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).

The M4 Field is located north of Central Luconia Province in the Sarawak Basin, East Malaysia. The reservoir is approximately 2000 m below sea-level where the water depth is approximately 120m. An integrated geomechanical study for CO2 geological storage has been conducted to evaluate the feasibility of injecting and storing CO2 in the M4 depleted carbonate gas reservoir. The storage feasibility of M4 reservoir is impacted by interaction of the reservoir rock with carbonic acid formed by dissolution of injected CO2 in the water which has risen close to the cap-rock. The geomechanical study needs to assess the risk of CO2 leakage from the reservoir due to degradation of the integrity of the cap-rock by the injection operations, and interaction of the injected CO2 and carbonic acid with the cap and reservoir rocks. A scope of work incorporating data review and integration, downhole log and image interpretation, 1-D in-situ stress and pore pressure analyses, rock property determination and 3-D coupled reservoir geomechanical modeling was conducted. In addition, laboratory rock mechanics tests and petrophysical measurements were conducted on core samples before and after injection of CO2 saturated brine solution, and the results were used to develop material strength, elastic and petrophysical property degradation models due to carbonic acid-carbonate interaction. A coupled geomechanical modeling was subsequently performed, which incorporates reservoir pressure and CO2 saturation from dynamic simulation, and subsequent changes in effective stress and the associated changes in porosity and permeability are calculated by a geomechanical modeler which were then passed back to the dynamic reservoir simulation. In addition, modifications were also made to geomechanical and petrophysical rock properties based on the carbonic acid-carbonate interaction degradation models. The paper describes the staged works from 1-D Mechanical Earth Model construction to comprehensive laboratory rock mechanics testing, 3-D geomechanical model construction, pre-production stress modeling and various injection scenario predictions. Examples of key results and utilization of the results and findings from the geomechanical study to develop recommendations for optimizing the CO2 injection and storage in the M4 Field in order to achieve optimal geological storage management and direct cost savings will be presented and discussed. Introduction Along with capacity and injectivity of CO2, containment is a primary function in geological storage performance. Controlling the trapping of CO2 in the subsurface, i.e. storage containment, is of fundamental importance for safe geological storage of CO2. Rock formations can be impervious enough to act as flow barriers to CO2 over geological periods of time. Delineating such a seal, safeguarding its integrity under operational conditions, and verifying its isolation effectiveness are key objectives in achieving a successful CO2 storage project.

Initiatives on improved hydrocarbon gas recovery are usually facing challenges which are directly related to representative understanding of reservoir and fluid flow characterization as well as the depletion strategies. A field case study has been used as a platform to explain such challenges and the adopted integrated methodologies toward improving the hydrocarbon recovery from the field. Reliable water saturation modeling, validation of the production allocation, reevaluation of the trapped/residual gas saturation (S gr ), modeling of regional aquifer behavior and its support, connectivity and pressure communication among the fields, fluid contact movement, gas expantion, smeared oil rim and condensate drop-out within the reservoir and appropriate production scenarios (i.e., off take rate, strategy, etc.) are among the challenges. This paper presents on methodologies how to manage the challenges in improving the hydrocarbon gas recovery (i.e., IGR) through an integrated subsurface-surface study for the evaluation of giving a gas reservoir second life i.e. field rejuvenation. The adopted methodology integrates both simple analytical approaches including the the material balance and volumetric assessment as well as the 3D simulation modeling to evaluate the reservoir performance and examine some of these uncertainties and challenges and subsequently identify the value of the field second life through the optimum prediction and production strategies. As the studied field is in pressure communication with neighboring fields in a mega platform carbonate build-up through a stronge regional aquifer, the results from an isolated model for the field showed to be unrealistic and, therefore, an integrated mega platform model is constructed for the purpose of the study.The study integrates the 4D seismic results, regional aquifer and trapped gas saturation modeling, reservoir performance monitoring, and its application to obtain a better history matching results and reliable prediction strategy for recovery factor improvement. By incorporating the 4D seismic results and representative water saturation and residual gas saturation modelling, the resulting history matched model is significantly improved with less degree of uncertainty and it is believed to be more reliable to produce optimum prediction scenarios, which focused on recovery factor enhancement. Various prediction scenarios together with advanced modeling of the regional aquifer effect in a merged mega platform model have been examined using the final history matched model. By doing this, the field recovery factor is improved by 16% and thus giving the field the second live to continue its production.

