Evaluation of the First Field Piloting of the THAI Process: Athabasca Pilot

Objectives/Scope: The paper describes the first field piloting of the Toe-To-Heel Air Injection (THAI) process - a novel in-situ combustion (ISC) process, which uses vertical air injectors and horizontal producers. The pilot (located in the Athabasca region of Canada) consisted of three adjacent pairs of vertical-horizontal well arranged in a direct line drive configuration, with a lateral spacing of 100m. Methods, Procedures, Process: An in-depth analysis was performed for the determination of the ignition delay during initiation of the ISC via enhanced spontaneous ignition using a steam slug injection. Numerous bottomhole temperatures (BHT) recordings from 11 observation wells allowed to establish the loss of air in an upper formation and the lateral development of the burned zone. Using many thermocouples located along the horizontal section of producers, the determination of the ISC front advancement along the horizontal drains was possible. Based on this, a rough estimate of the volumetric sweep was made. Due to sand influx and other factors, all horizontal producers were replaced; the period of air injection stoppage - during the drilling of replacement producers – caused a substantial modification of the process. Results, Observations, Conclusions: Initiation of ISC process by spontaneous ignition occurred within 1-2 months. A robust ISC was generated and sustained during the five-year piloting. Oxygen utilization was almost 100%, indicating a remarkable stability of the process. The lateral development of the burned zone was limited, within 20 m laterally off the horizontal producer trajectory. The ISC front advanced approximately half of the distance toe-heel, along the horizontal section. Loss of air vertically, led to the formation of a secondary ISC front in an upper formation with no production outlets (production wells). This loss of air and the presence of a thin bottom water zone (from which a significant amount of water was produced) decreased the efficiency of the process. During the air injection interruption (3-4 months), a pronounced pressure decline in the burnt-out zone caused its oil re-saturation leading to coke deposition and blockage; subsequent ISC front propagation along the replacement wells was not satisfactory and these wells did not improve the performance. The pilot confirmed the laboratory findings regarding the production of in-situ upgraded oil and, revealed that THAI generates significant amounts of hydrogen as part of the produced gases. Upgrading of the produced oil was in the range of 2° to 8° API., while the oil viscosity decreased 220 times, from 550,000cp to 2500cP The oil recovery from the pilot was estimated at 7%. In its stabilized phase, the effective oil rate per well was 10-20 m³/day, as compared with 3-4 m³/day in all previous old ISC trials in Athabasca.