The art of well planning

Such was the case in the early development of BP's well planning toolkit (WPTK), which was born out of necessity in 2000 after BP had made a number of significantly large oil discoveries in the deep waters of the Gulf of Mexico. Reaching these prizes, the largest of which was the Thunder Horse field with reservoirs lying some 5500m to 8000m below the seabed, required a number of deep appraisal and development wells to be drilled through complex subsurface geology, compounded by the deep water above. In short, a way had to be found to achieve extraordinary performance while working at the technical limits of drilling at that time.

BP's approach to the challenge brought drilling information and a comprehensive description of the subsurface together for the first time. The move was to initiate a step change in the level of cross-functional interaction in well planning, one which has gone on to deliver many positive business impacts for BP - for example, a decade later Thunder Horse has become the most productive deepwater field in the world (see feature 'Deepwater horse power', this issue).

Faced with the unusual subsurface challenge, BP petroleum systems analyst Stephan Duppenbecker, working in co-operation with earth modelling specialist Jean-Sebastian Hall and team colleague Amal Vittachi, set about developing a 'toolkit' which brought together several aspects of well planning that had previously been considered separately in isolation.

Fast forward over the 10 years between then and now, and the WPTK software has grown both in functionality and in its application. Whether running on a laptop computer or seen at its best on the 'big screen' when viewed through 3D spectacles in a HIVE, the toolkit is giving BP's subsurface teams an advantage in designing, viewing and modifying wells.

'The net result is shorter well planning cycle times and improved cross-functional interaction between subsurface and drilling teams - leading to significantly reduced risks and costs in drilling wells,' observes Nigel Last, senior drilling advisor within BP's exploration and production technology group at Sunbury.

When production commenced in 2005, that first phase development encompassed a relatively small area of the overall reserves, accessed from a single offshore platform. While early near-vertical wells drilled into the reservoir below the platform proved relatively straightforward, the level of challenge rose sharply as both the distance and angle from the vertical increased on wells needed to reach the more distant hydrocarbon targets.

'The BP team needed to find a way to calculate, within a relatively narrow band, the weight of the drilling mud required to drill and stabilise the wellbore,' explains structural geologist David Barr. 'Too low in weight and the wellbore would collapse, too high and the weak rock formations could fracture.'

This proved to be a huge challenge as the early wells showed that Clair did not display the rock behaviour predicated by conventional analysis. A dedicated investigation team identified an unusual condition known as weak-plane failure as the likely cause, and began the laborious process of calculating the critical points along the proposed route of a well aimed at part of the reservoir some four kilometres from the platform.

The well was a success, and the weak-plane model has since been implemented in the WPTK where it now allows accurate mud weight calculations to be carried out quickly and easily.

Clair is by no means the only development to seize the potential within the WPTK to compress the time required for well planning and to enable human resources to stretch further.

One significant characteristic of Valhall's reservoir is compaction of the chalk formation - compaction in excess of 10m has occurred in places. Another is seabed subsidence, exceeding six metres in some areas of the field.

'Valhall has become more and more difficult to drill over time,' explains Tron Golder Kristiansen, geomechanics advisor in BP Norway. 'Our challenge now is to predict the spatial and temporal impact of the compaction and subsidence to find the low risk well trajectories during the remainder of the field's life.'

'People didn't need to worry too much about rock mechanics 20 or 30 years ago when the oil fields were in accessible and well understood places,' says Last. 'But the industry is moving into new areas where the way rock behaves is more uncertain. Fortunately, BP now has the WPTK to enable us to model complex wells and visualise them in three dimensions, while the shared earth model has become the place where all the mechanical properties of the rock formations are stored in a standardised format. This will be useful to the reservoir engineers who manage the field during operations once the drilling campaign has been completed.'

Much of the focus behind the development of the toolkit, which is now being taken forward by Randy Hickman and Steve Willson in BP Houston, has been to achieve cost savings throughout BP's global drilling programme by eliminating 'drilling surprises' and reducing non-productive time. Progress towards both these targets stands to receive significant impetus if it proves possible to link the WPTK to real-time data acquired as a well is actively being drilled - BP's first offshore trial is likely to take place soon. This link, an advance on conventional measurement-while-drilling (MWD) techniques, will enable pre-drilling models to be checked and updated using real-time data from the well, which in turn would provide early warning of the need to change drilling parameters should the data not match expectations.

With WPTK now firmly established, BP is truly benefitting from the synergy of the whole being greater than the sum of the parts.