A Novel Expert System for Multilateral Well Completion

Abstract

Abstract In this study, we assembled and put under scruitiny variables that affect the drilling and completion of multilateral and horizontal wells. We then formulated reservoir candidate screening criteria for applying multilateral technology, and implemented these criteria in an expert system for designing optimal multilateral well configuration and completion using the leading edge technologies of simple and complex multilateral junctions. The system features the use of fuzzy logic for handling ambiguous completion scenarios. The main benefits of horizontal wells consist of lowering the development and lifting costs per barrel of oil produced. Multilateral technology multiplies these benefits by adding more reservoir exposure per well, and more exposure per drilling and completion dollar spent. The recent evolution of multilateral technology introduced a large variation of completion options for multilateral wells. Many of these options are simple and basic while others are complex. The development of this fuzzy expert system allows the use of multilaterals in a much wider range of well scenarios, and allows to account for a large number of production-styles constraints and rock-properties conditions. Introduction The drilling and completion options when drilling a well are numerous. Some of the basic options are: drill a vertical well with open or cased and perforated completion, a horizontal well, or perhaps multilateral wells. In multilateral applications, two or more horizontal wells are drilled from a single parent mainbore, enabling production from multiple reservoirs. These laterals may be horizontal, vertical, or deviated. They may be in the same, or in different planes. The application of multilateral technology has been driven mainly by complex reservoir and geologic settings for which laterals may be used to produce by-passed or separated pockets of hydrocarbons in depleted, faulted, layered and heavy oil reservoirs. The purpose of this study is toassemble and formulate screening criteria that can be used as guidelines for selecting possible reservoir candidates for the application of horizontal and multilateral drilling and completion;develop an expert system that incorporates these screening criteria to handle ambiguous drilling and completion scenarios. There are various reservoir characteristics that affect the success or failure of horizontal and multilateral wells.1,2,3 What follows is a discussion of some of these characteristics. Coning Water and gas coning are serious undesirable events that can take place during the development of oil fields. Coning increases the cost of production, reduces the depletion mechanism efficiency, and therefore, decreases the overall recovery. Several factors have been shown to have a direct effect on coning.4 Some of these factors include density difference between fluids being produced, fluid viscosity, reservoir thickness, anisotropy ratio, and pressure drawdown. Among these factors, pressure drawdown is believed to have the most significant impact on coning. A vertical well, for instance, exhibits a large pressure drawdown near the wellbore. On the other hand, horizontal wells exhibit minimum pressure drawdown near the wellbore. The presence of water coning into a well may result from the pressure drawdown being too high near the well. If the pressure drawdown, ?P, is larger than the gravity pressure differential, which tends to keep the oil on top of the water, then coning may occur. A variety of researchers concluded that horizontal well technology is an adequate remedy for the coning problem.4,5,6 It has been estimated that 40% of all horizontal wells are drilled to arrest coning problems. Permeability Anisotropy One of the most important of these reservoir factors, influencing horizontal or multilateral well performance, is the permeability/permeability anisotropy. This includes both the vertical to horizontal permeability anisotropy and the permeability anisotropy in the azimuthal orientation. As a rule of thumb, horizontal and multilateral drilling becomes a viable option as the ratio kv/kh approaches unity.