The Lisburne Group is a Pennsylvanian-missisippian age carbonate sequence which underlies the main Permo-Triassic producing horizon at Prudhoe Bay. During the last three years, delineation and appraisal efforts have accelerated leading to financial commitment to field development. Production startup is scheduled for December 1986.
This paper discusses the details of well planning and implementation necessary for the evaluation of this complex, fractured, low permeability limestone and dolomite reservoir. well operations and testing have included techniques previously uncommon or unused in the Alaskan Arctic such as lightweight mud and cement, bland and pressure coring, open-hole tests with open-hole solvent stimulations, nitrified acid treatments, nitrogen lift of cased hole tests and fracture treatments. These methods helped ensure conclusive tests in a formation that is characteristically difficult to test and also established procedures to affect commercial well completions.
Methods used in the analysis of well results are also discussed briefly to provide some insight into the use of the well data in the overall evaluation process.
The Lisburne initial development area consists of roughly 30,000 acres (1.21 × 108 m) underlying Prudhoe Bay and surrounding onshore tracts. In plan, it is coincident with the northeast corner of the prolific Permo-Triassic reservoir as shown in Fig. 1. The Lisburne prolific Permo-Triassic reservoir as shown in Fig. 1. The Lisburne monoclinal trap is sealed against the Niakuk Fault to the north and truncated by the Lower Cretaceous Unconformity to the east (Fig. 2). The Lisburne itself is divided into upper and lower sections by a "Green Shale" marker. The Upper Lisburne is a stratified section of limestones and dolomitic limestones with interbedded cherts and sands and is oil productive in the initial development area. The Lower Lisburne productive in the initial development area. The Lower Lisburne consists of a zone of porous dolomite stringers interbedded with tight limestones and a zone of more uniform dolomites with good to fair intercrystalline porosity and matrix permeability. These zones have tested oil and water in the initial development area. Fig. 3 shows a typical Lisburne stratigraphic section.
Investigations of the Lisburne reservoir date back to ARCo's Prudhoe Bay State 1 discovery well. This 1967 - 68 test Prudhoe Bay State 1 discovery well. This 1967 - 68 test confirmed productive hydrocarbons in the Lisburne section. Additional wells drilled in this area from 1969 to 1978 served to further delineate the reservoir and confirmed the existence of a Lisburne gas cap.
Interest in the Lisburne increased in the early 1980's as a result of increasing oil prices and anticipation of the Permo-Triassic reservoir decline. Extended production tests were conducted to establish that the Lisburne could indeed sustain oil production at an economic rate. Reservoir performance information was obtained which provided additional justification for committing to field development in 1984.
During delineation drilling and testing, it became apparent that standard techniques employed in North Slope clastic formations would not be acceptable for operations in a fractured carbonate environment. Operational problems included lost circulation, formation breakdown during cementing, effective stimulation design, and logistics associated with testing several zones during a limited season.