In view of the influence of fractures on tool responses, and of their contribution to the productivity of formations, it is appropriate to devote a whole chapter to the study of fractured formations.
11 .l. INTRODUCTION Fracture is a general term that indicates all breaks or ruptures in a rock, whether accompanied by a displacement ornot. It corresponds to a surface along which there is a loss of cohesion. These ruptures are caused by tectonic forces (tension, compression or torsion), or by changes of temperature, by drying out, or by leaching in the or schistosity. plane of stratification Generally grouped in the category of fractures are : crack is a partial or incomplete fracture; fissure is a surface of fracture or a crackalong which there is a distinct separation, often filled with crystals; - joint is “a surface of fracture without displacement; the surface is usually plane and occurs with parallel joints to form part of a joint set” (Glossary of Geology, 1980); - gash is a small-scale tension fissure of several centimetres to a few decimetres in length, and several millimetres to a few centimetres in width. Itmay be gaped or, most often, filled with crystals. Several gashes are most frequently arranged in en dchelon (Fig. 11-1). They are produced by simple shear; fault is “a fracture or a zone of fractures along which there has been displacement of the sides relative to one another parallel to the fracture” (Glossary of Geology, 1980). Calling a joint or fault a fracture depends on the scale ofobservation. The fractures may be cemented (filled with crystalline material) or open. Clearly it is the open
Deformed by simple shear
Fig. 11-l. En &helm tension gashes produced by simple shear. (a) : Theory. (b) : Phatograph of an actual case (from Ramsay. 1967). fractures which are of interest for production, because they create substantial permeability, and a preferred flowpath for the fluids. The latter are largely caused by tension or torsion, while closed fractures are generally associated with compression. Fractures are usually perpendicular to the plane of stratification, and are usually more or less planar. Moreover, the occurrence of fractures is not random (Fig. 11-2). In a constrained formation, the fractures appear as interconnected systems, each systemconsisting of a group of more or less parallel fractures. They result in the rock being
broken up into small volumes or parallelepipeds which can be broken off by the drill-bit or the rotating drill-pipe. The average gap of a fracture, or fracture aperture, is often less than 0.1 mm, and so the porosity of fractures is generally negligible [less than 2%). Boyeldieu et al. (1982) haveestimated that, if the fracture system breaks the rock into cubes with 10 cm edges, a gap of 1 mm would be necessary to create a porosity of 3 %. Fractures appear predominantly in brittle rocks, hence in consolidated formations. Very often they disappear on entering formations which are more plastic (clays or halite), or friable (sands). 11 .l .1 Fracture Orientation
However, although theorientation may be statistically significant, it must be remembered that there can be considerable dispersion. 11 .1.2. Importance of Fractwes
It has frequently been observed that the fracture system, or network, in a given region tends to have the same orientation as the fault system.
In formations of low porosity and permeability, the production potential relies on an extensive system of openfractures. The productivity will vary greatly according to the number. extent and opening of the fractures and to the porosity and permeability of the matrix. As already mentioned, the porosity of fractures is insignificant in all but a few exceptional cases (highly compacted rocks), and makes no significant contribution to the reserves. However, the presence of fractures may significantly enhance...