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GCTesting Curvature’s Impact on Fractures*
Evan Staples1 and Kurt Marfurt2
Search and Discovery Article #41147 (2013)
Posted July 22, 2013
*Adapted from the Geophysical Corner column, prepared by the authors, in AAPG Explorer, July, 2013.
Editor of Geophysical Corner is Satinder Chopra ([email protected]). Managing Editor of AAPG Explorer is Vern Stefanic
1ConocoPhillips, Houston, Texas
([email protected])
2University of Oklahoma, Norman, Oklahoma
Fracture zones can be critical to improving or creating sufficient porosity and permeability in hydrocarbon reservoirs - with strain, along with lithology and thickness being the major controls. In this article we will use layer curvature as a proxy for strain induced by layer bending. We test our hypothesis using small-scale clay models, which provide a time history of folding and fracturing, and are commonly considered analogs for large-scale field structures. The experimental apparatus consisted of a horizontal table with one moveable sidewall, one stationary sidewall and a deforming base.
We present four experiments
- one extensional and three compressional.
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Clay We use wet clay with dimensions of 20 centimeters long, 15 centimeters wide and five centimeters thick. A laser scanner positioned above the clay cakes captured 3-D surface images. A typical experiment lasted approximately 30 minutes with a laser scan occurring every two minutes; this short duration eliminated clay drying as a variable. The curvature of the clay surface was calculated from the laser scans using commercial software, and the fractures were mapped on digital photographs of the clay surface. Curvature over the deformation area was calculated in each stage by placing three polygons at fixed locations and averaging the curvature within each polygon. The fracture intensity (FI) was calculated for these polygons by dividing total measured fracture length in each polygon by its area. In the extensional experiment, a basin formed above the moving plates with a synthetic normal fault on one side, and a system of antithetic-normal faults, fractures and a flexure on the other side (top of Figure 1). Curvature and fracture intensity measurements were conducted on the later side of the basin. The bottom of Figure 1 displays a suite of positive curvature images from initial through final stages.
Fractures were first visible on the clay surface with a measured curvature of 2.53 x 10-3 cm-1. We observed that curvature and fracture intensity increased with time, but was dependent on fault movement. Normal faults started as individual segments and eventually joined up to form faults that spanned the entire clay Compressional In the three compressional
During the
Curvature increased over time with systematic deformation occurring in the extensional, 15-degree compressional ramp and 30-degree compressional ramp
Calculated FI increased with curvature and correlations show a strong-positive linear relationship (Figure 3); however, fracturing did not occur at the same curvature value in each experiment. The extensional We interpret differences in horizontal strain as the main reason there is such a disparity in fracturing and curvature values at fracture initiation. In the extensional experiment, we assume that horizontal strain occurred throughout the experiment, since the basement plates were constantly moving apart - though we only measured vertical displacement. In the compressional settings, however, an anticline had to develop and grow before horizontal strain was great enough on its crest to induce fracturing. The clay In a future article we will show the correlation of curvature computed from 3-D seismic data to fractures seen in horizontal image logs. We would like to thank AAPG member Ze’ev Reches from the University of Oklahoma for help with this project and guidance in clay |