Optimised Field Development Planning in a Complex Geological and PSC Environment

Abstract

The paper describes an optimisation methodology used to aid development planning for the A-18 block in the Malaysia Thailand Joint Development Area (MTJDA). Development of this block is complicated by the large areal extent, gas residing in up to fifteen different major reservoir sequences, large depth range with the deeper sequences being HPHT intervals, and CO2 contents varying from 1 to 60%. An optimised field development plan has to maximise economic value from a volume based gas sales agreement which generates a heating value based revenue stream linked to CO2 level, and prioritises condensate production. The interplay between gas and liquids considerations and a myriad of potential platform and development drilling options within a PSC environment makes the selection of an optimum development scenario difficult. The paper describes how a decision framework was used to maximise the economic value of the asset. This custom built software automatically generates possible development scenarios, and then uses branch-and-bound and outer approximation solution techniques to analyze and efficiently "prune" the scenario tree to eliminate sub-optimal candidates. The results illustrate the effectiveness of applying optimisation technology to this complex asset, as development plans can be quickly and robustly constructed for different GIIP and recovery scenarios and for different objectives such as maximising sales gas volume or NPV. Introduction The A-18 block includes 11 discovered gas fields within an area of nearly 3000 sq km (Figure 1) and contains a total certified 2P GIIP of 12 Tcf. There are 4 fields currently under development: Cakerawala, Bumi, Suriya and Bulan. The asset sits in the area straddling Malaysian - Thai waters and is jointly owned and regulated by both countries through the Malaysia Thailand Joint Authority (MTJA). Exploitation has been entrusted to a joint venture between Petronas Carigali Sdn. Bhd. and Hess Oil & Gas Sdn. Bhd. and takes place in a PSC regulatory environment. There are two PSC's which cover the A-18 area, one for development in the Cakerawala field, and the other covering the remaining fields; they have different capital recovery terms. The fields are depositionally complex and key reservoirs vary from very high quality thick continuous sequences in the shallow zones of Cakerawala to poorer quality heterolithic intervals in areas of Bumi. The A-18 Miocene hydrocarbon system consists of a thick interval (~7000 feet) of interbedded sands and shales which progress from normally pressured between 3500–6000 feet, to over pressured and hot (8500 psi/390 OF) in the sequence XV sands at 10,000 feet. CO2 percentages in individual sub-sequences vary from below 1% to 60% and condensate yields, although predominantly light, show variations between 5 and over 40 stb/mmscf. Development to date has followed the two major Gas Sales Agreements (GSA). The Phase 1 development comprised 3 platforms in the Cakerawala field with a Central Processing Platform (CPP) and an export pipeline via southern Thailand into Malaysia. First gas was exported in 2005 under a GSA for 390 mmscf/d of sales gas. The Phase 2 development which underpins the second GSA for an additional 400 mmscf/d sales gas to Thailand, comprises expansion of the CPP and development of the Bumi, Bulan and Suriya fields via individual platforms (BMA, BLA and SYA). This was completed during 2007/8. There are a total of 36 Cakerawala production wells, with a similar number distributed between the other fields. The GSA allow for export of gas with up to 22% CO2 content, although price is dependant on heating value, and facilities design work concluded that optimum sales gas export was at 15% CO2 content. An additional key value driver is to maximise the condensate offtake, which is exported via a nearby Floating Storage and Offloading vessel (FSO) and shuttle tankers.