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Building Better Fracture Models By Combining Structural Analysis and Bayesian Updating to Quantify the Relationship Between Multiple Fracture Causing Agents

Peter Hennings, Jason McLennan, and Tricia Allwardt
ConocoPhillips Subsurface Technology, Houston, TX

Gaining a better understanding of the impact that fractures have on reservoir hydraulic behavior often requires interpolating between areas of limited geologic control, such as wellbore or outcrop data. A common approach taken is to perform structural analysis on a properly constructed 3D model of the reservoir to generate attributes that relate to the deformation. Structural attributes can be generalized into classes: morphologic--relating to the present-day shape of the reservoir, kinematic--relating qualitatively to incremental or cumulative deformation, and mechanistic--relating quantitatively to incremental or cumulative stress or strain. It is common to discover that a combination of attributes best explains patterns of fracture occurrence; however, these attributes are usually calibrated to fracture intensity and used to infer the distribution between points of control one at a time. To provide the best possible model describing fracture occurrence, the multiple structural attributes should be combined simultaneously in a mathematically unbiased and consistent manner.

Bayesian Updating is a statistical theory relating conditional probabilities through multivariate correlations. In considering the prediction of a primary event A with two secondary events B and C, Bayesian Updating provides the conditional probability of A given events B and C. This is done by combining B and C into a likelihood that updates A. In this application A is fracture intensity and B and C are structural attributes.

To illustrate this technique we assessed the distribution of tectonically-produced fractures in the Frontier #1 Sandstone flanking Oil Mountain Anticline in central Wyoming. The intensity of folding-related fractures, which are easily separable from those that predate folding, were obtained by continuous scan-line measurement of fracture spacing along a 5 km outcrop extending from both flanks of the anticline and around the plunging nose. We compared fracture intensity against 13 morphologic and kinematic structural attributes and find that a measure “flexural-folding strain”, as a single attribute, best explains fracture intensity. Convolving that attribute with a properly filtered measure of curvature improves the correlation. We use the combined attribute to populate the entire anticline with tectonic fractures (as well as pre-tectonic fractures) and consider the result to be a viable analog describing fracture development in similar lithotectonic cases.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado