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A carbonate-depleted carbonate gas field located offshore Sarawak has been identified as potential candidate for CO2 sequestration site in conjunction with another high CO2 gas field development for commercialization efforts. The field has undergone a feasibility study to evaluate potential geochemical reactions specifically on trapping mechanisms and storage capacity associated with CO2 injection. A detail 3D reactive transport modelling study was conducted to quantify on the four different trapping mechanism: structural, residual, solubility, and mineral trapping during and post-injection. The model was developed by first converted the available field history matched black oil simulation model into compositional 3D model, in which CO2 is treated as separate component in the reservoir through the production and injection processes. The study covered 22 years of gas production history forecast followed by 27 years of injection and 1000 years of post-CO2 injection in the gas column reservoir. The results show that the field has potential to store and sequestrate CO2 up to for 79% structural trapping, 19% residual trapping, 3% solubility trapping and no mineral trapping after 1000 years of post-injection period.

Abstract Modified Cam-Clay model is often used for simulation of soft geomaterials such as soil or sand. In this model, Young’s modulus continuously changes via the bulk modulus. Poisson’s ratio can change or be kept constant. If the Poisson’s ratio is kept constant, shear modulus will change with time. If the shear modulus is kept constant, Poisson’s ratio is allowed to change with time. In general, the effective stress tensor at time t+1 can be computed when all state variables at time t are known. The material can be either in elastic or plastic mode depending on the stress state. In the case of plasticity, there are many steps involved to obtain a correct stress so that the equilibrium force balance could be satisfied within a tolerance. In the case of plastic deformation when the stress is out of a yield surface at time t+1, the stress must be brought back to the yield surface. The stress computation procedure during loading and unloading has an important effect on the results of deformation and rebounding after a plastic deformation. In this study a new approach for stress computation during loading and plastic unloading path is introduced. In the new proposed method, the relationship between mean effective stress change and volumetric strain change is used as a parameter to determine the amount of plastic deformation during unloading. The new approach will be appropriate for any case with large residual strain and a proposed exponential parameter (n) will determine based on the value of residual strain. A sensitivity analysis was carried out for different values of the exponential parameter which showed the displacement to be gradually less sensitive when the value of the parameter “n” is 1.003 or higher during the unloading stage. This method was then implemented and successfully tested for a study on matured offshore Malaysian field.

There are many gas fields associated with large amount of CO2 concentrations (>50 mol%) in offshore Malaysia. Development of the contaminated gas fields is only possible if a geologically safe storage site is identified for the storage of produced CO2. The critical component of CCS project field development plan is monitoring of the storage site for the long-term containment and conformance security. 4D time- lapse seismic is key in monitoring, measurement, and verification (MMV) plan as it can demonstrate the periodic movement of stored CO2 within storage reservoir. Time-lapse seismic response depends on several parameters ranging from reservoir rock properties, initial fluid and saturation of CO2 during injection period. This study focuses on the different characteristics on 4D seismic attributes for carbonate and clastic reservoirs and discusses to effectively utilize for MMV. The time-lapse 4D seismic has been traditionally used to identify the potential threats associated with reservoirs with enhanced oil recovery plan. The application of 4D seismic would benefits CCS project for monitoring CO2 plume migration along existing or induced fractures. The study was conducted on identified CO2 storage sites in both depleted carbonate gas reservoir and clastic saline aquifer. It highlights two methods applied in the 4D AVO analysis, that quantifies the changes in CO2 saturation within reservoir during injection. The Aki and Richards' approximation were analyzed for the effect of changes in gas saturations on the seismic amplitudes and the Smith and Gidlow's approximation for the reflection coefficient relationship between the AVO gradient/intercept attributes and reservoir fluid/properties. The application of 4D seismic would benefits CCS project for monitoring CO2 plume migration along existing or induced fractures. The study was conducted on identified CO2 storage sites in both depleted carbonate gas reservoir and clastic saline aquifer. It highlights two methods applied in the 4D AVO analysis, that quantifies the changes in CO2 saturation within reservoir during injection. The Aki and Richards' approximation were analyzed for the effect of changes in gas saturations on the seismic amplitudes and the Smith and Gidlow's approximation for the reflection coefficient relationship between the AVO gradient/intercept attributes and reservoir fluid/properties. This study focuses on the different characteristics on 4D seismic attributes for carbonate and clastic reservoirs and discusses to effectively utilize for MMV.

Carbon Capture and Sequestration (CCS) has potential to provide an amicable solution in reducing CO2 emissions to the atmosphere for perpetuity with zero-degree failure. Geological storage sites are conventionally considered to be safe for CO2 sequestration and are thoroughly simulated for any potential leakage due to loss of structural integrity. The offshore CO2 storage site, if leaks, could potentially affect the marine environment eventually escaping into atmosphere. A prior modelling of potential leakage from identified threats and its impact on marine environment will ensure the safety and substantially aid site- specific and adaptive Monitoring, Measurement & Verification (MMV) planning. Subsurface integrity study of the storage site integrated with coupled modelling contemplates longterm security of CO2 storage. For a leak to occur through the plugged &abandonment (P&A) wells, one or more barriers must fail. P&A wells integrity feasibility reveals the impact of CO2 interaction with non- CRA composite structure identifying possible leak paths. The probability of a leak from subsurface to the environment through each of the P&A well, depending on its attribution, was estimated. Barrier's failure probabilities in P&A wells are understood under various well leak estimation scenarios. Range of potential leakage rate, in the event of loss of well integrity, was evaluated. To demonstrate the impact of potential CO2 leakage in marine environment through the loss of subsurface or well integrity from identified locations, integrated CO2 dispersion simulation was carried out for multiple potential leakage scenarios and changes in marine water pH were analyzed. Based on barriers reliability function, the well leakage modelling results suggest that minimum rate of CO2 leakage through the well path that can possibly reach to the seabed is 6-10 tCO2e/year and for extreme case it would be 500 tCO2e/year. The CO2 dispersion modelling were performed for these leakage rates at three different well locations. Three scenarios were considered and simulated for escaped CO2 gas in the water column and its impact on marine environment. The observed change in marine water pH values in near and far-field region were negligible or undetectable. Any reduction in the pH values was predicted to be within the natural variation of the seawater acidity at the storage site with the varying climatic conditions. The CO2 gas bubbles were predicted to be fully dissolved in the water column, no CO2 gas bubbles would reach the surface and escape into the atmosphere for the modelled leakage scenarios in water depths of 140m. In this paper, the integration of well leakage rate modelling with CO2 dispersionsimulation results are discussed to ameliorate the MMV planning for legacy wells to ensure that injected CO2 in the reservoir is intact and safely stored for hundreds of years post injection.

