Abstract

A case study is presented from an oil field in the Sultanate of Oman, operated by Petroleum Development Oman. To date the field has seen limited production under natural depletion (~1% of STOIIP produced). Most of the oil is located in 20–25 m thick oil rims with large gas caps in the reservoirs of the Gharif and Al Khlata formations. A Field Study was kicked off in PDO in 2004, triggered by the results of recent appraisal wells and has substantiated significant additional field potential.

This paper extends previous work [Ref 1] in which we set out the use of Experimental Design for subsurface uncertainties analysis and development options evaluation for an oil field in the Sultanate of Oman. Here, we demonstrate for the first time how the multi-scenario ED modeling results can be taken one step further in Field Development optimisation. We introduce the concept of "Well Maturity Index" which quantifies the impact of subsurface uncertainties on EUR for each well in the selected development. The Well Maturity Index was used to guide decisions on the phasing of development and appraisal drilling and to help formulate a data acquisition programme for the field.

This approach has resulted in an auditable and quantified decision-based plan for maximizing NPV and minimising risk, and adds significant value to the development planning process. This work also provides a natural stepping stone for a quantifiable Value of Information analysis, and scenario planning based on appraisal outcomes. The methodology used is general and should find application in many field development studies..

Introduction

The Case Study field is located in onshore North Oman and is operated by Petroleum Development Oman. The relatively light oil is located in stacked oil rims beneath large gas caps in four different reservoirs (AK, LG, MG and UG). The field is associated with a mainly dip-closed, low relief faulted anticline, with its crest at a depth of 3000 m. Intervening shales and tight limestones act as intra-reservoir seals, and the UG, MG and LG/AK formations have separate fluid contacts. To date the field has seen limited production under natural depletion with three phase flow to remote processing facilities. A schematic of the field is given in Figure 1. A Field Development Plan (FDP) was derived after an extensive multi-scenario modeling approach using Experimental Design [Ref 1], followed by deterministic optimisation and by testing of the development against subsurface uncertainty. Key elements of the FDP are:

  • Initial drilling to fill existing and committed facilities with low GOR oil by developing the AK oil rim with horizontal wells, coupled with appraisal of the overlying LG MG and UG reservoirs.

  • Installation of full field facilities. Gas separation and compression with gas and oil/water routed to dedicated processing facilities. The increased capacity to be filled with production from potentially higher GOR LG and UG reservoirs.

  • Final gas cap blow down will be carried out at the end of economic oil production.

The multi-scenario ED results have also created a wealth of useful data, that can be further explored. A more detailed quantification of the impact of subsurface uncertainties can be pursued for the final development, on a well by well basis. This paper describes how this has further refined the phasing of development and appraisal drilling and how this has tailored the data acquisition programme to be more specific to each of the wells. Also, ranges for key surface facility design parameters were defined and were used for further optimisation the design specifications.

Experimental Design Results

Experimental Design is a mathematical technique which aims at representative sampling of the full parameter space with a relatively small number of parameter combinations. It is now widely recognized in the oil industry that ED provides a robust and time efficient way of handling multi-factorial problems such as volumetric calculations, history matching and development screening [Ref 1- 8].

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