A Deepwater Well Construction Alternative: Surface BOP Drilling Concept using Environmental Safe Guard

Abstract

Abstract Over the recent years, the oil industry has been drilling wells in ever increasing water depths. Drilling these deepwater wells is performed under specific conditions namely a combination of extreme thermodynamic conditions (low temperature and high external pressure at the mud line level) and a narrow margin between pore pressure and fracturing pressure. These conditions lead to a number of drilling difficulties, especially in terms of well control. Traditionally, these challenging wells are drilled using a sub-sea BOP stack connected at the mud line with a low-pressure riser to the surface. From a well control point of view, this configuration presents several limitations: BOP closing time may be long enough to allow part of the influx to migrate inside the riser, long kill and choke lines add significant friction losses which sometimes jeopardize the kick control, BOP components are subject to extreme thermodynamic conditions and therefore are prone to frequent failures which increase drastically the rig NPT. In an attempt to overcome the drawbacks of using a sub sea BOP stack and reduce the costs associated with an expensive rig package, the industry developed a technique using the BOP stack on surface in a "jack-up type" configuration and a high pressure drilling riser. Primarily used in the Far East, this technique presents certain limitations, the main one being the absence of a secondary barrier at or just below the mud-line in case of riser failure. Total was considering to use this technique in 2000m water depth on its recently-acquired Donggala deepwater block, offshore Mahakam straight, Indonesia. A Quantitative Risk Assessment on surface BOP stack drilling was performed. This highlighted the necessity to have a shut-off device at the mud line level in order to reduce the environmental impact in the event of riser failure. Total, in association with Cameron, designed and developed this sub sea shut-off device (hereafter referred to as ESG - Environmental Safe Guard) and with Transocean upgraded a rig to allow safe and efficient drilling on the Donggala block. This paper outlines the development of the ESG in parallel with the required riser design and well architecture as an alternative to deepwater well construction. An operational feedback on the successful running of the ESG is provided. Introduction Over the past several years, the industry has developed a technique allowing deepwater wells to be drilled with a semi-submersible drilling rig and the BOP located at surface. Drilling with surface BOP stack presents a certain number of advantages compared with traditional sub sea drilling, such as:Cost saving: drilling within 2000m of water and a sub sea BOP stack usually requires the use of a 5th generation drilling rig with typical day rate rising up to 200,000US$/day. This generates very high total well costs. The surface BOP technique allows the use of 3rd generation drilling rigs with lower day rates (typically 80,000US$/day) and thus the potential for lower well costs. Reductions in high mob / demob costs are also possible especially for deepwater locations remote from the traditional operating areas of the 5th generation rigs.Optimize drilling operation: The potential to save rig time as no time is spent running and pulling the BOP together with the riser in deep water.Reduce Non Productive Time: having the BOP at surface actuated by a simple control system reduces the risk of BOP failure resulting in significant downtime to pull the BOP to surface for repair and re-run it to the mud line.Well control situation: the kick or well control situation is simplified by not having several kilometers of choke / kill line (and their corresponding high frictional pressure losses) while applying the simpler surface BOP well control techniques.