There are many gas fields associated with large amount of CO2 concentrations (>50 mol%) in offshore Malaysia. Development of the contaminated gas fields is only possible if a geologically safe storage site is identified for the storage of produced CO2. The critical component of CCS project field development plan is monitoring of the storage site for the long-term containment and conformance security. 4D time-lapse seismic is key in monitoring, measurement, and verification (MMV) plan. The time-lapse 4D seismic has been traditionally used to identify the potential threats associated with reservoirs with enhanced oil recovery plan. The application of 4D seismic would benefits CCS project for monitoring CO2 plume migration along existing or induced fractures. The study was conducted on identified CO2 storage sites in both depleted carbonate gas reservoir and clastic saline aquifer. It highlights two methods applied in the 4D AVO analysis, that quantifies the changes in CO2 saturation within reservoir during injection. The Aki and Richards’ approximation were analyzed for the effect of changes in gas saturations on the seismic amplitudes and the Smith and Gidlow's approximation for the reflection coefficient relationship between the AVO gradient/intercept attributes and reservoir fluid/properties. The application of 4D seismic would benefits CCS project for monitoring CO2 plume migration along existing or induced fractures. The study was conducted on identified CO2 storage sites in both depleted carbonate gas reservoir and clastic saline aquifer. It highlights two methods applied in the 4D AVO analysis, that quantifies the changes in CO2 saturation within reservoir during injection. The Aki and Richards’ approximation were analyzed for the effect of changes in gas saturations on the seismic amplitudes and the Smith and Gidlow's approximation for the reflection coefficient relationship between the AVO gradient/intercept attributes and reservoir fluid/properties. This study focuses on the different characteristics on 4D seismic attributes for carbonate and clastic reservoirs and discusses to effectively utilize for MMV.

Carbon Capture & Storage (CCS) has globally been recognized as the way forward to manage & reduce Green House Gas (GHG) emissions. However, for every geological CO2 storage project there are two inherent yet independent storage risks: loss of containment and loss of conformance. For early detection and mitigation of these risks, a robust Monitoring, Measurement and Verification (MMV) plan is of supreme importance. While several matured, newly developed and under trial monitoring technologies addressing various aspects of MMV are currently available but it's crucial to evaluate them in terms of applicability, durability, ease of operation & maintenance, and cost to the project. This study focuses on the screening and selection monitoring technologies to achieve a site-specific, adaptive, and cost-effective MMV plan and design. Identification of critical containment & conformance risks were achieved through integrating information from various elements viz. CO2 plume migration forward modeling, 3-way coupled modeling, well integrity analysis, overburden risk analysis, CO2 marine dispersion modeling, environmental impact assessment (EIA), regulatory requirements, etc. Based on identified risks, the specific MMV elements were selected along with designing of their scheduling and frequency. Subsequently, an inventory of all available technologies addressing explicit MMV needs was prepared. These technologies were put through rigorous analysis on their capability of detection, operability, tool/technology longevity, availability, and associated costs. High-ranking technologies were then knitted for the final MMV planning & design. As a part of the arduous screening, permanent ocean bottom (OB) monitoring technologies were analyzed and some of the alternate low-cost technologies surfaced that can cater to continuous and long- term data acquisition needs. Knitting of 3D Distributed Acoustic Sensor – Vertical Seismic Profile (DAS- VSP) hybrid with 3D seismic surveys would help optimizing the logistic requirements and reducing cost of monitoring survey acquisition over the project lifecycle. During the entire screening and selection process, Multi-Fiber Optic Sensor System (M-FOSS) technology emerged as the leading monitoring technology, which is cost-effective, easy to operate and has the flexibility to be deployed at well and/or on seabed. An exhaustive inventory of available/emerging MMV technologies and the cost versus benefit matrix was developed that can be used for MMV planning and design across CCS projects. The study focuses to streamline the long-term sustainability of the adapted MMV planning to enhance the project economics by ensuring the optimum value propositions of implemented technologies.

Storing CO2 in geological formations is gaining greater importance as various companies start transitioning towards a carbon neutral future. CO2 storage in depleted hydrocarbon reservoirs and saline aquifers is considered an effective and secure option to reduce atmospheric CO2. Once underground, four different mechanisms keep the supercritical CO2 securely stored. The mechanisms, in increasing order of storage security are, 1. Structural/stratigraphic trapping, 2. Residual trapping, 3. Solubility trapping, and 4. Mineral trapping. Optimization of injector design to increase the amount of CO2 trapped in one of the more secure mechanisms is desirable. Structural trapping is the most dominant and least secure trapping mechanism for CO2 storage. Any opportunity to move structurally trapped CO2 into one of the other trapping mechanisms is preferable from the standpoint of storage security. Mechanistic models are used to study ways to improve amount of CO2 trapped by certain mechanisms. Residual trapping is affected by several factors including path traveled from perforation to the top of the structure. Similarly, solubility trapping is influenced by several factors including the amount of contact CO2 has with water. Injected CO2, due to buoyancy, rapidly rises to the top of the structure. There is potential to increase residual trapping and solubility trapping by optimizing the injector design to increase volume of reservoir contacted by CO2. Mechanistic modeling study shows that residual trapped and solubility trapped CO2 volume can be increased by optimizing injector design. There is up to 50% improvement observed in both trapping mechanisms depending on the reservoir characteristics and injector design. Interestingly, lower permeability reservoirs are more sensitive to injector design compared to higher permeability reservoirs. Of the injector designs studied, horizontal injectors placed at the bottom of the structure show the most improvement in both residual and solubility trapping mechanisms. Pressure of the reservoir also influences trapping mechanisms. At higher reservoir pressures, density difference between CO2 and water is smaller. This affects how CO2 plume migrates in the reservoir.

