A knowledge base development tool (Resolver) that consists of various utilities for developing and maintaining the rules, and implementing inference mechanisms was used in developing the expert system for selecting a candidate UBD technique. The principal attractive feature of ReSolver is that it supports fuzzy logic among other confidence modes. Membership functions were defined to assist the expert system in making decisions when the decision variables fall in a "gray area". Membership functions were constructed for variables such as lost circulation, clay swelling, fines migration, hard drilling potentials, cost benefit, gas influx and water influx potentials, fire potential, and stuck pipe potential. When the final outcome consists of a set of drilling fluids rather than a single one, these drilling fluids options were even screened further by the expert system to assure that the UBD fluid density is adequate within the pressure window. If the expert system still recommends more than a single drilling fluid option, a confidence level is given with each option.

Drilling underbalanced offers several benefits compared to conventional drilling techniques. These include, increased penetration rate and bit life, reduced probability of sticking the drillstring downhole, minimized lost circulation, improved formation evaluation, increased well productivity, and the reduction of stimulation treatments. The extent to which it is possible to achieve any of these benefits is generally controlled by the properties of the target reservoir and the overlying formations, and in some instances even by the specific characteristics of the well being drilled. The need for a fuzzy expert system that guides the user to optimally drill a well underbalanced becomes evident when we consider the delicate balance between the numerous coupled factors affecting the success of this operation.


Recently, underbalanced drilling (UBD) has been used with increasing frequency. The purpose of using UBD is to minimize problems associated with invasive formation damage which often greatly reduces the productivity of oil and gas reservoirs.1 When properly designed and executed, UBD minimizes problems associated with the invasion of particulate matter into the formation as well as a number of other problems such as adverse clay reactions, phase trapping, organic and inorganic precipitation, and emulsification.2 These effects may be caused by the invasion of incompatible mud filtrates in an overbalanced condition. Benefits of UBD result from the reduction in drilling time, greater rates of penetration, increased bit life, a rapid indication of productive reservoir zones, and the potential for dynamic flow testing while drilling. However, UBD is not a solution for all formation damage problems. Indeed, damage caused by poorly designed and/or executed UBD programs can exceed that which may occur with a well-designed conventional overbalanced drilling program. Potential downsides and damage mechanisms associated with UBD include increased cost and safety concerns, difficulty in maintaining a continuously underbalanced condition, spontaneous imbibition and countercurrent imbibition effects, glazing, mashing, and mechanically induced wellbore damage.3

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