Experimental Design as a Framework for Multiple Realisation History Matching: F6 Further Development Studies

Abstract

Abstract History matching is traditionally complex and time-consuming: multiple parameters influence the match and their inter-dependency produces effects that are difficult to predict. Defining the match itself can be challenging, since various indexes or responses can be used: water breakthrough timing, pressures, layer contributions etc… Consequently, whilst multiple realisations methodologies are routinely applied for "green" field development planning, most of the time incremental activity screening on "brown" fields is done on a single matching realisation -"the" matched model - with little confidence that the full range of uncertainties is captured. Experimental design provides a well-suited framework to tackle the challenge of multi-realisation history matching, following these key steps:Selection of key parameters with variance analysis,Reduction of dimensionality by creating hybrid parameters, using techniques related to principle component analysis,Predicting matching domains: combination of parameters levels (once discretised) that are likely to generate a match. This greatly helps the likelihood of finding multiple matching realisations, covering the range of parameter variation. This methodology was successfully applied in the F6 subsurface studies, aimed at screening field redevelopment opportunities. F6 is the largest gas field in the Central Luconia carbonate province, offshore Sarawak (Borneo), having a GIIP of more than 7 Tscf. With over half the reserves produced, well capacity is now threatened by the rising aquifer. In order to safeguard and possibly increase the reserves, a field review was undertaken to identify further development opportunities, and a multi-realisation approach was chosen to capture the effect of key subsurface uncertainties on those activities. A total of 28 matching realisations were generated, covering the variation range of the identified key seven parameters whilst optimising the number of runs performed, thus saving time. Key to the success of the method lies in the integration of disciplines to allow the upfront identification of parameters and their ranges. The screening of redevelopment options against those realisations allowed to establish the range of expected incremental reserves, assess risks, and form a sound basis for business decisions. Introduction F6 is the largest gas field within the Sarawak Shell portfolio, with a GIIP of over 7 Tscf. The field covers a large area of approx. 168 km2 and has a gas-bearing interval of over 850 feet thickness. It is an elongated carbonate build-up of Miocen age, which has steep flanks and a generally flat crest. Two main units make up the gas accumulation: the Upper (Zone 1 and Zone 2) reservoir and Lower (Zone 3) reservoir, separated by an extensive baffle (figure 1). The Lower reservoir contains almost two-thirds of the gas in place. Production started in 1987 and the initial field performance indicated a weak aquifer drive, with a slow water rise. Water breakthrough occurred in late 2001, and capacity went under threat since then, and triggered the need to look at infill drilling and redevelopment opportunities. To this purpose a comprehensive subsurface review was undertaken, using the latest information, notably a 3D seismic dataset shot in 2002, but also innovative technologies such as multi-attribute imaging to map the internal architecture, and Experimental Design techniques to perform a multi-parameter history match, aimed at delivering matching subsurface realisations covering the whole range of uncertainties.