Development of Hard-To-Recover Hydrocarbon Resources by Improving Integrity: Integration of Multi-Well Steam Injection Assisted by CO2 and Cluster Drilling Technology

Abstract The geological reserves of heavy oil worldwide are estimated to be approximately 815 billion metric tons. The global abundance of heavy oil is noteworthy, as heavy oil, extra-heavy oil, and bitumen collectively make up approximately 70% of the world's total oil resources. China has currently discovered over 70 heavy oil fields in 12 basins, with proven reserves of 4 billion metric tons. Heavy oil may be produced by cold methods or thermal wells. The most common thermal well production technologies include cyclic steam stimulation (CSS), continuous steam injection, in-situ combustion (ISC), steam assisted gravity drainage (SAGD) and vapor extraction (VAPEX). However, poor production performance has been noted in interwell interactions, such as steam channeling and high water cut of offset wells by condensed steam. As a more advanced mechanism, sequential multi-well CSS has been initiated by injecting steam into a group of wells simultaneously, in order to avoid interwell steam channeling and extend the heating area and increase reservoir pressure. Various injection sequences displace the oil in different directions in order to improve sweep efficiency. A group of 7 wells were selected and the overall steam stimulation was simulated by software. Natural gas, nitrogen and CO2 with excessive doses were injected with crude oil extracted from Liaohe Oilfield into a PVT apparatus to analyze their properties with different dissolution ratios. Heat exchange model of liquefied CO2 in the borehole was simulated to determine the best injection speed and CO2's temperature. The average sweep efficiency and oil production rate of wells in the group were compared with that of adjacent wells. The multi-well steam stimulation technology which depends on injecting steam into a group of wells and creating a uniform temperature field is useful, as steam channeling caused by longitudinal heterogeneity in the heavy oil reservoir and repetitive steam injection may be overcome. CO2 has the best role in reducing the oil viscosity, while natural gas and nitrogen follow it. Hence CO2 is the most appropriate EOR gas. CO2's dissolubility declines as temperature goes up and improves as pressure increases. Temperature of liquefied CO2 varies a lot with different injection speeds, in that the heat diffusion time is different. The faster CO2 is injected, the shorter the heat diffusion time is, which makes downhole temperature change less. As CO2 is injected into formation, it dissolves rapidly with heavy oil and makes it expand. Steam is injected then to heat the borehole, while CO2 diffuses rapidly and its dissolubility declines as temperature goes up, which makes CO2 separated from oil and diffused by scale. Prior to deployment of the multi-well steam stimulation technology, it is important to carry out cluster drilling, which requires optimum placement of wells, based on the distribution of targets. Based on "the minimum total horizontal displacement of all trajectories", a software may be developed to place wellheads automatically. Conventional steam stimulation used to be adopted by scale in Liaohe Oilfield, while the daily production rate began dramatically decreasing after 4 to 5 rounds of steam injection. The integration of multi-well steam stimulation and CO2 technology improves heat utilization and sweep efficiency. The cluster well placement software is encouraged to be deployed on green fields where heavy oil has not been developed by scale, in order to improve economic benefits.