The problem with conventional drilling is that it requires a higher pressure to be maintained in the well bore than exists in the reservoir in order to control the well. This means drilling fluid is effectively injected into the reservoir, causing damage to the pores in the rock and making it more difficult for the oil and gas to flow.

Also, when reservoirs reach maturity and reservoir pressures fall it becomes technically more and more difficult to drill using conventional techniques. Eventually, there comes a point when it is no longer cost-effective or even technically possible to drill new wells, and the reservoir is abandoned.

However, the development of new drilling techniques such as UBD can overcome these problems and breathe new life into depleted or damaged fields. UBD is one of the fastest-growing technologies in the drilling industry and is capable of delivering substantial increases in initial well productivity and ultimate hydrocarbon recovery. Typically, increases of between 200-400 per cent in productivity and 10-20 per cent improvement in ultimate recovery can be achieved in susceptible reservoirs.

With UBD the pressure in the well bore is lower than the pressure in the reservoir being drilled. This allows hydrocarbon fluids to flow from the reservoir into the well bore during drilling. Not only does this avoid damage to the reservoir from invasion of the drilling fluid, it also increases production as well as final hydrocarbon recovery by allowing the hydrocarbon fluids to more easily pass through the pores in the reservoir rock.

Lower well bore pressures also prevent drilling fluid loss and differential sticking in depleted reservoirs, and offers improved penetration rate and extended bit life, all of which accelerate the time taken to drill the well. Ultimately UBD offers better value for money when drilling in depleted reservoirs.

In the early 1990s in Canada, there was a resurgence in UBD, to combat drilling fluid losses and differential sticking of drill pipe, using modern equipment designed to control oil and gas flow to surface during drilling. It was very quickly observed by UBD pioneer companies such as PetroCanada and the Royal Dutch/Shell Group that wells drilled using UBD techniques produced more oil and gas than wells drilled conventionally.

UBD advantage

A quantum leap was made in 1996 when Shell Expro applied UBD techniques to the drilling of offshore wells in the challenging arena of the southern North Sea. However, UBD remains less costly on onshore locations and lends itself particularly to land well drilling in areas such as the Middle East.

After an 18-month project to prove the benefits of UBD on the maturing Nimr and Saih Rawl oil fields operated by the Shell joint venture company Petroleum Development Oman (PDO), UBD could now be about to take off in the region.

During the project, which was project managed by Frontier Group, PDO found that UBD not only increases production levels from mature reservoirs, but also has the advantage of providing early information on reservoir performance.

Nimr and Saih Rawl are both mature fields and have characteristics indicating that production is not being optimised due to reservoir damage, hence the reason for adopting UBD.

Nimr is a sandstone reservoir with high viscosity oil. Fourteen wells were drilled to evaluate the benefits of UBD in improving productivity in this reservoir. These wells were drilled horizontally with crude oil and nitrogen as the drilling fluid.

At Saih Rawl, a carbonate oil reservoir, four six-leg multilateral wells were drilled to provide an evaluation of productivity improvement. These wells were drilled horizontally with crude oil and natural gas as the drilling fluid.

PDO was aware of the potential benefits of UBD prior to the project and had drilled some wells using this technique as early as 1992. However, single-well trials of UBD in the past had been very expensive because of the costs of mobilising equipment to the region, and had also proven inconclusive in confirming benefits. As a result the technique was not adopted for ongoing drilling operations.

Recent proven successes using UBD techniques elsewhere by Shell, and an enlightened approach from PDO in the adoption of two one-year trial programmes, together with a zero cost implementation strategy mean that the increased production from the wells paid for the cost of implementing UBD.

This cost-controlled strategy was formulated to ensure that the business benefits outweighed the costs involved, with excellent results for PDO. The improvements in productivity and projected ultimate recovery and additional reservoir knowledge more than justified the decision to use this emerging technology.

Initial results suggest that UBD will have a profound effect on the way oil companies drill reservoirs in the future. The PDO project has been a success in terms of proving the value of UBD and delivering extra oil production. PDO intends to continue applying UBD as its standard drilling method in Nimr, Saih Rawl and several other fields. Current plans are for UBD activity with PDO to increase during 2003, with the number of rigs drilling UBD wells to double to four. n