The Influence of Jointed Fabric and Stress Ratio on Deformation and Fluid Flow
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Date
2017-01-24
Authors
Pirayehgar, Atena
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
There are remarkable volumes of hydrocarbon resources in low-permeability naturally fractured rock masses (NFRs) characterized by naturally occurring discontinuities. Natural fractures are one of the most important factors controlling the hydraulic behavior of the rock masses and most low-permeability rocks are very strong, stiff, and fractured. Hydraulic fracturing in low-permeability hard NFRs has recently gained popularity both in the petroleum and mining industries, with different goals.
A strong understanding of the behavior of natural fractures provides engineers with better insight into the hydraulic fracturing technology and development strategies. To gain this knowledge, different methodologies and mathematical codes have been introduced to numerically model jointed systems. The first step for every model would be generating geometries that realistically represent the naturally fractured rock mass. Consequently, generating fabrics with different geometric attributes and assessing fabric effects on fluid flow and deformation are points of interest in this research. In addition, stress fields play a prominent role in hydraulic and mechanical responses of natural fractures, and this forms another core direction of the current project.
This research presents some attempts to simulate and emulate hydraulic fracturing in hard low-permeability naturally fractured rock masses. In a rock with a hard matrix, only pre-existing fractures may re-activate during the process of pressurization and no new fractures are created. Analysis may lead to a better physical and empirical explanation for how different fabric patterns and deviatoric stress conditions affect the hydro-mechanical behavior of NFRs during injection and after shut-in.
A commercial software, Universal Distinct Element Code (UDECTM), is used to generate NFRs in the study. Various geometries have been generated in a two-dimensional framework and subjected to biaxial stresses. Among all generated fabrics, three of the primary contributors to the study include: Voronoi tessellation, Cross-joints and Cross-cuts. Given stipulated differences in in-situ stresses, pore pressure distribution during injection is monitored and shear and normal displacements of joints are investigated. Some important concepts such as size of the stimulated zone and induced shear events were studied in conjunction with this data collection.
In order to calibrate numerical emulations with field data, two case studies of waste disposal were modeled. The first attempt was to find the closest geometry and subsequently match the bottom-hole pressure with the fluid pressure at the injection point. Some success in gaining a reasonable and good correspondence indicates that the studies can contribute to greater understanding of such complicated subjects and are of essential importance in qualitative and semi-quantitative evaluations.
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Keywords
Hydraulic Fracturing, Discrete Element Method, Numerical Modeling, Natural Fractures, Fluid Flow, Mechanical Deformation