Pore-scale flow simulation of supercritical CO2 and oil flow for simultaneous CO2 geo-sequestration and enhanced oil recovery

Molecular Insights into Soaking in Hybrid N2-CO2 Huff-n-Puff: A Case Study of a Single Quartz Nanopore-Hydrocarbon System

Hybrid N2-CO2 huff-n-puff (HnP) has been experimentally demonstrated to be a promising approach for improving oil recovery from tight/ultratight shale oil reservoirs. Despite this, the detailed soaking process and interaction mechanisms remain unclear. Adopting molecular dynamic simulations, the soaking behavior of hybrid N2-CO2 HnP was investigated at the molecular and atomic levels. Initially, the soaking process of fluid pressure equilibrium after injection pressure decays in a single matrix nanopore connected to a shale oil reservoir is studied. The study revealed that counter-current and cocurrent displacement processes exist during the CO2 and hybrid N2-CO2 soaking, but cocurrent displacement occurs much later than counter-current displacement. Although the total displacement efficiency of the hybrid N2-CO2 soaking system is lower than that of the CO2 soaking system, the cocurrent displacement initiates earlier in the hybrid N2-CO2 soaking system than in the CO2 soaking system. Moreover, the N2 soaking process is characterized by only counter-current displacement. Next, the soaking process of fluid pressure nonequilibrium before the injection pressure decays is investigated. It was discovered that counter-current and cocurrent displacement processes initiate simultaneously during the CO2, N2, and hybrid N2-CO2 soaking process, but cocurrent displacement exerts a dominant influence. During the CO2 soaking process, many hydrocarbon molecules in the nanopore are dissolved in CO2 while simultaneously exhibiting a substantial retention effect in the nanopore. After pure N2 injection, there is a tendency to form a favorable path of N2 through the oil phase. The injection of hybrid CO2-N2 facilitates the most significant cocurrent displacement effect and the reduction in residual oil retained in the nanopore during the soaking process, thus resulting in the best oil recovery. However, the increase rate in total displacement efficiencies of the different soaking systems over time (especially the hybrid N2-CO2 soaking system) was significantly larger before than after injection pressure decays. Additionally, the displacement effect induced by oil volume swelling is significantly restricted before the injection pressure decays compared to the soaking process after the injection pressure decays. This study explains the role of CO2-induced oil swelling and N2-induced elastic energy played by hybrid N2 and CO2 at different stages of the hybrid N2-CO2 soaking process before and after pressure decays and provides theoretical insights for hybrid gas HnP-enhanced recovery. These pore-scale results highlight the importance of injection pressure and medium composition during the soaking process in unconventional oil reservoirs.

Influence Factors and Feasibility Evaluation on Geological Sequestration of CO2 in Coal Seams: A Review

The geological sequestration of CO₂ in coal seams holds significant implications for coalbed methane development and greenhouse gas mitigation. This paper examines the principles, influencing factors, and evaluation methods for geological CO₂ sequestration in coal seams by analyzing relevant domestic and international findings. Suitable geological conditions for CO₂ sequestration include burial depths between 300 and 1300 m, permeability greater than 0.01 × 10^-3 μm², caprock and floor strata with water isolation capabilities, and high-rank bituminous coal or anthracite with low ash yield. Geological structures, shallow freshwater layers, and complex hydrological conditions should be avoided. Additionally, the engineering conditions of temperature, pressure, and storage time for CO₂ sequestration should be given special attention. The feasibility evaluation of CO₂ geological storage in coal seams necessitates a comprehensive understanding of coalfield geological factors. By integrating the evaluation principles of site selection feasibility, injection controllability, sequestration security, and development economy, various mathematical models and "one vote veto" power can optimize the sequestration area and provide recommendations for rational CO₂ geological storage layout.

Economic feasibility and policy incentive analysis of Carbon Capture, Utilization, and Storage (CCUS) in coal-fired power plants based on system dynamics

Carbon Capture, Utilization, and Storage (CCUS) is an important potential technical way for coal power plants to achieve near-zero carbon emissions with the current energy structure in China being dominated by coal. However, CCUS is still at the early demonstration stage, and there are many uncertainties in the business model and policy incentives that the traditional method can no longer handle. At the same time, few studies have analyzed these issues. Taking coal-fired power plants as the research object, we adopt the system dynamics method (SD) to compare the economic feasibility, stability, and CO₂ emission reduction effect of the CCUS in the vertical integration model and the CCUS operator model. Furthermore, this paper also studies the effect of different incentive policies on CCUS. Results show the following: (1) Both models are economically viable. From the stability perspective, the CCUS operator model is superior to the vertical integration model. (2) In terms of the effect of individual policies, the government's preference for subsidies to the coal-fired power plant can significantly reduce CO₂ emissions. Increasing the additional government subsidy and carbon tax can accelerate the CCUS system into a relatively stable state, but beyond a certain range, the marginal CO₂ capture rate, marginal CO₂ emission reduction rate, and marginal profit rate of each department will be significantly reduced. The storage enterprise highly depends on the government subsidy. Low government subsidy allocation, low additional government subsidy, and a low carbon tax will lead to continuous negative profits for the storage enterprise, making the CCUS system unsustainable. (3) The impact of combination policies on CCUS is not simply the sum of the effects of individual policies. Each policy has a counteracting or reinforcing effect on the other.