The Effect Of Vertical Fractures Upon The Performance Of Horizontal Wells When Coning Can Occur

Abstract

Abstract A common application of horizontal wells for oil production is in reservoirs where there is underlying water or a gas cap, or both. In such situations, a main objective is to minimize or eliminate coning. An important factor that affects performance is the presence of fractures. In some reservoirs these occur as parallel repeated systems and in others they are isolated. In this paper, theoretical predictions are made of the effect of such fractures and of their orientation. Cases where the interface is stable (i.e., the production rate below the critical) and cases where the operation must be above the critical rate for economic reasons are both examined. It is shown, for example, that a vertical fracture that is along the wellbore and that extends into a gas cap has a disastrous effect on performance if operation above the critical rate is required, but that there is only a relatively small reduction in the critical rate. Vertical fractures that are normal to the well affect both the horizontal and vertical effective permeability and, as a result, promote the lateral flow of oil as well as the vertical flow of gas and/or water. It is shown that, in this situation, the performance is often improved by the presence of fractures. Introduction This paper is concerned with the comparison of the performance of vertical fractures and horizontal wells in reservoirs where the coning of either water or gas can occur. Attention is also given to situations where horizontal wells penetrate one or more fractures that are either natural or artificially induced. Similarities and Differences Between the Effects of Horizontal Wells and Fractures on Reservoir Performance Fractures are commonplace in petroleum reservoir rock. They may be either naturally occuring or created artificially to stimulate the performance of production wells. The stimulation of vertical wells by artificial fracturing is practised extensively to enhance the contact of the wellbore with the reservoir and increase productivity. The direction that is taken by fractures depends upon the in situ stresses within the reservoir rock. Generally, they extend as planes that are perpendicular to the direction of the minimum compressive stress since, in this direction, the rock can be parted with the application of minimum pressure. In some circumstances, usually in very shallow reservoirs, the minimum compressive stress is vertical and horizontal fractures are formed. However, this is not commonly found and, in this paper, it will be assumed that the fractures are vertical and that, if there are multiple fractures, these exist in parallel, vertical planes. With horizontal wells, it is possible to create such systems of fractures by successively fracturing the reservoir from positions spaced along the length of the horizontal well(l). The performance of a horizontal well can resemble, and in some cases very closely, the performance of a vertical well with a vertical fracture extending from the wellbore. There is, however, a significant difference.