--> Abstract: Mechanical Evolution Of Multi-Scale Compartmentalization In Porous Sandstone: From Joints To Faults, by R. Myers, W. L. Taylor, A. Aydin, and D. D. Pollard; #90928 (1999).

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MYERS, RODRICK, W. LANSING TAYLOR, ATILLA AYDIN, and DAVID D. POLLARD
Rock Fracture Project, Dept. Geol. & Env. Sci., Stanford University, Stanford, CA 94305-2115

Abstract: Mechanical Evolution of Multi-Scale Compartmentalization in Porous Sandstone: from Joints to Faults

Reservoir rocks can be compartmentalized by joints and faults at scales that range from centimeters to kilometers. Knowledge about the geometry and hydraulic properties of compartment bounding structures is important for their accurate representation in simulation models. We have identified a mechanical process of fault growth in porous sandstone where successive generations of joints are formed by slip on pre-existing joints creating a hierarchical fracture network. This hierarchy continues at larger scale where faults are formed by shearing of joint zones ' and new joints are formed in the fault periphery. In the initial stages of the deformation process, small hydraulic compartments (i.e. 10s m3) are formed when connected joint zones surround a block of matrix. The amount of fluid exchanged between fractures and compartments depends on the effective permeability of the joint zones, which is controlled by the spatial distribution of joints within the zone, their individual hydraulic properties, and the matrix permeability. With increasing slip magnitudes and at intermediate scales (i.e. 100s m3 shear across zones of joints produces secondary fractures at high angles to the initial joint zones. Permeability along individual sheared joints decreases due to development of fault gouge while fracture connectivity increases due to secondary fracturing. At this evolutionary stage, fault zones and the associated joint zones consist of heterogeneities with both enhanced and reduced permeability. At larger slip magnitude and large scales (i.e. 1000's m3), faults are composed of a well developed gouge core and a broad associated damage zone. The gouge core reduces the fault normal permeability, while high fracture density in the fault periphery increases fault parallel permeability. It is this dichotomy of structures that characterizes reservoirs compartmentalized by the process of dilation associated with shearing.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas