A Novel Approach For Estimating Critical Rate And Horizontal Well Placement In Oil Rim Reservoirs

ABSTRACT

Coning tendencies in oil rim reservoirs pose serious hindrance to optimizing oil production in the thin oil beds as unwanted fluids tend to replace the oil in the production stream, which invariably limits the ultimate oil recovery. In this study, a semi-analytical method of estimating critical production rate (qoc) and optimum horizontal well placement (hopt) was developed for reservoirs with thin oil zones sandwiched between gas cap and bottom water. Also, proxy models was generated for qoc and hopt from analysis carried out with the developed semi-analytical method. Accurate estimation of qoc and hopt can help control coning tendencies in oil rim reservoirs, thereby improving the cumulative oil recovery. The developed semi-analytical method was done by applying the principle of nodal analysis to graphically combine the two Joshi (1991) models of estimating critical rates in gasoil reservoir system and oil-water reservoir system respectively. The resultant graphical solution can therefore estimate the qoc and hopt for gas-oil-water reservoir system (i.e. oil rim reservoirs). Estimates made from applying the semi-analytical method to a set of oil rim reservoir data obtained from the Niger Delta oil field, gave the critical rate and optimum horizontal well placement from GOC, as 6600stb/d and 14ft respectively. When this result was compared with the result calculated from the existing Papatzacos et al (1989) method, the developed semianalytical result shows a favorable comparison with 2.5% difference in obtained values. Experiment design was further performed using Plackett-Burman design to evaluate the individual effect of the reservoir parameters in estimating qoc and hopt in oil rim reservoirs. The result indicates that, among the parameters considered, horizontal permeability, kh; oil formation volume factor, Bo; and the oil column thickness, h; have the most significant effect in estimating critical rate, while two parameters, oil column thickness, h, and oil density, ρo, have the most significant effect in estimating optimum horizontal well placement in oil rim reservoirs. The advantage of the semi-analytical method developed in this study is that, unlike other models, it can estimate hopt with a corresponding qoc which an oil rim reservoirs may be produced without the incidence of water and/or gas coning.