Carbon dioxide (CO2) capture and geological storage is one of the best alternative methods of produced CO2 disposal rather than venting it into the atmosphere. However, there are numerous geomechanical challenges and risks associated with CO2 injection and geological storage that are required to be addressed. The key challenges include delineating the geological seal barrier and maintaining the seal integrity throughout the injection operation and storage life. The assessment of geomechanical-related challenges and risks to manage CO2 containment and mitigate leakage risks require a coupled geomechanical study that needs to be conducted as part of feasibility evaluation of injecting and storing CO2 in a field. In the geomechanics-dynamic coupled modelling, at selected stress steps the reservoir model passed data including pressure, CO2 concentration within the plume, water saturation and temperature to the geomechanical model. Changes in stresses, rock mechanical and rock petrophysical properties based on laboratory CO2-rock interaction test data, displacements, strains and deformations were computed by the geomechanical model using the data from the reservoir model. Updated petrophysical properties such as permeability and porosity were passed by geomechanical model back to the reservoir model and used in the next dynamic simulation step. The coupled geomechanical modelling assessed CO2 leakage risk associated with fault re-activation, failure of cap rock, CO2-rock interaction, and cooling of injected CO2 on caprock and reservoir rock. Axial loading on completions due to reservoir compaction could cause buckling collapse and/or cement failure. Fault re-activation was based on induced fault plastic shear strain throughout the injection programme which may pose risk of CO2 leakage. The evaluation of induced plastic shear strain and failure state of caprock, effect of CO2 interaction and cooling on the caprock and reservoir rock integrity, and compaction and associated subsidence were also evaluated. Recommendations from geomechanical perspective can subsequently be developed from the results and findings for the CO2 injection programme and storage operation.

Depleted hydrocarbon reservoirs are considered inherently safe for carbon sequestration, but high well density penetrating the CO2 storage reservoir could compromise the containment performance in a carbon, capture & sequestration (CCS) project. Based on the available well data, it is crucial to understand the age of the well, materials used for wellbore construction, cement quality, barriers performance, and well integrity. A risk management methodology can be incorporated to evaluate primary and secondary barriers in existing plugged and abandoned (P&A) and development wells to ensure long-term fate of CO2 sequestration project. Existing P&A wells and development wells in a depleted field were drilled 3–5 decades ago. The wellbore construction utilized non-corrosive resistant materials. Health of all wells that ever penetrated the CO2 storage reservoir need to be analyzed from long term perspective of storing CO2. Throughout the lifespan of wells, subsurface barriers should maintain hydraulic isolation to prevent leakage happening from subsurface to environment of reservoir fluids and injected CO2. Deterioration of strength of wellbore construction material due to corrosion, induced by downhole pressure and temperature conditions, should be considered. This study investigated 3 exploration and 21 development wells. Risk register was developed for each well describing causes and CO2 leakage risks, impacts and consequences. Metrics were defined for parameters such as well age, well head materials, wellhead functional test and leak test, sustained casing pressures for risk determination. Wells were risk rated individually based on the assessment. Wells with low risk can be utilized for well conversion. While for high-risk wells, an opportunity risk matrix was developed to mitigate risks in all the wells. This study evaluates the well integrity and CO2 leakage risk along the wells that penetrated the CO2 storage reservoir. The improved rigorous risk assessment exercise evaluates well barrier failure causes and impacts along with estimating the risk number per well. The well risk assessment score calculated was between 9.24 and 13.35 for 21 development wells. Out of these 21 wells, 4 wells with risk score

The Malay Basin is located at the southern part of the Gulf of Thailand between Vietnam and Peninsular Malaysia. The produced gases in the basin contain varying amounts of CO2. High CO2 concentrations (>50 mol%) are, in typical of reservoirs, mainly derived from inorganic sources. In order to develop these fields with reduced or zero greenhouse gas emissions for sustainability development, some geological storage sites were screened to store the future CO2 production. One of the important geological storage sites is the saline aquifer. There are many elements to assess the quality of the CO2 storage sites, such as caprock integrity and storage capacity. This study focused on one of the caprock characteristics, i.e., caprock velocity characteristic, and its significance for CO2 storage in a saline aquifer. The studied saline aquifer is one of a numerous grabens or sub-basins at the southern part of Malay Basin. There is one abandoned exploration well with 2D and 3D seismic surveys and there are several types of velocity data, including sonic log, checkshot and seismic stacking velocity. The formations are composited of multiple thick sandstones and shales. Based on velocity analysis, including average velocity and inter-val velocity analysis. the interval velocity in caprock zone is related to the lithology, normally expressed by the sandstone percentage in the caprock zone. The lithology variation will affect the geomechanics characteristics, so, the velocity in caprock zone presents the caprock quality. The caprock is confined within the sub-basin and sandstone percentage is less than 40%, which shows a good seal capacity, com-bined with the caprock thickness distribution. The velocity in shale zone (caprock) is higher than overlying and overlain sandstones. There is strong positive reflection (peak) at the top of shale zone (caprock) and negative reflection (trough) at the bottom of caprock, i.e., top of sandstone aquifer. There would be strong class III AVO feature at the top of sand-stone aquifer on time-lapse 4D seismic reflection once CO2 is injected. These features (negative reflection with bright spots) and class III AVO at the top of aquifer) are useful for MMV to monitor the CO2 plume migration.

