Delivering Giant Wells in Angola
Authors: Stewart Charles 
Published: October 2008

The several fields discovered in Block 18 during the first phase of exploration during 1999-2000 presented a major development challenge. Although collectively the fields had reserves potential similar to the larger accumulations in other Angolan deepwater blocks, their wide geographic separation tended to drive up the development cost.

Initial studies showed that the simultaneous development of the fields using typical deepwater strategies would result in unacceptable economics. Too many conventional wells needed to be drilled by multiple rigs in order to deliver an economic plateau production rate and ultimately the reserves expected of such high quality reservoirs.

A multi-disciplinary team approach was employed to revisit the problem. The team's conclusion was that a single, large FPSO, fed from fewer wells drilled by a single, large rig, could deliver the optimum outcome. However, exceptional well performance would be required.

Of recent major deepwater developments in Angola, Greater Plutonio was the first to use just one drilling & completion rig. The principal benefits were lower cost and less demand for increasingly scarce technical and operational staff. Less obvious, but equally important, the measured pace at which wells were delivered allowed the subsurface team to acquire and interpret reservoir data, which has been used to optimize subsequent wells. The challenge of using just one rig was the timely delivery of sufficient wells to sustain the plateau production rate of at least 200,000 B/D.

To date, all wells have successfully targeted their reservoir objectives and the efficiency and flexibility of the dynamically positioned 5th generation semi-submersible has kept well construction on track. Moreover, all openhole gravel pack completions have been pumped with 100% pack placement. Wells test analysis has confirmed mechanical skins of zero have been consistently achieved. This combination of factors has delivered several wells which individually have the potential to produce well in excess of 50,000 B/D to the FPSO, making these the operator's most productive oil wells anywhere in the world.

This exceptional result was delivered through the use of carefully selected drilling & completion technologies and the application of technical and operational rigour by a talented team, throughout the planning stage and into execution at the wellsite. These project choices and their outcome are described in the context of People, Well Design & Well Performance.

Introduction

The optimum development solution arrived at for Greater Plutonio was a single, very large FPSO, with a subsea well development of 5 geographically dispersed fields (Fig. 1). The adoption of an all wet-tree development strategy, with wells being constructed from a single rig, placed a significant burden of the overall project risk on drilling & completions.

The initial approved development phase of 43 wells represented the largest subsea development ever undertaken by the operator. However, at start-up, it was planned to have just 13 wells available, of which 9 would be oil producers. This was an aggressive tactic, given that other mega-projects in the region had dedicated at least 2 rigs per development. The key lever in the decision to contract just one rig was the belief that wells of exceptional productivity could be designed and delivered. This would enable plateau production to be achieved quickly and maintained for at least 4 years, at minimum cost.

The reservoirs have a number of characteristics that offer the potential of high rate wells-principally low viscosity oil and high permeability sands. However, counteracting those positive attributes is the relatively low net thickness (for the basin) of the sandstone pay intervals and especially the unconsolidated nature of the sands. To protect the wells and facilities infrastructure, sand control completions were required.