Application of Aerial Photographic Interpretation to the Study of Reservoir Natural Fracture Systems

Abstract

Aerial photography, through skillful interpretation, can be a potential source of information on natural fracture orientation in a reservoir. The feasibility of the approach is discussed here on the basis of an investigation conducted in the Midland basin of West Texas. Introduction Well developed natural fracture systems, forming intersecting sets of parallel planes, are extremely common within the rocks of the earth's crust. Extensive and systematic fracturing, generated by large stress fields of natural origin, is widespread in all geomorphological divisions of the earth, including those that are tectonically stable where most of the world's known oil producing basins are located. Over the years, considerable scientific inquiry has been directed toward natural fracture systems found in surface formations. Yet little emphasis has been placed on the fact that natural fractures, far from placed on the fact that natural fractures, far from being restricted to the surface of the earth, are equally prevalent in subsurface rocks at all depths. Thus until prevalent in subsurface rocks at all depths. Thus until recently, the petroleum industry has been less than enthusiastic about acknowledging natural fracture systems as routine structural components of producing formations. Indeed, engineering experience in producing formations. Indeed, engineering experience in the field reveals increasingly that the incidence of natural fracturing in reservoirs is much more common than is generally thought, extending well beyond the classic examples cited in the literature. As is universally recognized, the presence of fractures can enhance productivity in dissolved gas drive reservoirs. If it were not for naturally formed fractures, or for the practice of hydraulically inducing "parting" in formations to increase their productivity, many low-permeability reservoirs would perhaps never be commercial. On the other hand, in a reservoir where a frontal displacement mechanism is in effect, fractures can be highly undesirable. In various papers dealing with this subject, analytical studies papers dealing with this subject, analytical studies supported by field evidence have shown that in a reservoir with a rising water table the presence of fractures will result in lower recovery. Perhaps the most significant aspect of natural fracture systems is that they invariably create troublesome channeling problems in reservoirs undergoing recovery by fluid injection. Many investigations have shown that when fractures are present, volumetric sweep efficiency in a reservoir is adversely affected by, among other controlling factors, the degree of fracturing and the orientation of the fracture sets with respect to the alignment of injection-production well pairs. Consequently, in planning fluid injection programs or in dealing with reservoirs that produce by programs or in dealing with reservoirs that produce by frontal displacement, it is important to establish the spatial attitude of the reservoir fracture planes so that their adverse effects can be avoided or at least minimized. Unfortunately, the investigation of fracture conditions in the subsurface remains a technological frontier. No current engineering or logging method, when used alone, is entirely effective in determining conclusively either the presence or the systematic attitude of fractures in a reservoir. JPT P. 37