Rock develops fractures as a result of brittle failure caused by stress and pore pressure. Fractures have an important effect on permeability and the mechanical and seismic properties of rock. Naturally occurring fractures in hydrocarbon reservoirs can increase the porosity and permeability of the reservoir, and knowledge of the orientation and density of fractures is necessary for optimizing production. In reservoirs that have low matrix permeability, fractures often act as the primary flow paths; in higherpermeability formations, fractures act as shortcuts for flow. Because natural fractures show preferred orientations, they can lead to significant permeability anisotropy in the reservoir. Injection and depletion cause changes in pore pressure and in effective stress. An increase in effective stress can lead to partial closure of fractures because of the increasing compressive stress that is applied normal to the fracture face. The response of fractures to an applied stress is therefore of interest to reservoir engineers.
Fractures are also important in drilling because they can cause loss of drilling fluid and thus reduce the pressure exerted by wellbore fluid on the borehole wall. That reduced pressure can create the potential for a kick (influx of formation fluid) or for wellbore instability. Fortunately, fractures affect propagation of seismic waves, and there is great interest in using seismic waves to characterize naturally occurring fractures in sedimentary basins.
In designing the layout of wells in fractured reservoirs, it is important for optimum drainage to space producer wells more closely along the direction of minimum