： Methane Hydrate (MH) is well known as one of the unconventional resources, which is con rmed to exist abundantly offshore in Japan. A variety of studies have suggested that the depressurization is the most effective method for developing MH from the commercial and technical points of view. In the development of MH by the depressurization method, the dissociation and production of MH must greatly influence the geo-mechanical behaviors such as compaction/deformation of formations and vice versa. Hence, to rigorously predict the MH reservoir performances, it is essential to take account of the ow and geo-mechanical behaviors simultaneously.First, we developed a program for coupling the MH flow simulator (MH21-HYDRES) and the geo-mechanics simulator (COTHMAs) , which were developed under The Research Consortium for MH Resources in Japan (MH21 Research Consortium) , enabling simple explicit coupling (ECM) and iterative coupling (ICM) . As a result of the simulations using this program, it was confirmed that ICM could precisely predict MH reser voir performances in conjunction with geo-mechanical behavior, but the computation became far slower than uncoupled simulations by MH21-HYDRES.Second, we incorporated two functions into the program to improve the practicality, that is hybrid coupling (HCM) and dual-grid system (DGS) . In the simulations using the HCM, the ICM is applied only to the grid blocks (or elements) with noticeable strain/deformation, while the ECM is applied to those with negligible strain. In the DGS, the sizes of ow simulation grid blocks and geo-mechanics simulation elements can be defined independently. The simulation studies using the program revealed that the HCM could remarkably shorten the computational time without compromising calculation accuracy. It was also con rmed that the combination of the HCM and the DGS could dramatically reduce the computational time, although the calculation results were slightly different from those with smaller elements

Carbon dioxide capture, utilization and storage (CCUS) has been recognized as a key technology to reduce CO2 emission. Among various CCUS technologies, CO2 enhanced oil recovery (EOR) has been widely implemented at an industrial scale in the E&P sector. However, it is well-known that CO2-mixed oil would cause asphaltene precipitation resulting in flow assurance troubles. Therefore, more advanced asphaltene-risk-managing technology can be an enabler to improve robustness of CCUS projects. This paper presents a case study for a comprehensive series of asphaltene flow assurance pre-risk evaluation in Arabian Gulf Carbonate Oil Field at where the CO2 EOR is recognized as one of the highest potential technologies for full-field implementation. At first, sampling location was carefully selected considering the target reservoir's feature because the reliability of asphaltene study highly depends on sample representativeness. After the QA/QC of collected sample, asphaltene onset pressures (AOP) were measured at multiple temperatures under the CO2 mixing conditions in a straightforward experimental-design optimizing manner so that not only the evaluation accuracy could be improved but also the experimental cost could be minimized. The AOP measurements showed clear potential risks associated with CO2 injection. Subsequently, the numerical model analysis was conducted with Cubic-Plus-Association (CPA) EoS model to identify the risk area during CO2 injection. The analysis suggested that a risk would be caused at not only near-wellbore region at the sampling location but also tubing section / surface facility, furthermore, more seriously at the deeper location of target reservoir. Finally, CO2-induced asphaltene formation damage risk was investigated from the viewpoints of precipitated asphaltene particle size and pore throat size in the porous media. As a result, the clogging risks by CO2-induced asphaltene were estimated high in the target reservoir. By virtue of the above comprehensive series of pre-risk evaluation, the asphaltene flow assurance risk associated with CO2 injection was identified field-widely. The evaluation findings suggested moving on to future actions such as more detailed formation damage risk evaluation and mitigation plan development. The phased approach for evaluating asphaltene flow assurance risk and the reverse engineering of sampling operational design from the experimental design made a worthy demonstration to reduce unnecessary cost and time while obtaining the key information to drive the project. The procedure in this work can contribute to establish a subsurface part of guideline for CCUS from viewpoints of asphaltene flow assurance risk evaluation.

To enhance oil production (EOR) in tight carbonate oil reservoirs, gas EOR can be a promising option from injectivity viewpoints compared with water-basis chemical EOR. CO2 is the most attractive injectant with higher recovery factor expectation while responding to the recent decarbonization demands; however, CO2 is also known to accelerate asphaltene flow assurance risks. In actual fact, the previous work revealed a high asphaltene flow assurance potential risk for CO2 injection. Therefore, a further pre-risk evaluation case study was conducted to extract adequate type of potential injection gas among CO2, enriched gas, and lean gas. This study focused on subsurface gas-induced asphaltene risks in an offshore heterogeneous oil field showing unstable asphaltene colloidal instability index (CII=2.2), where a crestal lean gas injection has been applied without any asphaltene issues. A single phase bottomhole sample was taken from appropriate candidate well keeping representative reservoir fluid in a clear asphaltene-gradient field: i.e., lesser asphaltenes in shallow depth but more in deeper section. As a result of this study, no asphaltene onset was detected from original reservoir fluid while asphaltene onset pressures (AOPs) were detected from mixture of reservoir fluid and CO2 or enriched gas or lean gas at two temperatures representing reservoir and wellhead conditions. Experimental gas mixing ratios were carefully designed to distribute the AOPs broadly in operating pressure range: from in-situ reservoir and near wellbore to bottomhole, for securing higher numerical model accuracy by avoiding data extrapolation. A numerical model, calibrated with experimental outputs, predicted risk magnitude by reservoir depth. A comparative analysis revealed higher asphaltene precipitation risks in CO2 injectant than enriched gas and lean gas. Finally, asphaltene particle/aggregate size were discussed for formation damage risks. The visually measured asphaltene solid particle size varied from 1-10 μm for CO2 injection and 1-4 μm for enriched gas injection while no visible for lean gas. From the past injection water quality analysis, the reservoir has threshold particle size between 0.5-1.0 μm to cause plugging. Therefore, it was concluded that risk of asphaltene-induced formation damage is lower in enriched gas injection compared with CO2 injection. In general, higher oil recovery is expected by order of CO2, enriched, and lean gases. In conclusion, even CO2 EOR is being attractive rapidly from decarbonization viewpoints, the study highlights an importance of balancing business case opportunity and risk from the aspect of injection gas-induced asphaltene flow assurance potential risks.