CO2 sequestration is a process for eternity with a possibility of zero-degree failure. One of the key components of the CO2 Sequestration Project is to have a site-specific, risk-based and adaptive Monitoring, Measurement and Verification (MMV) plan. The storage site has been studied thoroughly and is understood to be inherently safe for CO2 sequestration. However, it is incumbent on operator to manage and minimize storage risks. MMV planning is critical along with geological site selection, transportation and storage process. Geological evaluation study of the storage site suggests the containment capacity of identified large depleted gas reservoirs as well as long term conformance due to thick interval. The fault-seal analysis and reservoir integrity study contemplate long-term security of the CO2 storage. An integrated 3D reservoir dynamic simulation model coupled with geomechanical and geochemical models were performed. This helps in understanding storage capacity, trapping mechanisms, reservoir integrity, plume migration path, and injectivity. To demonstrate that CO2 plume migration can be mapped from the seismic, a 4D Seismic feasibility study was carried out using well and fluid data. Gassmann fluid substitution was performed in carbonate reservoir at well, and seismic response of several combination of fluid saturation scenarios on synthetic gathers were analyzed. The CO2 dispersion study, which incorporate integration of subsurface, geomatic and metocean & environment data along with leakage character information, was carried out to understand the potential leakage pathway along existing wells and faults which enable to design a monitoring plan accordingly. The monitoring of wells & reservoir integrity, overburden integrity will be carried out by Fiber Optic System to be installed in injection wells. Significant difference in seismic amplitude observed at the reservoir top during 4D seismic feasibility study for varying CO2 saturation suggests that monitoring of CO2 plume migration from seismic is possible. CO2 plume front with as low as 25% saturation can be discriminated provided seismic data has high signal noise ratio (SNR). 3D DAS-VSP acquisition modeling results show that a subsurface coverage of approximately 3 km2 per well is achievable. Laboratory injectivity studies and three-way coupled modelling simulations established that three injection wells will be required to achieve the target injection rate. As planned injection wells are field centric and storage site area is large, DAS-VSP find limited coverage to monitor the CO2 plume front. Hence, surface seismic acquisition will be an integral component of full field monitoring and time-lapsed evaluations for integrated MMV planning to monitor CO2 plume migration. The integrated MMV planning is designed to ensure that injected CO2 in the reservoir is intact and safely stored for hundreds of years after injection. Field specific MMV technologies for CO2 plume migration with proactive approach were identified after exercising pre-defined screening criteria.

Measurement, Monitoring & Verification (MMV) is crucial to ascertain both containment and conformance in Carbon Capture & Storage (CCS) projects. The magnitude of parameters to be monitored along with the technologies to be adopted could be very cost intensive and impact overall project Net Present value (NPV). To rationalize the associated costs and maximize the value propositions of existing infrastructure, the development wells in depleted field provide the opportunity to reduce the MMV cost by converting them into observation wells. However, the wells are to be analyzed for their strategic location in the reservoir, fit for purpose plug & abandonment plan and the apt technologies that can be implemented for both reservoir & overburden monitoring. Development wells in the identified depleted field are 30-40 years old and were not designed considering high CO2 concentration. In consequence, the possibility of well leakage due to accelerated corrosion channeling, cracks, along the wellbore cannot be ignored and requires careful evaluation. Rigorous process has been adopted in assessing the feasibility for converting existing producers into observation wells. Wells basis of designs disparity between the producer and the required observation well governs the selection for conversion to observation wells or plugging and abandonment. The reservoir simulation and coupled modelling predict that CO2 plume will reach all wells penetrating the storage reservoir during the initial injection phase. Out of 9 available producers, 2 strategically located wells have been evaluated for conversion based on end injection reservoir pressure of ∼3450psi. Quantitative CO2 leakage through the observation wells has been numerically computed based on all possible pathways for risk characterization. The permeable/perforated zones in these two wells are to be isolated along with the cap-rock restoration technique at deepest depth of ∼4000ft TVDSS. This will ensure the wells are safe & accessible for monitoring CO2 plume migration, CO2 leakage and well integrity by analyzing acquired DAS-VSP, DTS, DPS data and well logs. This paper elaborates unique challenges associated with identifying strategic wells for conversion to observation wells. Minimum plug setting depths, ranging from 3720-3880ft TVDSS, for abandonment of 9 development wells are derived based on fracture gradient and maximum horizontal stress. 2 observation wells require deeper plug setting depth to make caprock accessible at ∼4000ft TVDSS to be restored by utilizing either perforate-wash-cement (PWC) or section milling. Based on the subsurface illumination modelling, deployment of fiber-optics sensors in observation wells promises a cost-effective solution for monitoring CO2 plume migration and leakage by acquiring 4D DAS-VSP survey. Conversion of producers to observation wells promises cost effective MMV application for CO2 plume migration and leakage monitoring along with periodic temperature, pressure, and CO2 concentration measurement in overburden.

