ABSTRACT:

Sarawak Shell Bhd has a number of offshore gas fields that produce from the Luconia carbonate formation, which can exhibit high compressibility pore collapse deformation. Accelerated subsidence observed several years ago above several of these fields gave extrapolated final subsidence values well above previous estimates. This motivated reviews and updates of the subsidence estimates for several of these fields. This paper describes a field/core calibrated finite element geomechanical model developed to predict future subsidence and lateral movements for the platforms of the adjacent M1 and Jintan fields, two Luconia fields offshore Sarawak, Malaysia. The model is calibrated to continuous GPS measurements of field platform movement. The Luconia carbonate formation is modeled as a non-linear elasto-plastic material, for which the material parameters are calibrated to laboratory core measurements and field platform subsidence This work illustrates the importance of integrated geomechanical core testing, field monitoring measurements and modeling to accurately predict compaction and subsidence effects in highly compacting environments.

1. INTRODUCTION

The main objective of this study is to provide updated subsidence predictions for Sarawak Shell Bhd?s M1 and Jintan fields. The study was motivated by observations of accelerated subsidence at other gas fields in the area, and the need to insure platform subsidence remains below that included in the platform design. The M1 and Jintan fields are carbonate reservoirs at the top of a single large carbonate build-up located offshore Sarawak, Malaysia. A top of carbonate map of the ?Mega Platform? area that contains the M1, Jintan, M3, M4, Saderi and Serai fields is shown in Figure 1. The Luconia carbonate formation has several facies, but its compaction properties are dominated by the mouldic limestone facies, which undergoes pore collapse with depletion [1]. Production started in 1996 in the M1 field and in 2004 in the Jintan field, respectively. Permanent GPS sensors were installed in 2000 and 2003 on the production platforms in the M1 and Jintan fields, respectively. The movement of the production platforms has been recorded since then (on a monthly basis since October 2005). The measured subsidence of the platform above the Jintan reservoir follows a linear trend with respect to depleting reservoir pressure at reservoir datum depth (5300 ft) directly below the platform. Meanwhile, the relationship between the seafloor subsidence above the M1 reservoir dome and the reservoir pressure is nonlinear, which is indicative of increasing compaction due to pore-collapse in the reservoir. The measured data will be presented below, together with the results of the modeling exercise. The M1 and Jintan reservoirs are in pressure communication and have to be modeled together. In order to accurately predict the seafloor subsidence resulting from the expected depletion of the reservoir pressure in both M1 and Jintan fields, a geomechanical model was constructed. It is a 3D finite element model with four formations with different material properties. The reservoir carbonate formation is modeled as elasticplastic, while the over- and underburden are assumed linear-elastic. The material parameters are estimated from well logs or laboratory core measurements, and the field model is calibrated to the seafloor subsidence data.

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