In a steam-assisted gravity drainage process, two horizontal wells, superposed 16.4 ft (5 m) apart, are placed near the bottom of the formation. Steam is generally circulated through the wells to establish hot communication paths between them and to initiate gravity drainage. However, this steam circulation phase delays oil production. Results of a research program to investigate two novel methods to eliminate steam circulation and enhance the recovery of bitumen or heavy oil through horizontal wells are described in this paper. In the first procedure, paired horizontal wells were linked transversely with vertical channels to improve liquid drainage, and in the second procedure, naphtha was used as a steam additive to accelerate oil drainage and to increase ultimate oil recovery. The physical models used in this study were designed according to scaling techniques which insured that gravity drainage was the major mechanism of recovery. The scaling technique also ensured that field bitumen could be used in the model. Geometric similarity was maintained, and time was appropriately scaled.
Six experiments were conducted which indicated that more than 50% of the initial bitumen in place could be recovered in less than four years. Using vertical channels to enhance the drainage of fluids from the injection well to the production well improved oil recovery from paired horizontal wells. When one vertical channel was used for a 65.6 ft (20 m) portion of the horizontal wells, the occurrence of a maximum oil-steam ratio was observed 180 d before that of a baseline experiment and the final recovery increased from 52% to 64%. When two channels were used for a 65.6 ft (20 m) portion of the horizontal wells, the final recovery was lower portion of the horizontal wells, the final recovery was lower than that for one channel (56% versus 64%) while the same early production was achieved. Hot fluids bypassed the region between the two horizontal wells, reducing recovery from that region.
The presence of naphtha in the steam further improved the rate of oil production and final recovery. When 5 wt% (0.05 kg/kg) naphtha in steam Was used. the final recovery Was 82% compared With 64% for a baseline experiment using steam-only injection with one vertical channel. When the naphtha concentration in steam was increased to 10 wt% (0.10 kg/kg), the final recovery was lower than that for the test with 5 wt% (0.05 kg/kg) naphtha (71% versus 62%). A high, non-condensible gas saturation in the steam chamber blanketed the heat transfer interface between the steam and bitumen, reducing heat transfer across the interface.
The application of horizontal wells in thermal recovery of heavy oil and bitumen has gained considerable interest in recent years as a result of advances made in horizontal well drilling technology and more efficient implementation of the gravity drainage process. In addition, horizontal wells nave greater contact with the reservoir formation than vertical wells and, therefore, appear to be more economical.
The steam-assisted gravity drainage process using horizontal wells has been field tested at the Underground Test Facility (UTF) site in Alberta, Canada, where the success of the process for highly viscous oils has been demonstrated. In this process, developed by Butler, two horizontal wells superposed 16.4 ft (5 m) apart, are placed near the bottom of the formation. The top horizontal well is used to inject steam which rises in large gravity cells above the well, and the bottom well is used to collect the produced liquids (formation water, condensate, and oil). Steam is fed continuously into a growing steam chamber which forms above the injection well.