Global Optimization and Uncertainty Assessment for History Matching the Valhall Field

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 100946, "History Matching of the Valhall Field Using a Global- Optimization Method and Uncertainty Assessment," by B.R. Al-Shamma, SPE, Imperial College, London, and R. Teigland, Total E&P Norge, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. A study was made of a global-optimization method to history match a complex reservoir model. Evolutionary algorithms (EAs) were applied, and methods for improving the convergence of the optimization cycle were used. Sensitivity parameters, correlations, and parameter trends were identified in a global search space, and an uncertainty assessment of the most recent history match was carried out with an experiment-design matrix. Introduction The focus of this study was to assess the Valhall field reservoir-simulation model by use of EAs and uncertainty assessments to improve the manual history match of the model. The Valhall field is complex with rock compaction as the main drive energy, which contributes to many challenges, including the reduction of porosity (Ø) and permeability (k) with time and continued change of reservoir thickness caused by the compaction. The compaction can affect different parts of the reservoir differently. For example, the reduction in the pore volume can vary between regions, resulting in problems with the history-matching process in which changing a particular parameter can have opposite effects on wells or regions. Fig. 1 shows the workflow used for uncertainty assessment. The Valhall field is an oversaturated Upper Cretaceous chalk reservoir in the North Sea approximately 290 km offshore Norway in 69 m of water. Discovered in 1975, oil production began in 1982. Oil originally in place was estimated at 4.3×108 m3. Total production at the time this paper was written was 7.9×107 m3 of oil equivalent. The primary reservoir is the Tor formation, with a secondary reservoir in the Hod formation. The thickness of the Tor formation varies, abruptly ranging from 0 to 80 m. Approximately 50% of the drive to date has been from rock compaction, observed by porosity reduction in infill wells. The porosity is greater than 50% in some places; some fractured permeability exists, but matrix permeability is low (less than 10 md). The average solution-gas/oil ratio (GOR) for the field is approximately 20 std m3/m3, with an increase in the field GOR after 20 years' depletion of only 35%. The main challenges experienced during drilling and production included chalk and solids production, well failures and stability problems, fracture closure during depletion, and a subsiding seafloor around the original production platform.