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Abstract
M4 carbonate field, a depleted gas field located offshore Sarawak, has been identified as potential candidate for CO2 sequestration site in conjunction with another high CO2 field development and commercialization efforts. The field has undergone a feasibility study to evaluate potential geomechanical issues associated with CO2 injection. A detail 3D simulation analysis was conducted to quantify the effective storage capacity in M4 field, identify the optimum CO2 injection scheme and evaluate the trapping mechanism in M4 field. Reservoir geomechanical study was also performed for M4 field to evaluate the associated geomechanical issues pre, during and post CO2 injection to assure a safe and long term CO2 sequestration in the field.

First, the available field history matched black oil simulation model was successfully converted to compositional 3D model, in which CO2 is treated and can be tracked as a separate component in the reservoir throughout the production and injection processes. A detail study has then been conducted to understand the containment and analyze the effective CO2 trapping mechanisms. Different types of trapping mechanisms including the hydrodynamic trapping, residual or capillary trapping, solubility trapping, and mineral trapping have been studied in detail. Hysteresis effect on CO2 sequestration and different trapping mechanism during and post CO2 injection has been also studied. In addition, various CO2 injection schemes have been also conducted to optimize the injection rate, sustainability, capacity, location, number of the wells and favorable trapping mechanism for long term sequestration. The study covered 20 years of gas production history and forecast followed by 10 years of CO2 injection in the selected optimum scheme and then monitoring part more than for 100 years after injection to assure the safe sequestration and potential CO2 leakage. Constraining to the initial reservoir pressure to assure cap rock integrity and potential leakages, the study showed that the field has potential to store and sequestrate CO2 up to 40% bigger standard volume than gas initially in place (GIIP).

Introduction
A feasibility study on CO2 injection and sequestration in depleted gas field was conducted in conjunction with Alpha field development in Malaysia. The Alpha field, a carbonate gas field, is located approximately 250 km offshore, Sarawak, East Malaysia. Based on reservoir fluid analysis, the Alpha field contained approximately 70% CO2 content. Vast CO2 production is expected from the field when it comes into production and as part of CO2 mitigation plan, the produced CO2 will be transported, injected and sequestrated into depleted nearby gas fields. M4 field, a depleting gas field, has been identified as one of the potential candidates for CO2 injection and sequestration site to support Alpha field gas monetization.

The M4 field, a carbonate gas condensate reservoir, is part of mega platform carbonate build-up formation that consists of seven fields and is located in Central Luconia province, approximately 250 km offshore Sarawak, East Malaysia [1 - 2], and about 170 km to the North North East (NNE) of the Alpha field (Fig. 1). The field was discovered in 1980 by A-1X exploration well and was later appraised by A-2 well drilled in 1992 with estimated initial reservoir pressure of 3860 psi at Original Gas Oil Contact (OGOC). The field was developed with two horizontal sub-sea wells namely AA-1 and AA-2, positioned at the crest of the reservoir structure, approximately 24 ft below the top of carbonate. Production from the field is tied back to M3 production facility, located approximately 10 km to the South, and first gas production came on stream in 2002.