A Multi-Scenario Modelling approach was used to select a field development option that is robust for most if not all of the sub-surface realisations. This approach was adopted for the Jintan carbonate gas field, located offshore Sarawak, Malaysia in the South-China Sea. The Jintan field will be developed as a satellite to the nearby M1 offshore gas production facility for the supply of gas to the MLNG Tiga plant, which is currently under construction. First gas is in July 2003. Significant value has been added through an integrated, multi-discipline sub-surface study which provides the basis for the field development plan:cost savings of more than 60% were realised through integration of Jintan in Shell's existing gas infrastructure of Sarawak and by reducing the number of wells from 12 to 6,the expectation initially-in-place volumes increased by approximately 1 Tscf to 4 Tscf of gas. Key uncertainties for the Jintan field are gross rock volume (GRV) and porosity, permeability (tight streaks, karst and fault features), the initial pressure (due to possible depletion from the nearby producing fields) and the effective aquifer strength. The applied workflow to model these uncertainties included structural uncertainty modelling, stochastic inversion, static reservoir modelling and dynamic simulation. The deliverable of the (Jason) stochastic inversion study were 93 combined depth/porosity models. Five of the 93 depth/ porosity models were selected for the final porosity modelling and exported to the 3D static reservoir simulation software. The static modelling resulted in 11 different static models, which were used for the dynamic modelling. In the dynamic simulator, the initial pressure uncertainties were combined with the 11 static models into 33 sub-surface realisations which were then combined with different aquifer strengths and karst realisations. For each of the realisations an optimal development option was selected based on the currently expected capacity/offtake requirements and possible future changes/flexibility. The resulting field development plan is robust for the following reasons:The selected development option caters for the uncertainty range of almost all modelled subsurface scenarios.Dependent on the subsurface scenario, 4 to 7 wells will be drilled. The exact number of wells required to provide a given capacity will be determined during the drilling phase.Well head compression is required in almost all scenarios but never within the first years therefore allowing sufficient time to monitor the reservoir performance and, if required, execute a compression project. The described workflow, with a high level of integration between the subsurface disciplines, demonstrates the value of multi-scenario subsurface uncertainty modelling in situations where large uncertainties exist. Introduction The objective of a field development plan is to cover the key (subsurface) uncertainties to optimize the economic value of an asset. In the early phase of a field's life, only limited information about the reservoir is available and consequently the potential impact of numerous uncertainties on a field development needs to be evaluated. The aim is to design a flexible development plan which is robust to potential downsides but can also rapidly respond to upside opportunities (Ref. 1). The Jintan field development plan (FDP) provides a case study where multi-scenario modelling has been used to address uncertainties in the static reservoir model as well as in the dynamic simulation model. Scenario modelling for field development planning is widely being used in the Shell Group, and it is being recognised inside and outside Shell, as a method that allows for improved, more flexible and proactive field development planning (Ref. 2). The approach is based on creating different independent subsurface realisations that are driven by the key subsurface uncertainties that could impact on the field development, i.e. on the well locations, surface facilities design, total supply forecast etc.

The field development plan for a Sarawak Shell Berhad operated gas field, located in the South China Sea, offshore Sarawak, Malaysia, specified drilling of horizontal wells into the Tertiary Miocene Carbonate reservoir. The wells were planned as high capacity producers of the Big Bore-Long Casing Flow design. The traditional well design dictated that, prior to entering the reservoir, a casing had to be installed to stabilise the hole in soft shale. The uncertainty of detecting the formation top resulted in premature casing commitment of at least 30 feet TVD above the top of the reservoir and the need to use an expandable liner to cover 300 feet of exposed shale above the reservoir. To obviate this problem, the capability of one of the components in the Logging-While-Drilling tool array, namely the Electromagnetic Wave Resistivity forward modeling technique, was used to detect the top of the carbonate formation (top reservoir), immediately prior to drilling into it. A standard Logging-While-Drilling tool is configured to prioritize Electromagnetic Wave Resistivity forward model response as the carbonate formation top is approached. This configuration, together with an appropriately designed bottom-hole-assembly and well trajectory, enabled the successful implementation of the plan to stop drilling approximately one foot true-vertical-depth above the carbonate top. At this point, a conventional 9 5/8-in. casing string was set at an optimum depth. This eliminated potential well control problems, costly remedial actions associated with lost circulation and inferior cementation of the 9 5/8-in. casing string. Thereafter, the wells were drilled horizontally in a conventional manner, into the carbonate gas reservoir. This paper compares pre-drilling Electromagnetic