This paper describes the safety and cost benefits as an onshore operator shifts from conventionally stacked (i.e. bowl and slip type wellheads) to unitized wellhead systems while transitioning from exploration to development phase. There was a paradigm shift in operational safety coupled with multi-million dollars of cost savings during the course of development drilling spanning across 500+ wells. This paper draws a detailed comparison between conventional and unitized wellheads elaborating on the safety as well as operational aspects.
The timing of full-fledged deployment of unitized systems by the operator coincided with development of the operator$s onshore Rajasthan block. The unitized wellheads had been previously deployed by the operator in a number of offshore platform wells. Hence, the operator had sufficient data to demonstrate that the overall economics was in favor of using unitized wellheads. Until 2007, mostly conventional wellheads were used by the operator across nearly 140 exploration wells. Thereafter, unitized wellheads were extensively used during development drilling in Rajasthan post 2009. The unitized wellhead plays a major role in ensuring two barriers throughout drilling and completion operations, which is a requirement in the NORSOK standards. Through careful scope definition and technical evaluation, the operator had ensured incorporation of a number of safety/operational critical specifications of the unitized wellheads including surface casing hanger landing rings, diverter adaptors, mandrel casing hangers, integral lockdown for casing hanger pack-offs, metal-end-cap seals, multi-stage compact bowls, quick make-up BOP adaptors, tubing hangers with integral body seals, TWCV (Two-way-check-valve) profiles, bonnet-LMV (Lower-master-valve) interface with dual seal provision etc. In contrast, the slip-and-bowl systems are associated with time consuming and relatively risky operations such as casing string space-outs, wait-on-cement, hot cuts, cold cuts, welding of riser/wellhead, nippling down of BOP with less than two physical barriers, BOP/wellhead housing installation after every hole section, dependence on cement sheath as sole barrier, risk of leaking lock down screws etc.
The time saving unitized wellhead has translated into well construction cost savings for Rajasthan development wells drilled by the operator. Though the cost of unitized wellheads is approximately 20%-40% higher than the conventional wellheads, the rig time savings and the resultant cost savings clearly offset the higher cost of unitized equipment/service in the Rajasthan onshore environment, both in development and exploration drilling. This is a step change from the previous belief that the unitized systems would only be economical for offshore operations. In addition, the usage of unitized wellheads offers safety advantages in terms of maintaining well barriers and brings greater efficiency and quality into well construction operations.