Uncertainty Analysis in Formation Evaluation: Rationale, Methods and Examples
- Philippe Gaillot (ExxonMobil) | Jerome Lewandowski (ExxonMobil) | Roza Nursaidova (ExxonMobil)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 60th Annual Logging Symposium, 15-19 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
- 3 in the last 30 days
- 160 since 2007
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Upstream Uncertainty Analysis (UUA) frameworks are designed to support geologic and engineering decisions with significant business risks. While those frameworks ultimately focus on the 3-dimensional (static) and 4-dimensional (dynamic) characterization of the subsurface, their implementation start at the 1-dimensional well scale with petrophysicists as key players.
This paper aims to highlight that petrophysical interpretations are not ‘hard’ data and that there is much to gain by estimating and communicating petrophysical uncertainties. Petrophysicists deal with incomplete and unknown information in all steps of the petrophysical analysis: well log acquisition, calibration, processing, and model development/interpretation. Each of these steps has uncertainties that can affect the final results. The key objectives for petrophysical analysis are to reduce the systematic errors, and to perform a consistent analysis between wells leading to the development of predictive models aiming at informing business decisions. Estimations of petrophysical uncertainties are an integral part of any petrophysical evaluation. Formation Evaluation Uncertainty Analysis (FEUA) attempts to include different sources of uncertainty (log measurements, sub-model approximations, heterogeneity, natural variability of the rocks), and efficiently communicate those uncertainties to team members and management.
This paper explains how uncertainties in formation evaluation (FE) can be practically quantified through various error propagation approaches, and proposes simple standards for communicating FEUA outputs. Direct benefits and limitations of FEUA are discussed in light of applications ranging from challenging multiwells petrophysical model development and assessment of farm-in opportunity to petrophysical support for rock physics modeling linking the physical properties measured at the well with petrophysical, elastic and seismic properties.
General and systematic use of FEUA helps petrophysicists take full ownership of well data and interpretations, including uncertainty, and conform to standards defined by UUA framework, i.e. software tools and workflows for systematic integration of data and interpretation to quantify magnitude and probable distribution of asset value. The most-direct and clear benefits of FEUA include (i) check that petrophysical results are within expected errors, (ii) alternative scenarios/interpretations have been considered early in the evaluation process, and (iii) inputs to sensitivity studies and geo-statistics for static and dynamic subsurface descriptions are produced in a timely and consistent manner.
|File Size||3 MB||Number of Pages||24|