EVOLUTION OBSERVED IN LABORATORY STUDIES OF MARCELLUS AND WOLFCAMP SHALE

Open Access
Author:
Schwartz, Brandon
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 05, 2018
Committee Members:
  • Derek Elsworth, Dissertation Advisor
  • Derek Elsworth, Committee Chair
  • Hamid Emami-Meybodi, Committee Member
  • Shimin Liu, Committee Member
  • Chris J Marone, Outside Member
Keywords:
  • permeability evolution
  • shale
  • pore structure
  • pore geometry
  • material stiffness
  • permeability enhancement
Abstract:
We explored the role of pore geometry and stiffness on the distribution of strain around pores for Marcellus and Wolfcamp shales. Relationships exist to model permeability evolution as well as bulk stiffness evolution—here we find a relationship relating these two variables to each other. Whereas bulk stiffness is determined by bulk mineralogy and initial pore structure, evolving bulk stiffness is determined by the evolution of the pore structure alone. Permeability evolution is also determined by the evolution of the pore structure. We cast the permeability evolution in terms of evolving material properties including the Poisson ratio, the crack density parameter, and the bulk modulus—all of which can be measured via acoustic waves. The end result is a method to measure permeability evolution via acoustic waves alone. We modeled the effects of fracture spacing, aspect ratio, and pore stiffness on the permeability evolution of an ellipsoid crack under uniaxial stress and varying pore pressure. We found that rocks undergoing identical compressional strain and pore pressure can undergo significantly different magnitudes of fracture closure or dilation based on these three variables. This is especially important is gas shales, where nano-porosity is challenging to characterize and heterogeneity between basins has led to disparate permeability responses in the field and in the laboratory. We found that the aspect ratio is the most sensitive parameter influencing pore compressibility. The fracture spacing becomes important when external stress is applied, but it has no significant effect when pore pressure is varied is the absence of external stress. To capture effects of mineral distribution around pores, we simulated mismatches between a pore’s skeletal stiffness and the surrounding matrix and determined that for a given strain soft pores relative to the bulk material experience greater permeability evolution than pores that are stiff relative to the surrounding matrix. While soft pores experience greater closure than stiff pores for a given applied stress, they also experience a greater amount of dilation when pore pressure increases. This highlights that while some shale basins such as the Marcellus can experience large permeability drops relative to other basins given the same production conditions, pressure maintenance may be the most important tool to preserve permeability. We compare the permeability response of Marcellus shale to Wolfcamp shale under changing strain to explore differences in pore structure between them. This work highlights that while magnitude of strain for a given stress is determined predominantly through a shale’s mineral composition, the response of transport properties to a given strain are dependent on fracture spacing, fracture geometry, and mineral distribution around pores. We dynamically stress samples of Marcellus and Wolfcamp shales and observed levels of compaction, creep, and permeability evolution. We characterize the differences between the two shales using bulk mineralogy, SEM imaging with elemental analysis, and the cubic law for permeability evolution. We find that the Marcellus shale is comprised predominantly of clays that leads to more deformation when stressed than the Wolfcamp shale which is composed predominantly of quartz and calcite. The level of creep and compaction are directly related to the amount of clay in each shale sample. Modifications to the cubic law for fluid flow reveal that Marcellus shale has a lower fracture density than the Wolfcamp shale, that the pore geometry more closely resembles slit-like pores, and that the mineral distribution around the pore space is soft compared to the Wolfcamp shale. These differences cause the Marcellus shale to experience much greater permeability reduction under the same compressive strain than the Wolfcamp. The result of our study is a unique strain-driven model to capture permeability evolution in shale due to differences in pore structure. We show that nitrogen flooding can double matrix permeability of gas shales. In laboratory experiments, nitrogen gas increased permeability in the bedding-parallel and bedding-perpendicular directions by 206% and 234%, respectively. Experiments are performed at constant stress, pore pressure, and temperature. We build a model to show that the permeability enhancement is controlled by the sorptive strain, pore geometry, and the spacing-to-aperture ratio. This work addresses how an organic-poor shale can experience large permeability changes driven by sorption induced strains. We plot methane and helium permeability curves as a function of pore pressure to isolate the portion of permeability evolution controlled by sorption. We independently build strain curves to solve for the sorptive strain and find good agreement between these two methods. This work demonstrates that matrix permeability in gas shales can be doubled, which suggests that ultimate recovery can be improved as well. We explore relationships among bulk modulus, crack density, and permeability through repetitive loading of Marcellus shale. Cumulative cyclic stressing (22-26 MPa with confinement of 24 MPa) is applied at a frequency of 0.05 Hz over 100,000 cycles. Changes in acoustic velocities are used to follow changes in dynamic bulk modulus, Poisson ratio, and crack density and to correlate these with bedding-parallel measurements of methane permeability. The shale is represented as an orthotropic elastic medium containing a dominant, noninteracting fracture set separated by thin laminae. An effective continuum model links permeability evolution to the evolution of the bulk modulus and crack density. Bulk modulus is linearly related to crack density by a scaling parameter representing rock fabric and fracture geometry. The Poisson ratio and bulk modulus of the intact, uncracked shale are deduced from our data. We propose a method for tracking permeability evolution of finely laminated shale using acoustic waves.