Generation of Probabilistic Reserves Distributions From Material Balance Models Using an Experimental Design Methodology

Overview of Malampaya Field: The Malampaya-Camago Gas-Condensate field is a Tertiary Carbonate build-up that is situated offshore to the northwest of Palawan Island (Philippines) below 800–1200m of water. It was discovered by Occidental in 1989 (Camago-1) and is operated by Shell (45%) with Chevron (45%) and the Philippines National Oil Company (10%) as equity partners. The field has been supplying gas from five subsea production wells since late 2001 to three Gas to Power plants on Luzon Island. Available subsurface data include a 2001 high resolution 3D seismic survey, five production wells and six exploration/appraisal wells with wireline and borehole image data, including spot core in selected wells, pressure and well test data. A schematic of the Malampaya gas to power project is shown below. Overview of Experimental Design (ED) Methodology An extensive reservoir characterisation study was carried out by Chevron on a non-operated asset in 2004 to determine gas-in-place and reserves distributions. This paper outlines the Experimental Design (ED) methodology used in the study which using material balance models integrated the static and dynamic reservoir uncertainties with the excellent reservoir pressure history data that exists for the field. The methodology incorporated a unique and "state-of-the-art" method for screening over 20,000 reservoir realisations to allow only those realisations that showed a close match to the pressure history to be included in the generation of GIIP and reserves distributions. This aspect of the ED methodology proved to be very powerful in generating reserves distributions in a "hands-off" approach without the requirement for traditional manual adjustment of reservoir parameters to get a "history match".