3D Numerical Dynamic Geomechanical Analyses of Compaction, Subsidence and Fault Reactivation Throughout Multiple Producing and Injecting Reservoirs of a Sarawak Offshore Oil and Gas Field, Malaysia
Authors: Fang Zhi , Younessi Ahmadreza , Hon Vai Yee , Refli Ikhwan , M. Piah Mohamad Faizzudin 
Published: May 2016

Abstract
A thorough 3D finite-element (FE) geomechanical study was over undertaken for an oil and gas field offshore Sarawak, Malaysia, to assist the development management. The field comprises eight major reservoirs with three of them planned for water injection to improve recovery. The historical well drilling data, wireline logs, production data, field tests and newly obtained laboratory test data provided inputs for deriving a wellbore-based 1D geomechanical model for the field. The 1D model was calibrated to well drilling observations before being up-scaled and populated in a 3D geomechanical model for the entire field. The model was implemented in an FE simulator to dynamically analyze reservoir compactions, platform site subsidence, fault reactivation potential and cap rock breaches that might be induced by reservoir depletion and injection in the full life cycle planning of the field management.
The results showed a maximum subsidence of 0.72 m above the shallowest depleting reservoir and a subsidence of approximately 0.62 m at the seabed in year 2032. Injection slightly reverses the subsidence induced by depletion and the subsidence at the centre of the seabed is approximately 0.58 m in year 2050. The corresponding subsidence at the platform sites reaches 0.58 m in year 2032 and reduces to 0.54 m in year 2050. Depletion tends to reduce fault reactivation potential within the depleting and close underlying intervals, but enhances fault slip likelihood near the upper bounding zones. Faults segments bounding the deepest producing reservoir, which contains the most depletion, is predicted to have the highest potential for fault shearing in the late stages of production. Injection reverses the trend of fault reactivation potential. There are no caprock integrity issues with the three main injection reservoirs for the entire field life.

Introduction
The study field is in offshore Sarawak, Malaysia and truncated by two sets of major faults with one set (Alpha) in the south and the other (Beta and Gamma) in the north of the field. There are dozens of hydrocarbon-bearing zones separately deposited in the upper to lower coastal plain environments of interbedded sandstones, shales and siltstones. The shallower zones comprise gas reservoirs; intermediate reservoirs exhibit small oil rims; and deep reservoirs show gas condensate. Eight reservoirs, labelled as R1, R2, … and R8, have been either in production, or in water injection, or within future production consideration. Fig. 1 shows the plan and cross-section views of the field structure.
After the field discovery in 1967, production commenced in 1972 from three major reservoirs: R4, R5 and R6, with R6 started water injection since 1994 follow by R4 and R5- in 2017. In addition, the other 5 reservoirs, including a deeper overpressured pure gas reservoir, R8, will begin production in 2017. Thereafter, depletion and recharge of pressure is expected from multiple reservoirs undergoing either production or injection.
Pressure depletion from production or pressure recharge because of injection induces stress changes in the depleting/injecting reservoirs and their bounding formations. Consequently, the stress changes in multiple reservoirs and bounding zones can induce various geomechanical problems, e.g., wellbore drilling instability, sand production, reservoir compaction, overburden subsidence, fault reactivation and caprock fracturing. Those geomechanical issues can impact well planning, platform facilitating and production management for future field development.