CO2 storage in a depleted field comes with the risk that is associated with wells integrity which is often defined as the ability to contain fluids with minimum to nil leakage throughout the project lifecycle. The targeted CO2 storage reservoir in offshore Malaysia has existing abandoned exploration/appraisal, and development wells. With a view of developing such CO2 storage sites, it is vital to maintain the integrity of the abandoned wells. High-risk characterized wells need to be analyzed and remedial action plan to be defined by understanding the complexity involved in restoring the integrity. This will safeguard CO2 containment for decades. Abandoned exploration/appraisal wells in the identified field are >40 years old and were not designed to withstand CO2 corrosion environment. Downhole temperature and pressure conditions may have further degraded the wellbore material strength elevating corrosion susceptibility. The reservoir simulation predicts that the CO2 plume will reach to these abandoned wells during the initial phase of total injection period. Single well was selected to assess the loss of containment through the composite structure along the wellbore and to determine the complexity in resorting the well integrity. CO2 leakage rates through all possible pathways were estimated based on numerical models and the well is characterized for its risk. For unacceptable leakage risk, the abandoned well needs to be re-entered to restore the performance of barriers. Minimum plug setting depth (MPSD) and caprock restoration considers original reservoir pressure(3450psia) anticipating the pressure buildup upon CO2 injection and is derived based on fracture gradient and maximum horizontal stress. This paper elaborates unique challenges associated with locating abandoned wells that are submerged below seabed. Top and side re-entry strategies are discussed to overcome challenges. Based on past abandonment scheme, leakage rate modeling calculates estimated leakage rate of ~460SCFD at higher differential pressure of around 3036psia at shallowest barrier and ~15SCFD for differential pressure of 1518psia at deepest barrier. Sensitivity analysis has been carried out for critical barrier parameters (cement permeability, cracks, fractures) to the containment ability and improving understanding of quality of barriers, uncertainties, and complexities for CO2 leakage risk. The paper proposes two(2) minimum plug setting depths (3550ft & 3750ft) derived based on fracture gradient and maximum horizontal stress. Perforate-wash-cement (PWC) and section milling were compared for operational efficiencies to achieve caprock restoration. for MPSD out strategic options to restore well integrity by remediating casing/cement barriers at by performing best fit abandonment technique to contain CO2 in the reservoir. Well integrity risk is assessed for existing plugged and abandoned (P&A) wells in a carbon storage site. Optimized remedial actions are proposed. Quantification of all the uncertainties are resolved that may affect long-term security of CO2 storage site.

CO2 sequestration is a process for eternity with a possibility of zero-degree failure. Monitoring, Measurement and Verification (MMV) planning of CO2 sequestration is crucial along with geological site selection, transportation and injection process. Several geological formations have been evaluated in the past for potential storage site which divulges the containment capacity of identified large, depleted gas reservoirs as well as long term conformance.Offshore environment makes MMV plan challenging and demands rigorous integration of monitoring technologies to optimize project economic and involved logistics. The role of MMV is critical for sustainability of the CO2 storage project as it ensures that injected CO2 in the reservoir is intact and safely stored for hundreds of years post-injection. Field specific MMV technologies for CO2 plume migration with proactive approach were identified after exercising pre-defined screening criteria.Marine CO2 dispersion study is carried out to confirm the impact of any potential leakage along existing wells and faults, and to understand the CO2 behavior in marine environment in the event of leakage. Study incorporates integration of G&G subsurface and Meta-Ocean & Environment data along with other leakage character information. Multi-Fiber Optic Sensors System (M-FOSS) to be installed in injector wells for monitoring well & reservoir integrity, overburden integrity and monitoring of early CO2 plume migration by acquiring & analyzing the distributed sensing data (DTS/DPS/DAS/DSS).Based on 3D couple modeling, a maximum injection rate of approximately 200 MMscfd of permeate stream produced from a high CO2 contaminated gas field can be achieved. Injectivity studies indicate that over 100 MMSCFD of CO2 injection rates into depleted gas reservoir is possible from a single injector. Injectivity results are integrated with dynamic simulation to determine number and location of injector wells. 3D DAS-VSP simulation results show that a subsurface coverage of approximately 3 km2 per well is achievable, which along with simulated CO2 plume extent help to determine the number of wells required to get maximum monitoring coverage for the MMV planning. As planned injector wells are field centric and storage site area is large, DAS-VSP find limited coverage to monitor the CO2 plume. To overcome this challenge, requirement of surface seismic acquisition survey is recommended for full field monitoring.An integrated MMV plan is designed for cost-effective long-term offshore monitoring of CO2 plume migration. The present study discusses the impacting parameters which make the whole process environmentally sustainable, economically viable and adhering to national and international regulations.

Ensuring long-term integrity of existing plugged and abandoned (P&A) and active wells that penetrated the selected CO2 storage reservoir is the key to reduce leakage risks along the wellpath for long-term containment sustainability. Restoring the well integrity, when required, will safeguard CO2 containment for decades. Well integrity is often defined as the ability to contain fluids with minimum to nil leakage throughout the project lifecycle. With a view to develop depleted gas fields as CO2 storage sites in offshore Sarawak, it is vital to determine the complexity involved in restoring the integrity of these P&A wells as well as the development wells. Leakage Rate Modeling (LRM) was performed to identify and evaluate the associated risks for designing the remedial action plan to safeguard CO2 storage site. The P&A wells in the identified depleted gas fields were drilled 35–45 years ago and were not designed to withstand high CO2 concentration downhole conditions. Corrosive-Resistant Alloy (CRA) tubulars and CO2 resistant cement were not used during well construction and downhole pressure and temperature conditions may have further degraded the material strength and elevated the corrosion susceptibility. As a proof of concept, single well was selected to assess the loss of containment along the wellbore and to determine the complexity in resorting the well integrity, multiple scenarios were considered in LRM and composite structure and barrier parameters were assigned to estimate possible leakage pathways. Detailed numerical models were simulated for estimating leakage from reservoir to the surface through possible leakage pathways. Risks were identified and remedial action plan was designed for restoring well integrity. Post remedial plan covers Marine CO2 dispersion modeling to design comprehensive monitoring and mitigation plan for potential CO2 leakage in the marine environment. This study summarizes the unique challenge associated with estimating well integrity and re-entering existing P&A wells. Leakage rate modeling along these wells involves uncertainties but when carried out with realistic parameters, it can be used as a predicting tool to determine the nature and complexity of leakage. Integrating with site survey results for any indication of gas bubbling, decision can be made to restoring the well integrity. The paper outlines the detail strategic options to safeguard CO2 storage by restoring well integrity using LRM and integrating with marine CO2 dispersion modeling. Assessing well integrity of P&A wells on individual basis, risk is assessed and identified. Proper remedial actions are proposed accordingly. Quantification of all the uncertainties involved needs to be conducted that may affect long-term security of CO2 storage site.

