Building Better Fracture Models by Combining Structural Analysis and Bayesian Updating to Quantify the Relationship between Multiple Fracture Causing Agents
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 #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009