RIASSUNTO
Abstract
Texaco UK North Sea is operator and 100% license holder of the Captain Field which contains a greater than 1.5 billion barrel, viscous oil accumulation. Oil production from the Captain reservoir will be dominated by the water / oil mobility ratio of 30, resulting in severe water coning from an aquifer underlying a large section of the field. For this reason the field will be developed with pumped horizontal wells with completion intervals of up to 6000 ft. Key elements which will affect reservoir and well performance are horizontal to vertical permeability ratio, critical water saturation, mobile transition zone height, and horizontal well inflow profile. To evaluate these parameters a 30 day vertical water coning test and a separate 90 day extended production test in a prototype horizontal development well, were performed and the measured water cut performance and bottom hole pressures matched with simulation models. Additionally inflow performance in the horizontal well was assessed by production logs which demonstrated that inflow occurred along the majority of the well length. These two tests were conducted in close proximity to each other in a reasonably homogeneous sand, providing the opportunity for simultaneous and consistent interpretation of the observed data. This integration of the results from two tests of different types has resulted in interpretations which have greater levels of confidence attached to them than would have been possible if the two tests had been performed in isolation.
Introduction
The Captain field is located in the North Sea in block l3/22a. It is located at a depth of 2800 ft in 350ft of water and contains about 1500 mmbbls of 20 API oil of 40 to 80 cp viscosity.
The field was discovered in 1977 by well 13/22-1 which was tested without artificial lift and failed to flow. Small volumes of oil were circulated to surface but proved to be so viscous that further appraisal drilling was delayed until 1989 with well l3/22a-2. This well was sufficiently encouraging for further drilling to occur, culminating in l3/22a-8 in 1990. This well was completed with a 1000 ft horizontal section and was tested at a rate of about 6500 bbls/day with the aid of an electric submersible pump. Based on the results of this well an accelerated appraisal drilling program was commenced in 1993. No fewer than ten wells were drilled, the most important of which were l3/22a-9A which discovered a small gas cap and l3/22a- 12 and l3/22a-10 on which were performed the extended well tests (EWTs) which are the subject of this paper.
Geology
The oil is contained in two distinct reservoirs; The Upper Captain Sand (UCS) and Lower Captain Sand (LCS). Additionally an Eastern Extension to the UCS is separated from the main UCS body by a scaling fault (Figure 1). The reservoirs are bounded to the north by an aquifer which is effectively infinite in extent, outcropping approximately 30 km from the reservoir. The main UCS has a gas cap over approximately one fifth of its area and offset to the east. The UCS dips and thickens to the north and rises and pinches out to the south. As a result, approximately half the UCS is underlain by shale and half by the aquifer. The LCS is a channel sand directly under the UCS and running northwest - southeast.
The petrophysical properties of the reservoir are excellent with 28 - 35 percent porosity and 6 to 12 Darcy permeability.
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