Probabilistic Subsurface Forecasting - What Do We Really Know?

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. The full manuscript of SPE 132957, ’Probabilistic Subsurface Forecasting,’ is available as a supplement to this article. Abstract The use of single-valued assessments of company portfolios and projects continues to decline as the industry accepts that strong subsurface uncertainties dictate an ongoing consideration of ranges of outcomes. Exploration has pioneered the use of probabilistic prospect assessments as being the norm, in both majors and independents. Production has lagged, in part because of the need to comply with US Security and Exchange (SEC) reserves-reporting requirements that drive a conservative deterministic approach. Look-backs continue to show the difficulty of achieving a forecast within an uncertainty band as well as the difficulty of establishing what that band should be. Ongoing challenges include identifying relevant static and dynamic uncertainties, efficiently and reliably determining ranges and dependencies for those uncertainties, incorporating production history (brownfield assessments), and coupling subsurface with operational and economic uncertainties. Despite these challenges, none of which are fully resolved, a systematic approach based on probabilistic principles [often including design-of-experiment (DoE) techniques] provides the best auditable and justifiable means of forecasting projects and presenting decision makers with a suitable range of outcomes to consider. Introduction Probabilistic subsurface assessments are the norm within the exploration side of the oil and gas industry, both in majors and independents (Rose 2007). However, in many companies, the production side is still in transition from single-valued deterministic assessments, sometimes carried out with ad-hoc sensitivity studies, to more-rigorous probabilistic assessments with an auditable trail of assumptions and a statistical underpinning. Reflecting these changes in practices and technology, recently revised SEC rules for reserves reporting (effective 1 January 2010) allow for the use of both probabilistic and deterministic methods in addition to allowing reporting of reserves categories other than ’proved.?? This paper attempts to present some of the challenges facing probabilistic assessments and present some practical considerations to carry out the assessments effectively.