CO2 sequestration in depleted carbonate reservoir stipulate incorporation of comprehensive and trailblazing monitoring technologies. 4D time-lapse seismic is sine qua non for Monitoring, Measurement and Verification (MMV) planning to demonstrate the migration of CO2 plume within geological storage. An ingenious, adaptive and site specific MMV plan for monitoring CO2 plume is paramount to minimize possible subsurface and project integrity risks. Integration of dynamic simulation with seismic forward modeling aggrandize the capabilities of 4D seismic in CO2 sequestration projects.Depleted carbonate reservoir has been thoroughly studied and its geomechanical and geochemical modeling results were coupled into dynamic simulation. Reservoir porosity and fluid properties along with CO2 saturation and injection pressure distribution within each reservoir level were generated. The dynamic simulation results were integrated with seismic forward modeling to demonstrate the CO2 plume migration and its impact on seismic amplitude. Fluid acoustic properties were computed for carbonate reservoir using FLAG method. Selection of wells was based on availability of superior quality acoustic logs as well as those representing the reservoir best. Gassmann fluid substitution exercise was carried using dry rock modeling. Several scenarios were generated, and results were analyzed to demonstrate the effect of CO2 saturation and pressure build-ups within reservoir on the seismic amplitude due to continuous CO2 injection.Synthetic seismic AVO gathers were generated for angles ranging from 5 to 50 degree. Near, Mid and Far seismic amplitude response at the top of carbonate reservoir were analyzed with respect to in-situ condition for each scenario. Results reveal that CO2 saturation as low as 25 - 30% in depleted carbonate reservoir can be distinguished from 4D time-lapse seismic. With continuous CO2 injection, the reservoir pressure increases and this in turn controls the properties of both in-situ and injected fluids. The gradual changes in fluid properties and their impact on bulk acoustic properties of reservoir were modeled to assess the feasibility of using 4D seismic as a predictive tool for detection of localized and provincial pressure build-ups. Modeling results show that although observed changes in amplitude on synthetic gathers were subtle, it is expected that 4D seismic with high signal-to-noise ratio possibly be able to image such localized pressure build-ups. To monitor CO2 plume migration as well as localized pressure build-ups, we recommend acquiring multi-azimuth (MAZ) surface seismic in combination with 3D DAS-VSP for superior subsurface imaging. The integrated modeling approach ensures that 4D Seismic in subsurface CO2 plume monitoring is robust. Monitoring pressure build-ups from MAZ surface seismic and 3D DAS-VSP will reduce the associated risks.

Carbon sequestration is the process of capturing and storage of atmospheric carbon dioxide. The objective of any carbon sequestration project is to store CO2 safely for hundreds or thousands of years with a goal of reducing global climate change. A depleted hydrocarbon reservoir is one of the potential storage sites being considered for long-term CO2 storage. The dynamic, geochemical, and geomechanics changes that occur during CO2 injection are inter-related. For example, when injected CO2 causes dissolution of reservoir rock, on one hand, porosity increases while rock strength decreases. On the other hand, reduced rock strength could cause additional compaction thus reducing porosity, whereas increase in pressure due to injection could cause dilation. Hence, it is critical to have an integrated model that captures effect of all changes on the storage capacity and integrity of the reservoir. Three major depleted gas reservoirs in Central Luconia field, located offshore Sarawak, are being evaluated for future CO2 storage. A 3-way coupled modelling approach that integrates dynamic model, geochemistry model, and geomechanics model is utilized to obtain cumulative effect of all three changes. This integrated model provides a more accurate estimate of 1) CO2 storage capacity, 2) Caprock integrity evaluation, 3) CO2 plume migration path, and 4) Volume of CO2 stored through different storage mechanisms (viz. hydrodynamic trapping, capillary trapping, solubility trapping, and mineral trapping). Apart from providing storage capacity, this model also provides inputs for evaluating integrity of caprock, fault reactivation study, MMV (Measurement, Monitoring, and Verification) planning, and estimating potential leak rates through plugged and abandoned wells. Using a 3-way coupled model, it is estimated that there is an average reduction in porosity of 5-10% (of initial porosity). This translates to an equivalent reduction in CO2 storage capacity of 5-10% compared to dynamic model. It is observed that pore collapse as a result of pressure depletion is primarily responsible for this reduction in porosity. It has also been observed that the injection can be continued till initial reservoir pressure is reached without breaching caprock integrity. CO2 plume migration path significantly affects MMV planning. Potential leak rate estimation is critical in mitigation and contingency planning.

Avoiding or reducing greenhouse gases emission in the atmosphere requires extensive application of technologies and one of them is underground CO2 sequestration. Capture and storage of CO2 in depleted hydrocarbon reservoirs can reduce greenhouse gases released into the atmosphere effectively. Hydrocarbon reservoirs are considered one of the ideal geologic storage sites as they have held hydrocarbons over millions of years. Their architecture and properties are well understood due to exploration and production activities from these reservoirs. Storage projects require a large depleted hydrocarbon reservoir with good reservoir properties and are affected by several factors including voidage created by hydrocarbon production, pressure, architecture, formation permeability, aquifer influx, subsidence and compaction, and rock compressibility to name a few. Thus, realistic estimation of the storage capacity of the reservoir is a key step in the evaluation of CO2 storage plan. A good history matched simulation model incorporating the geomechanical parameters is essential to estimate storage capacity of the reservoir.Three major depleted gas reservoirs in Central Luconia field, located in offshore Sarawak, are being evaluated for future CO2 storage. Reservoir simulation is used as a tool to estimate future CO2 storage capacity of these reservoirs. Reliability of forecast from a reservoir simulation model is dependent on the quality of history match achieved. Hence it is believed that CO2 storage capacity estimates obtained from a good history matched simulation model must be reliable. However, during history matching exercise in these reservoirs, it was observed that an acceptable history match could be achieved with a range of rock compressibility values and aquifer influxes. Generally, a constant value of rock compressibility is used in conventional simulation. For example, in order to obtain an acceptable history match, with a lower compressibility, a larger aquifer influx is needed and vice versa. Interestingly, a forecast using these history match cases yield different CO2 storage capacities. A closer evaluation shows that aquifer influx has a strong impact on future CO2 storage capacity.An acceptable quality of history match can be obtained for a range of rock compressibility values when aquifer influx is adjusted along with it. Sensitivity analysis shows that future CO2 storage capacity in depleted hydrocarbon reservoir is sensitive to rock compressibility used in the simulation model. A detailed sensitivity analysis along with multiple history match scenarios is necessary to understand the range in future storage capacity when evaluating CO2 storage plan.

