Cyclic steam stimulation (CSS) is a typical enhanced oil recovery method for heavy oil reservoirs. In this paper, a new model for the productivity of a CSS well in multilayer heavy oil reservoirs is proposed. First, for the steam volume of each formation layer, it is proposed that the total steam injection volume will be split by the formation factor (Kh) for the commingled steam injection mode. Then, based on the equivalent flow resistance principle, the productivity model can be derived. In this model, the heavy oil reservoir is composed of a cold zone, a hot water zone, and a steam zone. Next, using the energy conservation law, the equivalent heating radius can be calculated with the consideration of the steam overlay. Simultaneously, a correlation between the threshold pressure gradient (TPG) and oil mobility is also applied for the productivity formula in the cold zone and the hot water zone. Afterward, this model is validated by comparing the simulation results with the results of an actual CNOOC CSS well. A good agreement is observed, and the relative error of the cumulative oil production is about 2.20%. The sensitivity analysis results indicate that the effect of the bottom hole pressure is the most significant, followed by the TPG, and the effect of the steam overlay is relatively slight. The formation factor can affect the splitting of the steam volume in each layer; thus, the oil production rate will be impacted. The proposed mathematical model in this paper provides an effective method for the prediction of preliminary productivity of a CSS well in a multilayer heavy oil reservoir.

The desorption of heavy oil is one of the important indicators affecting the development efficiency of the remaining oil in nanopores. However, the study of the adsorption and diffusion mechanisms of heavy oil molecules in nanopores remains scarce. In this work, the influences of temperature on the adsorption and diffusion properties of the heavy oil four-fractions in quartz nanopore have been investigated via molecular dynamics simulations. Our results show that the heavy oil molecules will form a denser multilayer adsorption oil layer on the nanopore surface, and high temperature can alter the adsorption behaviors of the heavy oil four-fractions. As the temperature increases, the saturate molecules are desorbed from the nanopore surfaces, but the aromatic, resin, and asphaltene molecules maintain a tendency to aggregate towards the nanopore surface. In particular, the agglomeration behaviors of most saturate, aromatic and asphaltene molecules in nanopore can be suppressed by the confined space compared with the heavy oil molecules in oil droplet. In addition, the influence of temperature on the movement of heavy oil molecules in nanopore decreases compared with the oil molecules in a heavy oil droplet due to the confined space and adsorption effect. Interestingly, there is a competition phenomenon between the adsorption and diffusion of aromatic, resin, and asphaltene molecules in the nanopore, resulting in different adsorption behaviors with the increase in temperature. The results obtained in this paper will provide molecular-level theoretical guidance for understanding the adsorption and desorption mechanisms of heavy oil in nanopores.

Thermal recovery is still the most important means to increase heavy oil EOR. With the increase in the recovery factor and the difficulty of exploiting new exploration reserves, the efficient utilization of offshore heavy oil reserves has attracted much attention. However, due to the challenges of high development investments, high operating costs, platform safety factors, and high economic cumulative yield, the offshore heavy oil reserves of nearly 700 million tons have not been effectively utilized. In this paper, Chinese offshore heavy oil reserves were taken as the research object. The indoor one-dimensional experiments were carried out to optimize an applicable development method, and the superheated steam huff and puff was selected as the injection medium for high-speed and high-efficiency development of offshore heavy oil, which verified the great potential of the application of superheated steam in offshore heavy oil thermal recovery. A numerical simulation model for offshore heavy oil superheated steam injection development was established, and a dynamic model considering the thermal cracking of heavy oil was established through historical matching. Through the field numerical simulation models, the whole process development mode of a single sand body, thin interbedded reservoir superheated steam huff and puff turning to superheated steam flooding, and thick layer super heavy oil reservoir with bottom water sidetracking after superheated steam huff and puff for eight cycles was established. Through the numerical simulation method and grey correlation method, the main control factors of superheated steam development of different types of heavy oil reservoirs were determined, and the cumulative oil production charts of different types of reservoirs under the influence of the main control factors were built. The economic evaluation model of superheated steam development of offshore heavy oil was established. Combining multi-specialty of geological, reservoir engineering, drilling and completion, oceanographic engineering, economics, the economic limits of steam injection development under different reserve scales, and engineering conditions of offshore heavy oilfields were clarified. At last, we planned the economic production mode of undeveloped reserves and predicted the construction profile of superheated steam capacity of offshore heavy oil using the production charts and the economic charts. The research results clarify the great potential of thermal recovery development of offshore heavy oil, provide an important basis for the economic development of offshore heavy oil undeveloped reserves, and also provide an important decision for the sustainable and stable production of global heavy oil reservoirs.