Wave Resistivity forward modeling of the proposed well trajectory with the actual well data, whilst drilling. The pre-drilling and post-drilling modeled data is presented. The cost savings from employing this technique are variable, ranging from substantial - in the event of a well control situation and attendant high losses - to moderate if the need to set an expandable is eliminated. Introduction Sarawak Shell Berhad operates numerous gas fields in the Central Luconia area located in the South China Sea, offshore Sarawak, Malaysia. The field development plan for the M4 field (Fig. 1) specified drilling of two horizontal wells into the Tertiary Miocene Carbonate reservoir. The wells were planned as high capacity producers of the Big Bore-Long Casing Flow Design. In the Central Luconia area, the drilling of development wells is challenging as frequently mud losses or total loss of circulation are encountered due to karst and fractures in the carbonate reservoir. The carbonate reservoir is overlain by a thick layer of soft shale which needs to be drilled with high mud weight drilling fluids. A 9 5/8" casing string has to be set before drilling into the carbonate reservoir to avoid bore-hole collapse when drilling into the reservoir and the occurrence of severe mud losses. Since the shale section above the carbonate reservoir lacks any geological character or marker beds to help delineate the Top Carbonate; it was common practice to set the casing point at least 30 feet TVD above the Top Carbonate. Subsequent drilling left approximately 300 feet of exposed soft shale along the bore-hole before entering the carbonate reservoir which necessitated the installation of an expandable liner to isolate the soft shale. Therefore, a new methodology was looked for to minimize the length of the open bore-hole shale section. Well Plan and Strategy To set the 9 5/8" casing as close as possible to the Top Carbonate, the Electromagnetic Wave Resistivity (EWR) forward modeling technique is employed to "look ahead" of the drill bit.

Summary The field-development plan for a Sarawak Shell Bhd.-operated gas field, located in the South China Sea offshore Sarawak, Malaysia, specified drilling of horizontal wells into the Tertiary-Miocene carbonate reservoir. The wells were planned as high-capacity producers with a big-bore, long-casing flow design. The traditional well design dictated that before entering the reservoir a casing had to be installed to stabilize the hole in soft shale. The uncertainty of detecting the formation top resulted in a premature casing commitment of at least 30 ft true vertical depth (TVD) above the top of the reservoir and the need to use an expandable liner to cover 300 ft of exposed shale above the reservoir. To obviate this problem, the capability of one of the components in the logging-while-drilling (LWD) tool array, specifically the electromagnetic-wave-resistivity (EWR) forward-modeling technique, was used to detect the top of the carbonate formation (i.e., the top of reservoir), immediately before drilling into it. A standard LWD tool is configured to prioritize EWR forward-model response as the carbonate-formation top is approached. This configuration, together with an appropriately designed bottomhole assembly (BHA) and well trajectory, enabled the successful implementation of the plan to stop drilling approximately 1 ft TVD above the carbonate top. At this point, a conventional 9?-in. casing string was set at an optimum depth. This eliminated potential well-control problems, costly remedial actions associated with lost circulation, and inferior cementation of the9?-in. casing string. Thereafter, the wells were drilled horizontally in a conventional manner into the carbonate-gas reservoir. This paper compares predrilling EWR forward modeling of the proposed well trajectory with the actual well data while drilling. The predrilling- and post-drilling-modeled data are presented. The cost savings from employing this technique are variable, ranging fromsubstantial—in the event of a well-control situation and attendant high losses—to moderate if the need to set an expandable liner is eliminated. A minimum of U.S. $1 million per well was saved with this technique. Introduction Sarawak Shell Bhd. operates numerous gas fields in the central Luconia area located in the South China Sea, offshore Sarawak, Malaysia. The field-development plan for the M4 field specified drilling two horizontal wells into the Tertiary-Miocene carbonate reservoir. The wells were planned as high-capacity producers with a big-bore, long-casing flow design. In the central Luconia area, the drilling of development wells is challenging because frequent mud losses or total loss of circulation are encountered as a result of the karst phenomenon, which is the secondary erosion of limestone formation producing subterranean fissures, conduits, and caverns as well as fractures in the carbonate reservoir. The carbonate reservoir is overlaid with a thick layer of soft shale, which needs to be drilled with high mud-weight drilling fluids: 13.27 ppg. A 9?-in. casing string has to be set before drilling into the carbonate reservoir to avoid borehole collapse when drilling into the reservoir with lower mud weight to prevent severe mud losses. Because the shale section above the carbonate reservoir lacks any geological character or marker beds to help delineate the top carbonate, it was common practice to set the casing point at least 30 ft TVD above the top carbonate. Subsequent drilling left approximately 300 ft of exposed soft shale along the borehole before entering the carbonate reservoir, which necessitated the installation of an expandable liner to isolate the soft shale. Therefore, a new methodology was sought to minimize the length of the openhole shale section.