Cairn India Ltd & ONGC completed a joint venture appraisal of the Barmer Hill (BH) field in the Rajasthan block of India in 2015. The objective was to evaluate the effectiveness of horizontal multi-frac completions. The Barmer hill field is a moderate permeability (0.5 – 4 mD) oil bearing porcellanite with alternating sequences of tight shale. To produce this reservoir economically, hydraulic fracturing was the obvious choice of stimulation and was performed on a number of vertical wells (see Shaoul et al. 2007). To better evaluate the development strategy, completions using either transverse or longitudinal fracture treatments were successfully designed, executed, and evaluated. In the appraisal phase, four vertical and four horizontal wells were drilled to appraise the Barmer Hill reservoir in the Mangala field. Two of the horizontal wells were drilled along the maximum horizontal stress direction and completed with multiple hydraulic fracturing stages, which generated longitudinal fractures along the lateral wellbore. The fracture orientation with respect to the wellbore was confirmed with micro-seismic monitoring. This fracture treatment strategy provided the opportunity to decrease the number of frac stages and exhibited lower treating pressures as predicted by standard elastic rock mechanics theory.While the initial productivity of the transverse fractured wells was expected to be almost 2.5 times more than the longitudinally fractured well, it only produced about 40% more. In addition, the EUR of the longitudinally fractured well is almost the same as the transversely fractured well. Based on the generated fracture geometry, theoretically these longitudinally fractured horizontal wells may also provide better effective sweep efficiency when converted to a water injector from a field development concept.

Cairn India Ltd & ONGC completed a joint venture appraisal of the Barmer Hill (BH) field in the Rajasthan block of India in 2015. The objective was to evaluate the effectiveness of horizontal multi-frac completions. The Barmer hill field is a moderate permeability (0.5 - 4 mD) oil bearing porcellanite with alternating sequences of tight shale. To produce this reservoir economically, hydraulic fracturing was the obvious choice of stimulation and was performed on a number of vertical wells (see Shaoul et al).To better evaluate the development strategy, completions using either transverse or longitudinal fracture treatments were successfully designed, executed, and evaluated. In the appraisal phase, four vertical and four horizontal wells were drilled to appraise the Barmer Hill reservoir in the Mangala field. Two of the horizontal wells were drilled along the maximum horizontal stress direction and completed with multiple hydraulic fracturing stages, which generated longitudinal fractures along the lateral wellbore. The fracture orientation with respect to the wellbore was confirmed with micro-seismic monitoring. This fracture treatment strategy provided the opportunity to decrease the number of frac stages and exhibited lower treating pressures as predicted by standard elastic rock mechanics theory.While the initial productivity of the transverse fractured wells was expected to be almost 2.5 times more than the longitudinally fractured well, it only produced about 40% more. In addition, the EUR of the longitudinally fractured well is almost the same as the transversely fractured well. Based on the generated fracture geometry, theoretically these longitudinally fractured horizontal wells may also provide better effective sweep efficiency when converted to a water injector from a field development concept.

The increasing atmospheric concentration of carbon dioxide (CO2), a greenhouse gas (GHG) is creating environmental imbalance and affecting the climate adversely due to growing industrialization. Global leaders are emphasizing on controlling the production of GHG. However, growing demands of natural gas, industry is embarking on the development of high CO2 contaminant gas fields to meet supply gap. Development and management of contaminated hydrocarbon gas fields add additional dimension of sequestration of CO2 after production and separation in project management. CO2 sequestration is a process for eternity with a possibility of zero-degree failure. Monitoring, measuring and verification (MMV) of injected CO2 volume in sequestration is critical component along with geological site selection, transportation, storage process. The present study discusses all the impacting parameters which makes whole process environment friendly, economically prudent and adhering to national and international regulations.The migration of injected CO2 plume in the reservoir is uncertain and its monitoring is equally challenging. The role of MMV planning is critical in development of high CO2 contaminant fields of offshore Sarawak. It substantiates that injected CO2 in the reservoir is intact and safely stored for hundreds of years after injection and possesses minimum to no risk to HS&E. The deployment of Multi-Fiber Optic Sensor System (M-FOSS) promises a cost-effective solution for monitoring the lateral & vertical migration of CO2 plume by acquiring 4D DAS-VSP (Distributed Acoustic Sensor – Vertical Seismic Profile) survey and for the well integrity by analyzing DAS/DTS (Distributed Temperature Sensor)/DPS (Distributed Pressure Sensor)/DSS (Distributed Strain Sensor) data.Simulation results and injectivity test at laboratory for in-situ CO2 injection has demonstrated the possibility of over 100MMscfd/well injection in aquifer to meet the total CO2 injection of 1.2Bscfd for full field development while maintaining the reservoir integrity. Uncertainty & risk analysis shows possible presence of seismically undistinguished fractures and minor faults, an early breakthrough of injected CO2 cannot be ruled out. The depleted reservoir storage study divulges the containment capacity of identified carbonate reservoirs as well as conformance of potential storage sites. The fault-seal analysis and reservoir integrity studies determine the robustness of the long-term security of the CO2 storage. Injectivity study demonstrates the optimum and maximum possible rates of CO2 injection into these depleted gas reservoirs. VSP simulation results show that a subsurface coverage of 3-4 km2 per well is achievable, which along with simulated CO2 plume extent help to determine the number of wells required to get maximum monitoring coverage for the MMV planning. The deployment of M-FOSS technology is novel and proactive approach to monitor the CO2 plume migration and well integrity. First ever development of MMV Planning for CO2 Sequestration in offshore Sarawak, Malaysia using novel and cutting-edge M-FOSS technology for proactive monitoring of CO2 plume migration and well integrity.

