Fractured Reservoir Analysis

Abstract

Fractured reservoirs are among the most prolific reservoirs in the world because of the fractures’ ability to sustain very high flow rates. Examples where fracturing provides the primary production mechanism include the Tertiary Asmari formation in the fields of Southwest Iran (Lees, 1933; also Levorsen, 1967), the Maastrichtian/Danian chalks of the Ekofisk and other fields in the North Sea (Van den Bark & Thomas, 1980, Fritsen & Corrigan, 1990), the Permian Spraberry formation in West Texas (Wilkinson, 1953), the granitic basement in Southeast Asia (Areshev et al., 1992; Tandom et al., 1999), the Austin Chalk in Texas, the Monterey formation in California, and many more. In all cases, a fracture network has been established through shear or tensile rupture of the rocks by tectonic forces. Often, the actual hydrocarbon reservoir is provided by matrix porosity, not the fracture system, but the fractures are crucial in providing a permeable conduit to drain the fluids — a mechanism equally used in hydraulic fracturing. The matrix porosity may be very high as in the case of the North Sea chalk, where it reaches over 40%, or very low as in the basement rocks where it typically is no more than a few percent. Reservoir engineers call these “dual porosity systems” and have found various ways of simulating the flow in such reservoirs (Van Golf-Racht, 1982). Since fractures, and particularly large-scale shear zones, can extend very far, water breakthrough can occur very rapidly. It is, therefore, important to drain the reservoir carefully despite the temptation of having a high production from only a few wells. Completions are often bare-foot, i.e. without casing in the reservoir, because the fractures communicate well enough with each other and the wellbore. In cases where a dominant fracture orientation is established and the fractures are more or less vertical, horizontal wells oriented at a right angle to the fracture strike have proven successful as they intersect a maximum number of fractures.