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Underbalanced drilling technology (UBD) has increased production potential from many oil and gas reservoirs worldwide. Kuwait Oil Company (KOC) decided to pilot this technology in Kuwaiti reservoirs to assess the viability of this technique and its capability to optimize production and development of their reservoirs. To maximize the potential for successful implementation, a scientifically rigorous process was implemented to evaluate the reservoir candidates. The reservoirs include Upper Burgan, Mauddud, Tayarat, and Radhuma. Since there is a high degree of variation in reservoir quality, hydrocarbon properties, formation pressures and other properties depending on location, candidates from different locations were examined. In the first phase, a high order qualitative ranking of the candidates was performed. The basis of comparison included an examination of the reservoir and fluid properties, production data, review of the core, log and geological/geophysical data, drilling and operational information. The ranking of the candidates was based on production potential, drilling benefits, potential for instability, early water break through, and other operational, reservoir and drilling considerations. The top-ranked candidates were examined determining the range of potential reservoir behavior during UBD and the corresponding wellbore hydraulics (drilling the well on paper) was performed. It was concluded that UBD technology was suited for several of the reservoirs but that some of the candidates might not realize as much improvement as others to justify implementation in the first implementation stage. If UBD proves successful on the candidates chosen for the pilot study, the candidates showing a lesser degree of potential will again be considered. This paper describes the screening methodology used and discusses the results from the candidate evaluation process. Introduction KOC has several potential reservoir candidates which may benefit from underbalanced drilling. Data from seven of these candidates was examined and ranked. In underbalanced drilling, the wellbore pressure is lower than the reservoir pressure. This wellbore condition allows formation fluids to flow into the wellbore during the drilling process. The intent is to minimize formation damage by minimizing any invasion of the drilling fluid or solids into the reservoir. An additional advantage is that with proper instrumentation, data acquisition, and drilling procedures the data that is collected can then be interpreted and analyzed to extract information about the reservoir. Unlike conventional overbalanced drilling or even managed pressure drilling (MPD) with reduced overbalance margin, the underbalanced drilling environment provides a unique opportunity to gather reservoir data that have the potential to be interpreted and give a more detailed picture of the productive intervals present in the wellbore drilled. At inception, UBD technology focused primarily on drilling issues to facilitate drilling through difficult zones. However, UBD has seen increased application in cases where the primary objective is to avoid or minimize reservoir damage. This focus change means that recovery from the reservoir can be maximized along with data and information about the zones drilled.

The deep, sub-salt reservoir complex is tiered with fractured tight carbonate at bottom and top, with the two sub-units of "upper unconventional kerogen" and "lower inter-bedded kerogen-carbonate" in the middle. This depositional setting is challenging for horizontal well placement where the thicknesses of respective sub-units are about 50 and 30 feet with varying geomechanical and petrophysical properties. Additionally, this complexity poses limitations in completions and effective stimulation of the Kimmeridgian-Oxfordian reservoirs in several gas fields at development stage in Kuwait. A horizontal well is placed in the lower sub-unit of the laminated complex of unconventional kerogen and fractured carbonate reservoir as a Maximum Reservoir Contact (MRC) type well. A pilot mother-bore was drilled and logged to identify the lithological properties across the entire vertical domain - facilitates the optimization of horizontal drain-hole placement within the targeted reservoir units. No wellbore stability issues in drilling were predicted based on the geomechanical understanding where core-calibrated logs from offset vertical wells were considered. However, this modeling method did not have the functionality to integrate the impact of drawdown on the laminated formation which became unstable and collapsed during the short open-hole drill-stem test (DST) plugging the tubing prior to the final completions. An alternative "book-shelf" geomechanical model was considered at pre-drill stage for predicting the wellbore stability. Once the drilling was completed, the time-lapsed multi-arm caliper indicated the validity of the alternative methodology in predicting the unstable stack of laminations in kerogen-rich strata. The paper discusses an optimization methodology to enhance the understanding of static and dynamic geomechanical stability through the use of BHI data. Objective of the proposed method is to help improve the effectiveness of completions where wellbore stability due to geomechanical complexity in stacked-pay reservoirs is a primary wellbore challenge in deploying the completions and executing a subsequent stimulation and testing campaign.

Kuwait Oil Company (KOC) is considering adapting new technologies to ensure that strategic resources are optimally explored, developed and produced during the life cycle of their oil and gas fields. Among the various options available to achieve these objectives, Under-balanced Drilling (UBD) technology is considered to be one of the most effective methods when it is effectively applied in conjunction with horizontal and/or Multi-lateral well drilling and completion techniques Accordingly, KOC decided to pioneer the introduction of this new technology by employing a systematic scientific approach to screen the Mauddud reservoir in the Greater Burgan field for under-balanced drilling candidacy, in order to ensure a successful implementation of the pilot project. The screening process incorporated reservoir, production, drilling, Geology & Geophysics and operational data with risk analysis incorporated into a rigorous expert system aimed at maximizing the value of any UBD operation. The screening process was performed in two phases; a high level low resolution screening and in depth high resolution analysis. Phase I comprised of:Evaluating and ranking the Mauddud reservoir with respect to its risked probability of being successfully exploited with horizontal underbalanced drilling and completion techniques as opposed to conventional methods.Comparing candidate reservoirs against global database of analogues proven to be suitable with UBD. Phase II comprised of:Quantifying drilling-induced damage and determining the impact on the productivity on the Mauddud reservoir.Comparing under-balanced and overbalanced costs, production rates, projected revenues and Net Present Value (NPV). This paper describes the expert system methodology used in the screening process and provides a discussion of the results obtained. The Phase II study findings provided KOC with detailed damage analysis, production forecast and economic benefits that under-balanced drilling could offer as compared to conventional drilling & completion technologies. The UBD Screening Process This paper investigates the reservoir suitability of underbalanced drilling using an Expert System screening process that was developed in conjunction with leading industry consultants to increase success in candidate selection and to evaluate the benefits of underbalanced drilling over traditional drilling and completion technology in terms of reservoir performance. The screening process began by taking basic information for the Mauddud reservoir and processing the information through the expert system phase I software. This software rates the candidacy of the reservoir to help determine the likelihood of achieving success with underbalanced compared to overbalanced drilling and completion techniques. All data input was reviewed for completeness and consistency.