fax 01-972-952-9435. AbstractThe Jean Marie reservoir in the northeastern part of British Columbia, Canada, is one of the most active horizontally underbalanced drilled reservoirs in the world. The Jean Marie is a depleted, low permeability, fractured carbonate with low initial water saturation making it susceptible to formation damage using conventional drilling fluids. 1 Given these reservoir characteristics, horizontal underbalanced drilling is the current method of choice for operators active in the area.Although underbalanced technology has made tremendous improvements in maintaining and ensuring underbalanced conditions at all times during operations, overbalanced incidents do occur while drilling the horizontal section. These incidents can be much more damaging than drilling with a well designed overbalanced fluid.In some cases, an overbalanced event could permanently impair production from a well by as much as 75%.With the aid of real time downhole pressure gauges, these overbalanced incidents can be identified but very often go undetected, leading to a false sense of achieving the goal of minimal damage. The unidentified damage can lead to misinterpretation of production results and disappointment with underbalanced drilling.To exacerbate the problem of identifying damage, traditional pressure transient analysis of wells in the Jean Marie may yield incorrect valuation of the overbalanced incidents by underestimating the skin on the face of the horizontal well. This could be due to the difficulty of performing meaningful transient analysis on a horizontal well in a dual porosity reservoir and a general lack of knowledge of the impact of drilling related damage on fractured or highly heterogeneous reservoirs. This paper will show how, by combining transient productivity index data collected while drilling with the use of history matching techniques, overbalanced incidents can be easily identified and the impact on well productivity can be quantified.

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The general benefits of underbalanced drilling (UBD) can be explained in terms of:Drilling performanceReduced drilling problemsIncreased reservoir productivity and recovery by minimization of formation damage. However, the absence of an easily applicable method to predict which reservoirs will benefit leads to an under-utilization of UBD by the industry. For that reason, the fuzzy logic expert system (ES) introduced in this paper was developed specifically to screen underbalanced candidates with respect to maximizing reservoir performance in terms of productivity. The ES software considers basic reservoir properties and attributes such as fluid type, drive mechanism, lithology, porosity, water saturation, thickness, permeability, etc. Using algorithms and logic rules, various modules evaluate these input parameters to assess underbalanced candidacy based on the following criteria:The likelihood and severity of formation damageThe overall impact of the damage on productivity and recoveryThe ability to successfully treat this damage with conventional stimulation techniquesThe relative economics of UBD compared to conventional drilling, completion and stimulation techniques In addition to a description of the ES, this paper discusses several case studies in which the output of the fuzzy logic ES is compared with published successful and unsuccessful UBD campaigns. The results demonstrate that the ES model can accurately predict the suitability of UBD for a given reservoir. Introduction UBD is not a new idea. For decades, operators have drilled various geological formations with different drilling fluids designed to exert a bottomhole pressure lower than the formation pressure, thereby allowing oil, gas and water from the formation to flow into the wellbore as drilling proceeds. One may argue that the benefits of UBD are well documented, however, while few question its value, its perceived issues of safety and complexity - properly managing pressures and production during drilling and completion – are challenging the full acceptance of this drilling technique. In addition to these operational issues, it is more important to realize that UBD is not an economic solution for every formation. Anecdotal evidence suggests that applying UBD in unsuitable reservoirs can lead to more costly and unsuccessful wells. This paper introduces an ES that was developed to aid the successful selection of horizontal underbalanced candidate reservoirs. It is important to note that the proposed ES is specifically built to screen horizontal underbalanced candidates with respect to maximizing reservoir performance in terms of productivity and recovery.