Colombian Underbalanced Drilling Operations - Start Up Experience
Authors: Hogg T.W. , Boyle J.E. 
Published: December 2007

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Abstract
This is a study of BPX Colombia's initial experience introducing under balanced drilling technology to their Piedemonte licence. The intention was to prove that a step change in performance could be achieved by application of this technology. Thus significantly reducing traditionally very high operational costs due to the faulted and over thrust nature of this developing mountain region. The occurrence of over thrust sheets which contain very hard sandstone and shale sequences have in the past proven particularly difficult to drill, slow and therefore very expensive. BPX Colombia researched the options available in this developing technique considering, proven and developing technology in order to prepare a strategy leading to the evaluation of under balanced drilling on two wells in the Piedemonte Licence.

Introduction
BP Exploration Colombia in the past few years has made significant discoveries in the Colombian foothills. Building upon these discoveries, exploration has successfully continued North into the Piedemonte licence (figure 1). In order to develop the Cusiana and Cupiagua fields, BPX Colombia and it's Partners conducted significant research to provide solutions to the difficult drilling problems. However even with the application of that knowledge Piedemonte wells are significantly more difficult to drill due to the more complex geology. This is characterised by high horizontal stresses, steeply dipping beds, numerous faults and alternating sand shale sequences combined with the El Morro thrust structure containing the very hard Mirador and Barco sandstone which has proven particularly difficult to drill using optimised conventional techniques. The El Morro thrust sheet is not present in the Cusiana and Cupiagua fields. Traditionally drilling the El Morro thrust sheet alone has taken upwards of 72 bits and 100 days per well.
Under balanced drilling was identified as a technology which had the potential to deliver significant performance improvements, even by solely doubling the ROP which has been currently achieved. The decision was taken to investigate the potential for it on two wells which were in the planning stage. Initial planning consisted of a literature review and site visits to operations in South and Central America where various aspects of under balanced drilling operations were ongoing. In parallel with this effort all the contractors associated with BPX Colombia reviewed their own immediate in house experience in under balanced drilling. The net was then cast further to identify as much experience and technology as possible prior to determining what aspects of the technologies would be evaluated on these two wells. At a series of open meetings with all the contractors, they outlined the knowledge and technology which existed within their organisations. Each technology and technique was examined in order to identify where it could contribute towards a step change in performance in the Piedemonte licence. This identified at an early stage that only a few companies had specific knowledge in a narrow sphere, but no one company had all the experience. The air drilling Service Contractor had an overall perspective, however the Colombian Llanos foothills and specifically the Piedemonte licence was a new area to them, hence operational experience still had to be developed. It was recognised and acknowledged by BPX Colombia that the limited experience available indicated that under balanced drilling operations required a period of time to tailor it to a specific area, as experienced in other geological regions of the world. As this initial planning process progressed it confirmed the magnitude of the possible prize.
It was decided to evaluate under balanced drilling techniques using Foam, Aerated Water Based Mud (WBM) and Nitrified Oil Based Mud (OBM) on two Piedemonte wells which were at the planning stage. Several options identified other than the selected under balanced drilling techniques are still being monitored or pursued by BPX Colombia in order to provide additional step changes in performance.
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