Systematic Evaluation of Kuwaiti Reservoirs to Determine Optimum Candidates for Underbalanced Drilling

Abstract

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.