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2013-14 Tour Information
Western North America:
• March 3-14, 2014
Research Scientist at the Bureau of Economic Geology in the Jackson School of Geosciences, University of Texas at Austin
Funded by the AAPG Foundation
Julia Gale started her career in geology with undergraduate studies at Imperial College, London. She obtained a Ph.D. in Structural Geology from Exeter University, UK in 1987, working on the Archean of southern West Greenland. She taught structural geology and tectonics for 12 years at the University of Derby, UK, having research interests in the Dalradian of NE Scotland and the Mona Complex of Anglesey, NW Wales. Julia moved to the University of Texas at Austin in 1998, where she is a Research Scientist at the Bureau of Economic Geology in the Jackson School of Geosciences. Her research focus is on natural fracture characterization and prediction in shale and carbonate hydrocarbon reservoirs.
Using examples from shale reservoirs worldwide, I demonstrate the diversity of shale-hosted fracture systems and present evidence for how and why various fractures systems form. Core and outcrop observations, strength tests on shale and on fractures in core, and geomechanical models allow prediction of fracture patterns and attributes that can be taken into account in well placement and hydraulic fracture treatment design. Both open and sealed fractures can interact with and modify hydraulic fracture size and shape. Open fractures can enhance reservoir permeability but may conduct treatment fluids great distances, in some instances possibly aseismically.
We have addressed the challenge of incomplete sampling of subsurface fractures through comprehensive fracture data collection in cores and image logs and careful selection of outcrops, coupled with an understanding of how fractures and their attributes scale. We also use tested mechanistic models of how fractures grow in tight sandstones and carbonates to interpret fractures in shale. In order to predict fracture patterns and attributes it is helpful to understand their mechanism of formation and timing in the context of the burial and tectonic histories of the basin in which they are forming. A key variable is the depth of burial, and thereby the temperature, pore-fluid pressure and effective stress at the time of fracture development. For the most part the origin of fractures cannot be determined from their orientation or commonly-measured attributes such as width, height and length. The mineral fill in sealed fractures does provide an opportunity, however, and we use fluid-inclusion studies of fracture cements tied to burial history to unravel their origin.
Interaction with hydraulic fracture treatments may serve to increase the effectiveness of the hydraulic fracture network, or could work against it. Factors governing the interaction include natural fracture intensity, orientation with respect to reservoir stress directions, and the strength of the fracture plane relative to intact host rock. We tested the effect of calcite-sealed fractures in Barnett Shale on tensile strength of shale with a bending test. Samples containing natural fractures have half the tensile strength of those without and always break along the natural fracture plane. Yet in other examples the weakness is in the cement itself, partly because of retained fracture porosity.
Natural fractures in shales likely grew by slow, chemically assisted (subcritical) propagation and we use a subcritical propagation criterion to model the growing fractures. The subcritical crack index is a mechanical rock property that controls fracture spacing and an input parameter for the models. We measured the subcritical crack index for several shales. The index is generally high for Barnett Shale, in excess of 100, although it does show variability with facies. By contrast, subcritical indices in the New Albany Shale are much lower, and also show considerable variability. Barnett Shale subcritical indices suggest high clustering whereas New Albany Shale subcritical indices suggest fractures are likely to be more evenly spaced, with spacing related to mechanical layer thickness. We are investigating the variability in subcritical index in shale and how it might tie to other rock properties.
Natural fractures are a prominent and dramatic feature of many outcrops and are commonly observed in core, where they govern subsurface fluid flow and rock strength. Examples from more than 20 fractured reservoirs show a wide range of fracture sizes and patterns of spatial organization. These patterns can be understood in terms of geochemical and mechanical processes across a range of scales. Fractures in core show pervasive evidence of geochemical reactions; more than is typical of fractures in many outcrops. Accounting for geochemistry and size and size-arrangement and their interactions leads to better predictions of fluid flow.
Opening-mode fracture apertures commonly follow power-law size distributions with opening displacements ranging from approximately 1 µm to 1 m. A power law forms a straight line on a log cumulative frequency versus log aperture size plot. The slope of the line is the power-law exponent, reflecting the relative number of narrow and wide fractures in the set. The pre-exponential coefficient reflects the overall fracture intensity. We will examine the variation in power-law exponent and coefficient for fracture sets in carbonate and siliciclastic rocks and analyze why such variation occurs. Fractures may open in a single event or may repeatedly open and seal. During an opening event the rate of opening competes with the fastest rates of precipitation to determine if the fracture will seal before the next strain increment. Small fractures completely seal with cement precipitated synchronously with opening, whereas large fractures may retain some porosity. The aperture size at which porosity is preserved varies, and it is controlled by the temperature of the ambient fluid, the composition and texture of the host rock and precipitating minerals, and the length of time the fracture wall is exposed to mineral precipitation, which is dependent on burial history and fracture timing. If the widest fractures are not completely sealed before the next strain increment, they may act as planes of weakness, causing strain to progressively partition into fewer fractures, which will grow wider. The extent to which this process happens should partly govern the exponent in the power-law distribution. Cements deposited whilst fractures are growing may cause fracture size distributions to vary from those found in barren fracture arrays (including many of those in outcrop).
Geochemical and fracture size interactions may also affect fracture spatial arrangements. Fractures may be evenly spaced, but more commonly fractures occur in complex and, in some cases, fractal arrays of clusters. We have developed a method, based on a two-point correlation integral, to rigorously identify different types of spatial arrangement, including periodic, random, and clustered. Our method provides a measure of preferred spacing relative to that expected from a random ordering of spacings. I will show examples from outcrop data sets and from fractures interpreted in image logs in shale gas wells.