Production Forecasting of an Unstable Compacting Chalk Field Using Uncertainty Analysis

Abstract

Summary Past years' forecasting performance has demonstrated that single "most likely" (deterministic) prediction of future production, without quantifying the associated uncertainties, is inadequate for management and planning purposes. Based on this an effort was initiated to improve the forecasting methodology and procedures with special emphasis on quantifying the uncertainty in short-term production forecasting (STPF). The Valhall field, offshore Norway, presented special problems for production forecasting because many unusual events affect production, even for STPF. The soft compacting reservoir chalk causes well collapses and chalk influxes and makes drilling difficult. Modeling of reservoir fracturing and therefore well performance is also uncertain. A probabilistic forecasting approach was adopted using a customized spreadsheet and commercially available statistical analysis add ins which allowed deterministic forecasting, partially probabilistic analysis, or fully integrated uncertainty analysis. Uncertainties were characterized by distributions based on historical data where possible. Communication and integration of knowledge were also key success factors since the process required input from several departments and many individuals to ensure that the "company knowledge" was fully reflected. A "common language" to communicate uncertainty and an auditable process to ensure buy in and consistency were also critical.