The scope of the geomechanical study is to investigate the risk associated with different reservoir depletion strategies and to numerically simulate the geomechanical response of the reservoir rocks. The study focused on the large karstic distribution of the reservoir for the prediction of the best drilling direction and optimum well trajectories, and also to model the pore collapse behavior observed in the high porosity carbonate which will result in surface subsidence and impact the platform facilities placement. A methodological risk evaluation approach based on numerical simulations with stringent experimental programme has been applied to the field study. The regional geological understanding and operational experience of the nearby fields have been considered for the study via extensive assessment of constitutive models relating to pore collapse. Advanced 4D geomechanical simulations were carried out to incorporate the seismic-based karstic models and to strengthen understanding of the pore collapse phenomena during reservoir depletion. The obtained prediction results were compared to nearby fields and subsequently use for wells, facilities planning and engineering considerations. The results obtained in the study identified a few key outcomes which are being considered for detailed surface engineering design and well planning. The results have impacted the decision to place the location of the platform away from the center of the seabed subsidence bowl. The predicted reservoir compaction and subsidence described the rate and the magnitude of the subsidence which are use to design the height of the platform to mitigate potential damage induced by wave deck shearing. In addition, the distribution of karst has been mapped through seismic imaging and incorporated in the geomechanical modelling. The results are also used to determine the hazard of the weak zones in each formation and high stress anisotropy regions which are to be avoided for future well placement and to be used for well trajectory optimization. Key findings of the geomechanical-related risk have been identified and considered in the field development plan. Consequently, a Risk Ranking Criteria incorporating the results of advanced simulations and rock testing programme have been developed based on comprehensive weightage and the technical categories. The paper offers a detailed insight on the geomechanical risk evaluation obtained using 4D finite element coupled reservoir geomechanical simulations. The study addressed the challenging development of a highly karstified limestone reservoir by offering valuable inputs for the well design and facility engineering through prediction of reservoir compaction and seabed subsidence, best drilling direction and optimum well trajectories. This will avoid potential geomechanical related hazards and ensure adequate operational safety levels.

This paper presented an integrated CO2 injection and sequestration modelling study performed on a depleted carbonate gas reservoir, which has been identified as one of potential CO2 sequestration site candidate in conjunction with nearby high CO2 gas fields development and commercialization effort to monetize the fields. 3D compositional modelling, geomechanical and geochemical assessment were conducted to strategize optimum subsurface CO2 injection and sequestration development concept for project execution. Available history matched black oil simulation model was converted into compositional model. Sensitivity analyses on optimum injection rate, number and types of injectors, solubility of CO2 in water, injection locations and impact of hysteresis to plume distribution were investigated. Different types of CO2 trapping mechanisms including hydrodynamic, residual/capillary, solubility and mineral trapping were studied in detailed. Coupled modelling study was performed on base case scenario to assess geomechnical and geochemical risks associated with CO2 injection and sequestration process before-, during- and post- CO2 injection operation to provide assurance for a safe and long-term CO2 sequestration in the field. Available history matched black oil model was successfully converted into compositional model, in which CO2 is treated and can be tracked as a separate component in the reservoir throughout the production and injection processes. Integrating all the results obtained from sensitivities analyses, the proposed optimum subsurface CO2 injection and sequestration development concept for the field is to inject up to 400 MMscf/D of CO2 rate via four injectors. CO2 injection rate is forecasted to sustain more than 3 years from injection start date before declining with time. In terms of CO2 storage capacity, constraining injection pressure up to initial reservoir pressure, maximum CO2 storage capacity is estimated ~65 Million tonnes. Nevertheless, considering maximum allowable CO2 injection pressure estimated from coupled modelling study and operational safety factor, the field is capable to accommodate a total of ~77 Million tonnes of CO2, whereby 73% of total CO2 injected will exists in mobile phase and trapped underneath caprock whilst the other 24% and 3% will be trapped as residual/capillary and dissolved in water respectively. Changes of minerals and porosity were observed from 3D geochemical modelling, however, changes are negligible due to the fact that geochemical reaction is a very slow process. This paper highlights and shares simulation results obtained from CO2 injection and sequestration studies performed on 3D compositional model to generate an optimum subsurface CO2 injection and sequestration development concept for project execution in future. Integration with geomechanical and geochemical modelling studies are crucial to assess site's capability to accommodate CO2 within the geological formation and provide assurance for a safe and long-term CO2 sequestration.