This paper discusses important factors that influence the simulation of horizontal and deviated wells in fractured reservoirs. Although wellbore hydraulics affect the inflow along the well, they do not necessarily have an impact on the total well production. It is shown that it is the combined effect of wellbore hydraulics and relative location of injectors and producers that can significantly affect the production. For hydraulically fractured horizontal wells, it is shown that orthogonal fractures are more efficient than longitudinal fractures for very tight reservoirs. As the reservoir permeabilities increase, the effectiveness of orthogonal fractures lessen.
The use of horizontal wells to improve recovery has been accelerating and this technology may turn out to be one of the most important aspects of oil recovery in recent and probably future years. The accurate simulation of horizontal well performance requires the coupling of the wellbore flow with the reservoir flow. For practical purposes, it is imperative that the solution of the wellbore flow should only be a small fraction of the run time. The direct solution of the momentum equation for wellbore flow is computationally intensive and is not appropriate for large field-scale simulation.
Collins et al (1992) described a very efficient technique for modelling wellbore dynamics which is suitable for incorporation into reservoir simulators. In this technique, the wellbore flow equations were cast judiciously in a form similar to the reservoir flow equations. Thus, efficient techniques that were developed to solve the reservoir equations can readily be applied to solve the wellbore equations. Using this approach (referred to hereafter as "discretized wellbore" approach), Collins et al (1991a) examined the effect of horizontal wellbore hydraulics on simulation results. It was shown that the inclusion of wellbore hydraulics in the simulation yields an inflow profile along the wellbore substantially different from the case without wellbore hydraulics. This difference is more pronounced for higher-permeability reservoirs. Nevertheless, it was observed that the total well production appeared not to be affected by wellbore hydraulics. Subsequently, Collins et al (1991 b) examined the effect 01 wellbore hydraulics in heterogeneous reservoirs. They found that reservoir heterogeneities give rise to different wellbore pressure drops for different wellbore flow directions. However, they also reported that for the cases tested, the cumulative productions predicted with and without wellbore hydraulics were similar.
This paper examines the effect of wellbore hydraulics in naturally fractured reservoirs. Naturally fractured reservoirs have high-permeability channels (i.e. the fractures) that act as conduits for fluid flow. The pressure drops in these fractures are very small and comparable to the pressure drop in the wellbore. The relative locations of injectors and producers are also considered. It is shown how these factors can significantly affect recovery predictions for horizontal and deviated wells. Several black-oil and compositional runs are used to study this effect.
Hydraulic fracturing of horizontal wells has also received interest recently (Soliman et al, 1990; Dees et al, 1990). A discussion of the impact of wellbore / fracture orientation on the recovery is also given.