Carbon capture and storage (CCS) technology has been widely investigated to decrease the greenhouse effect. Geological CO2 storage sites are targeted mainly on depleted petroleum reservoirs or deep saline aquifers. However, CO2 leakage might take place through wellbores, cap rocks, reservoir fractures, or faults during or after the process of CO2 storage leading to environmental problems. To minimize these hazards, different kinds of sealants have been developed and applied. This review aims to summarize those materials applicable to CO2 leakage remediation. On the basis of the sealing mechanisms and compositions of different sealant materials, they were divided into seven major types: Portland cement, geopolymer cement, resins, biofilms barriers, gel systems, foams, and nanoparticles. For different types of sealants, their application background, chemical and physical properties, CO2 leakage remediation mechanism, impact factors of sealing performance, advantages, and limitations were summarized. Future development directions for these sealant materials are also recommended. To solve the problem caused by the weak acid-resistant performance of Portland cement, anti-CO2 materials should be developed and added to the formulation. Environmentally friendly materials need to be designed to replace some current user-hostile compositions in the geopolymer cement. Moreover, chemicals that can control the geopolymerization process are also required because of the high curing temperature requirement for the recent geopolymer productions. The injectivity of Portland cement and resin limits its application for in-depth CO2 leakage control; however, gels with relatively low viscosity during the injection can be a good alternative, although their thermal stability and strength need to be further enhanced. Biotechnology and nanotechnology are perspectives to be applied in the CO2 leakage control process. Foams with good stability might be used for CO2 leakage remediation in the porous medium without fractures, but their life cycle should be prolonged.

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Ultra-low permeability limestone reservoirs in the Middle East have huge untapped reserves. In Iraq, Field H has nearly one quarter of reserves in the S-Reservoir with permeabilities of (0.02-0.4 mD). It is difficult to identify optimal drilling locations, or sweet spots, that maximize recovery factors due to poor understanding of reservoir characteristics. Using the S-limestone reservoir, this study aims to clarify the environment of deposition, diagenetic evolution, reveal the development mechanism of ultra-low permeability, and predict the distribution of more favorable areas. Whole core, thin sections, scanning electron microscope (SEM), mercury injection (MICP), well logs, and seismic data were utilized for the analysis. Through core and thin section observation, lithology, biostratigraphic sequences, pore type, and sedimentary structures were determined. Through these observations the environment of deposition of the S-Formation is interpreted as mid to outer carbonate ramp. The lithology is mainly wackestone to packstone deposited in a low energy environment below fair weather wave base. Primary fossils are planktonic foraminifera tests as grains and coccolith microfossils in the matrix. Thin sections and MICP analysis helped delineate the porosity classes and diagenetic history. The pore throat size, determined by MICP data, was 0.03 - 0.1 μm. Primary porosity is around 10-25% and is driven by intragranular voids in foraminifera tests, followed by intergranular, and intercrystalline microporosity of the matrix. Foraminifera tests are generally intact and float in the mud matrix. As a result, large intragranular pores are well developed but not connected, causing overall medium-high porosity but ultra-low permeability. Early marine calcite cement is the primary diagenetic process which destroyed porosity and permeability. Little to no evidence of secondary dissolution was observed.

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Affected by the Covid-19 pandemic and low oil prices, OPEC members were forced to curtail production. The H oilfield in Iraq commenced production curtailment in early March 2020 and then oil production gradually decreased. By the end of 2020, production was less than one-third of the rate before curtailment. There are multiple sets of oil-bearing formations in the H Oilfield vertically. The developed oil reservoirs have a total of more than three hundreds development wells. The reservoir types are diverse, the relationship among multiphase fluids is complex, and the development methods are different. The reduction of the daily production will inevitably require a comprehensive strategy adjustment to cope with the new situation. Any intentional or unintentional shut-in has a price. Therefore, the key is how to reasonably control the production in many oil reservoirs and re-adjust the oil reservoir development plan at the minimum cost while meeting the overall changing production restriction target for each oil reservoir. In this study, the author established a simple and fast process for judging open and closed wells through years of experience in reservoir dynamic analysis and field management. Step 1: Wells are classified according to production characteristics. For pre-selected wells, some wells with unique functions that need to be opened and those that need to be closed for objective reasons should be excluded. Step 2: Conduct single well cost analysis with reference to production status. Respectively evaluate the performance of the production well under the state of opening and closing. Step 3: Establish the model with economic indicators as the objective function. According to different goals, the model established is slightly different. Step 4: Optimize the best solution based on actual needs. Solve the optimal solution under the target and optimize the number of reasonably configured wells in each reservoir. Through this process, combined with historical and current actual production conditions, different types of oil wells in all reservoirs are classified. Their priorities of reopening are evaluated to meet the needs of other production restriction targets and ensure the smooth transition of oilfield development.