Well Count Optimization Incorporating a Wide Range of Uncertainties for the Deepwater Agbami Field
Authors: Narahara G.M. , Spokes J.J. , Brennan D.D. , Maxwell G. , Bast M. 
Published: October 2008

This paper describes the methodology for incorporating uncertainties in the optimization of well count for the deepwater Agbami project. The lack of substantial reservoir description data is common in many deepwater discoveries. Therefore, the development plan must be optimized for a wide range of uncertainties. In the Agbami project, the design of experiments or experimental design (ED) technique was incorporated to optimize the well count across a wide range of subsurface uncertainties.

Multiple well count development plans (high, mid, and low) were developed and used as a variable in ED. Also, multiple geologic models representing the broad range in uncertainty in oil in place (greater than a factor of two) and in net-to-gross were built and used as a variable. Additional uncertainty variables, including permeability, fault seals, and injection conformance, were studied concurrently. The ED technique allowed multiple well counts to be quickly tested against multiple geologic models. With the net present value (NPV) calculated for each case, not only was the well countfor the overall highest NPV determined, but ED allowed discrete testing of each geologic model to determine the optimum well count for each model.

A methodology was developed for optimizing well count development plans over a broad range of uncertainties including a range of geologic models, which vary in oil in place and net-to-gross. The process allowed testing the robustness of any well count versus any uncertainty (or set of uncertainties).

A method was demonstrated quantifying the difference between perfect and imperfect knowledge of the reservoir description (geologic model) as it pertains to well locations.

A total well count of 38 was concluded to be the optimum well count for the Agbami project based on NPV. This well count proved to be robust across the full range of uncertainties tested.

Introduction

The Agbami structure is a northwest/southeast trending fourway closure anticline, and is located on the Niger delta front approximately 65 miles off-shore Nigeria in the Gulf of Guinea (see map in Fig. 1). The structure spans an area of 45,000 acres at spill point and is located in 4800 ft of water. The Agbami No. 1 discovery well was drilled in late 1998. The appraisal program was completed in 2001 and included five wells and one sidetrack drilled on the structure with each ncountering oil pay. These five wells and a sidetrack penetrated an average of approximately 350 ft of oil.

In Phase 3 of the development process, the key objectives are to construct a field development plan and to obtain sanctioning. With drilling depths of up to 10,000 ft below mudline in 4800 ft of water, well costs at Agbami will be at the high end of typical deepwater costs. Therefore, an important optimization parameter in the field development is the total well count.

Agbami is typical of many deepwater developments in that the seismic is less than perfect and the appraisal well data is sparse relative to the area coverage. Therefore, subsurface uncertainty is high.