Basic concepts in the theory of seepage of homogeneous liquids in fissured rocks [strata]

Evaluating Fractured Reservoirs Reservoir Management Detecting And Predicting Fracture Occurrence and Intensity Analysis Of Anisotropic Reservoirs Analysis Procedures in Fractured Reservoirs Appendix A: List of Documented Fractured Reservoirs Appendix B: Procedures Checklist Appendix C: Averaging Techniques

Geologic Analysis of Naturally Fractured Reservoirs

Natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low-porosity and low-permeability shale host rock. Many shale outcrops, cores, and image logs contain fractures or fracture traces, and microseismic event patterns associated with hydraulic-fracture stimulation have been ascribed to natural fracture reactivation. Here we review previous work, and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization, orientation, and size patterns. A wide range of shales have a common suite of types and configurations of fractures: those at high angle to bedding, faults, bed-parallel fractures, early compacted fractures, and fractures associated with concretions. These fractures differ markedly in their prevalence and arrangement within each shale play, however, constituting different fracture stratigraphies—differences that depend on interface and mechanical properties governed by depositional, diagenetic, and structural setting. Several mechanisms may act independently or in combination to cause fracture growth, including differential compaction, local and regional stress changes associated with tectonic events, strain accommodation around large structures, catagenesis, and uplift. Fracture systems in shales are heterogeneous; they can enhance or detract from producibility, augment or reduce rock strength and the propensity to interact with hydraulic-fracture stimulation. Burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated. The role of microfractures in production from shale is currently poorly understood yet potentially critical; we identify a need for further work in this field and on the role of natural fractures generally.

Natural fractures in shale: A review and new observations

More than 900 items of porosity and bulk density data for sedimentary rocks have been tabulated Most of the data are from the more accessible American, British, German, and Swiss literature The number of porosity determinations per item ranges from 1 to 2,109 The tabulation reflects the fact that more porosity than bulk density data are availabl_e for sedimentary rocks

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Porosity and bulk density of sedimentary rocks

"Regional" fractures may form slowly, at depth, in extension, and parallel to regional horizontal tectonic compression. Such fracture sets can be common in reservoirs despite the absence of flexure of the strata, and are commonly important contributors to reservoir permeability. Such fractures make good targets for deviated wells. High formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs (conditions conducive to fracturing) despite deep burial, but pore pressure does not exceed the least principal stress and does not cause tensile fracturing: the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets. Rather, many regional fractures form duri g far-field compression, initiate at locally induced tensile stresses caused by flaws in the rock, and propagate in the plane of the maximum and intermediate compressive stresses. The differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure. In the presence of high pore pressure, fractures can be held open at depth by a tectonic, basinwide dilatancy of the strata, and the open void space commonly becomes mineralized.

Regional Fractures I: A Mechanism for the Formation of Regional Fractures at Depth in Flat-Lying Reservoirs (1)

The Spraberry formation of West Texas is developed in the lower Leonard of middle Permian, restricted in most part to the Midland basin. The main producing structure is a fractured permeability trap on a homoclinal fold. This homogeneous mass is undifferentiated except as to alternate layers of sands, siltstones, shales, and limestones, deposited in a deep basin under stagnant conditions with hydrocarbons formed throughout the 1,000 feet of sedimentary rocks. Fractures were created by tensional forces after induration, probably during post-Leonard time. With storage of the oil reservoir in the sandstone matrix, the fractures serve as "feeder lines" to conduct the oil to the bore hole. Without these fractures commercial production would be from a seemingly "too-tight" reservoir rock. The producing area of the Spraberry formation is a "fairway" 150 miles long and 50 miles wide at an average depth of 6,800 feet. The main area, however, is 50 miles long, with width ranging from a few miles to 48 miles, thus creating a triangle of 488,000 proved and semi-proved productive acres.

Walter McCool Wilkinson

Papers

Fracturing in Spraberry Reservoir, West Texas