It is still a challenge to build a numerical static reservoir model, based on limited data, to characterize reservoir architecture that corresponds to the geological concept models. The numerical static reef reservoir model has been evolving from the oversimplified tank-like models, simple multi-layer models to the complex multi-layer models that are more realistic representations of complex reservoirs. A simple multi-layer model for the reef reservoir with proportional layering scheme was applied in the CO2 Storage Development Plan (SDP) study, as the most-likely scenario to match the geological complexity. Model refinement can be conducted during CO2 injection phase with Measurement, Monitoring and Verification (MMV) technologies for CO2 plume distribution tracking. The selected reservoir is a Middle to Late Miocene carbonate reef complex, with three phases of reef growth: 1) basal transgressive phase, 2) lower buildup phase, and 3) upper buildup phase. Three chronostratigraphic surfaces were identified on 3D seismic reflection data as the zone boundaries, which were then divided into sub-zones and layers. Four layering methods were compared, which are ‘proportional’, ’follow top’, ‘follow base’ and ‘follow top with reference surface’. The proportional layering method was selected for the base case of the 3D static reservoir model and the others were used in the uncertainty analysis. Based on the results of uncertainty and risk assessment, a risk mitigation for CO2 injection operation were modeled and three CO2 injection well locations were optimized. The reservoir architecture model would be updated and refined by the difference between the modeled CO2 plume patterns and The MMV results in the future.

Monitoring of CO2 plume migration in a depleted carbonate reservoir is challenging and demand comprehensive and trailblazing monitoring technologies. 4D time-lapse seismic exhibits the migration of CO2 plume within geological storage but in the area affected by gas chimney due to poor signal-to-noise ratio (SNR), uncertainty in identifying and interpretation of CO2 plume gets exaggerated. High resolution 3D vertical seismic profile (VSP) survey using distributed acoustic sensor (DAS) technology fulfil the objective of obtaining the detailed subsurface image which include CO2 plume migration, reservoir architecture, sub-seismic faults and fracture networks as well as the caprock. Integration of quantitative geophysics and dynamic simulation with illumination modelling dignify the capabilities of 3D DAS-VSP for CO2 plume migration monitoring. The storage site has been studied in detailed and an integrated coupled dynamic simulation were performed and results were integrated with seismic forward modeling to demonstrate the CO2 plume migration with in reservoir and its impact on seismic amplitude. 3D VSP illumination modelling was carried out by integrating reservoir and overburden interpretations, acoustic logs and seismic velocity to illustrate the subsurface coverage area at top of reservoir. Several acquisition survey geometries were simulated based on different source carpet size for effective surface source contribution for subsurface illumination and results were analyzed to design the 3D VSP survey for early CO2 plume migration monitoring. The illumination simulation was integrated with dynamic simulation for fullfield CO2 plume migration monitoring with 3D DAS-VSP by incorporating Pseudo wells illumination analysis. Results of integrated coupled dynamic simulation and 4D seismic feasibility were analyzed for selection of best well location to deploy the multi fiber optic sensor system (M-FOSS) technology. Amplitude response of synthetic AVO (amplitude vs offsets) gathers at the top of carbonate reservoir were analyzed for near, mid and far angle stacks with respect to pre-production as well as pre-injection reservoir conditions. Observed promising results of distinguishable 25-30% of CO2 saturation in depleted reservoir from 4D time-lapse seismic envisage the application of 3D DAS-VSP acquisition. The source patch analysis of 3D VSP illumination modelling results indicate that a source carpet of 6km×6km would be cos-effectively sufficient to produce a maximum of approximately 2km in diameter subsurface illumination at the top of the reservoir. The Pseudo wells illumination analysis results show that current planned injection wells would probably able to monitor early CO2 injection but for the fullfield monitoring additional monitoring wells or a hybrid survey of VSP and surface seismic would be required. The integrated modeling approach ensures that 4D Seismic in subsurface CO2 plume monitoring is robust. Monitoring pressure build-ups from 3D DAS-VSP will reduce the associated risks.

Underground storage of CO2 in depleted gas reservoirs is a greenhouse gas reduction technique that significantly reduces CO2 released into the atmosphere. Three major depleted gas reservoirs in Central Luconia gas field, located offshore Sarawak, possess good geological characteristics needed to ensure long-term security for CO2 stored deep underground. Long-term integrity of all the wells drilled in these gas fields must be ensured in order to successfully keep the CO2 stored for decades/centuries into the future. Well integrity is often defined as the ability to contain fluids without significant leakage through the project lifecycle.In order to analyze the risk associated with all 38 drilled wells, that includes 11 plugged and abandoned (P&A) wells and 27 active wells, probabilistic risk assessment approach has been developed. This approach uses various leakage scenarios, that includes features, events, and processes (FEP). A P&A well in a depleted reservoir is a very complex system in order to assess the loss of containment as several scenarios and parameters associated to those scenarios are difficult to estimate. Based on the available data of P&A wells, a well has been selected for this study. All the barriers in the example well have been identified and properties associated with those barriers are defined in order to estimate the possible leakage pathways through the identified barriers within that well. Detailed mathematical models are provided for estimating CO2 leakage from reservoir to the surface through all possible leakage pathways.Sensitivity analysis has been carried out for critical parameters such as cement permeability, and length of cement plug, in order to assess the containment ability of that well and understand its impact on overall well integrity. Sensitivity analysis shows that permeability of the cement in the annulus, and length of cement plug in the wellbore along with pressure differential can be used as critical set of parameters to assess the risk associated with all wells in these three fields.Well integrity is defined as the ability of the composite system (cemented casings string) in the well to contain fluids without significant leakage from underground reservoir up to surface. It has been recognized as a key performance factor determining the viability of any CCS project. This is the first attempt in assessing Well Integrity risk related to CO2 storage in Central Luconia Gas Fields in Sarawak. The wells have been looked individually in order to make sure that integrity is maintained, and CO2 is contained underground for years to come.