The Role of Geophysical Uncertainty in Field Development Concept Selection
Authors: Krasova Daria , Tresselt Sverre , Meisingset Ivar , Forde Thomas , Romundstad Stale 
Published: May 2013

The paper presents a parallel, probabilistic approach to field appraisal and development concept selection, rather than the conventional sequential approach. Instead of waiting for appraisal drilling to confirm and finalize the reservoir model, front end concept selection work is started at an earlier stage, based on a model with a high degree of uncertainty. Stochastic depth conversion uncertainty analysis is used to calculate P10 – P50 – P90 structure maps and gross rock volumes, thus quantifying the uncertainty. A series of field development concepts are being estimated to handle the entire uncertainty span. An optimized appraisal drilling program is then proposed, for the purpose of eliminating those uncertainties which would swing the field development concept selection. This combination of geophysical and engineering disiplines leads to a field development scenario with a minimal drilling cost spent on appraisal, and with an assurance that the optimal field development concept has been chosen.

Introduction

A number of cost intensive and technically crucial decisions need to be made in oil and gas field development. A broad range of issues are involved, within geology and geophysics (G&G), reservoir management, drilling/completion technology, production strategy, facilities size/solutions, infrastructure and transportation to the market. Deciding on the right field development option requires an organization that works closely together across the disiplines. Oil and gas companies have come a long way in using modern simulation and modeling tools which are suited for such cooperation.

We have, for the purpose of this study, constructed a synthetic data set, the Aker Field, which is in the early stages of field development planning. The latest exploration well has made a significant oil discovery. The field is located in the Norwegian North Sea. The reservoir is situated relatively shallow, at a depth of about 4675 ft. under 378 ft. of water. Current data indicate that the reservoir has excellent flow properties in clean sands with no indications of complex faults and barriers, but there is still significant uncertainty with regards to top reservoir depth, and as a consequence, the lateral extent of the field. Based on seismic mapping, and reservoir properties from wells in the area, the Aker Field looks very promising, and plans for field appraisal drilling and field development are being made.

The conventional (sequential) approach would be to start with appraisal drilling, confirming the reservoir model of the field, and then hand that model over to engineering as the basis for development concept selection. The alternative, which we are exploring in this paper, is instead to use a parallel, propabilistic approach, where early phase development concept selection is started before appraisal drilling, when the reservoir model still is very uncertain. This is challenging, because people from disciplines who normally do not interact closely have to cooperate, but it can be very rewarding, because the problems are being looked at from additional angles, pulling on expertise that normally is not used at this stage. It is very likely that this approach will lead to improvements in the appraisal program, and in the field development, and thus to significant economic gain.