Cyclic steam stimulation (CSS) is successfully applied to increase heavy oil recovery in heavy oil reservoirs in Bohai Bay, China. However, during the CSS processes, hydrogen sulfide (H₂S) was detected in some heavy oil reservoirs. The existing literature mainly focused on the H₂S generation of onshore heavy oil. There is no concrete experimental data available, especially about the level of H₂S generation during CSS of offshore heavy oil. In addition, there is still a lack of effective reaction kinetic models and numerical simulation methods to simulate H₂S generation during the CSS of offshore heavy oil. Therefore, this paper presents a case study from Bohai Bay, China. First, the laboratory aquathermolysis tests were conducted to simulate the gases that are produced during the CSS processes of heavy oil. The effects of the reaction temperature and time on the H₂S generation were studied. Then, a one-dimensional CSS experiment was performed to predict H₂S generation under reservoir conditions. A kinetic model for the prediction of H₂S generation during the CSS of heavy oil was presented. The developed model was calibrated with the experimental data of the one-dimensional CSS experiment at a temperature of 300 °C. Finally, a reservoir model was developed to predict H₂S generation and investigate the effects of soaking time, steam quality, and steam injection volume on H₂S generation during CSS processes. The results show that the H₂S concentration increased from 0.77 ppm in the first cycle to 1.94 ppm in the eighth cycle during the one-dimensional CSS experiment. The average absolute error between the measured and simulated H₂S production was 12.46%, indicating that the developed model can accurately predict H₂S production. The H₂S production increase with soaking time, steam quality, and steam injection volume due to the strengthened aquathermolysis reaction. Based on the reservoir simulation, the H₂S production was predicted in the range of 228 m³ to 2895 m³ within the parameters of this study.

Chemical-assisted steam flooding (CASF) is a promising method for heavy oils. However, few researches have investigated the CASF performance on offshore heavy oil reservoirs recovery after water flooding. In this study, a numerical simulation model was developed to simulate CASF processes for offshore heavy oil reservoirs after water flooding. Then, a comparison of CASF and various thermal methods was made to assess the feasibility of CASF in an offshore heavy reservoir of Bohai Bay, China. Finally, sensitivity analysis was performed to evaluate the effects of the gas-liquid ratio, foaming agent concentration, surfactant concentration, the size of nitrogen foam slug, the size of surfactant slug, and the number of chemical injection round on CASF performance by the developed model. The results showed that the developed numerical method can precisely simulate the CASF processes. CASF is a potential and effective method for offshore heavy oil reservoirs after water flooding. The most suitable gas-liquid ratio was around 2 : 1 under the simulation conditions. Considering the economic benefit, it is significant to optimize the CASF parameters, such as foaming agent concentration, the size of nitrogen foam slug, and the number of chemical injection round.

Supercritical multi-source, multi-component thermal fluid is a heavy oil thermal recovery method independently developed by China National Offshore Oil Co., Ltd (Beijing, China). It uses waste liquid at the production end of the production well as the water source, the injection medium temperature exceeds 374 °C, 22.1 MPa, and all the produced flue gas is re-injected. Compared with steam huff and puff technology, supercritical technology has the advantages of high enthalpy value, high heat utilization rate, good oil displacement effect, and being green and pollution-free. In addition, its oil–water treatment cost is low, it can realize the reuse of organic matter, it has a good cost advantage of water treatment under the background of low carbon, and it is a thermal recovery method with great application potential for offshore heavy oil. Therefore, it is necessary to carry out research on the adaptability and application potential of supercritical multi-source, multi-heat flow thermal recovery technology in the sea. Based on the laboratory one-dimensional displacement experiment, this paper reveals the mechanism of heavy oil supercritical multi-source multi-component thermal fluid displacement and the contribution of supercritical components to the displacement effect, and establishes the supercritical multi-source, multi-component thermal fluid numerical simulation characterization method. Combined with the characteristics of offshore heavy oil reserves, the main control factors affecting supercritical multi-source, multi-component thermal fluid development were established by numerical simulation and orthogonal test methods, and the adaptive screening method of offshore supercritical technology was established. The application potential of 670 million tons of offshore heavy oil reserves was evaluated and sorted, and KL 10-2 oilfield was selected as the pilot test oilfield. The results show that supercritical technology has great advantages in oil displacement and water treatment cost reduction, and the results play an important guiding significance for the development of offshore heavy oil technology system and the iteration of new technology.