With the advent of horizontal wells a distinct change is tacitly taking place in our approach to the improved recovery of heavy oil: from displacing mobilized oil in a flood pattern from injector to producers over long distances of the order of hundreds of meters to short-distance oil displacement (SDOD) processes (typically over a few tens of meters).
SDOD processes comprise Steam Assisted Gravity Drainage (SAGD), Vapor Extraction (VAPEX) and Toe-To- Heel (TTH) Displacement Processes, which comprise THAI (Toe-To-Heel Air Injection), with its variant catalytic THAI (CAPRI) and Toe-To-Heel Waterflooding (TTHW). Presently SAGD is commercially used, while VAPEX is under field testing. THAI has been under field testing for nearly two years, while TTHW for more than four years; at this time, plans for the field testing of CAPRI are at an advanced stage.
Steamflooding in a Toe-To-Heel (TTH) configuration (TTH steamflooding) uses vertical wells as injectors and horizontal wells as producers, arranged in a staggered line drive, with producers having their toes close to the shoes of vertical injectors; the horizontal section of producers is located at the base of the pay. The vertical wells are used for initiating the steam front, which subsequently is anchored at the toe of the horizontal producer; it is then propagated towards the producer's heel. In TTH steamflooding, the existing deficiency of conventional steamflooding schemes in terms of low vertical sweep is overcome by the beneficial use of gravity.
To investigate the potential of TTH steamflooding, some laboratory tests were conducted. The objective was to assess the feasibility of TTH steam and thermo-solvent flooding (steam +propane co-injection) by carrying out 3-D model experiments using heavy oil with a viscosity of 15,000 cp.
Laboratory results showed that the concepts of TTH Steamflooding and TTH Steamflooding with solvent are feasible. All in all, stability of TTH steamflooding was relatively good, while the stability of TTH steamflooding with the addition of nitrogen or propane was much better. Significant improvements in design and operation of these processes were needed in order to promote override during the early phase, and obtain a stable and efficient process. The improvements included: a cold (gas fingering) and a hot (steam based) communication phase; and controlling lateral spread of steam by using two additional vertical control wells (positioned laterally but close to the heel of the horizontal producer) for conducting a limited steamflood. Also, nitrogen was injected along with steam in the conventional steamflood; propane replaced nitrogen in TTH Steamflooding with solvent. With these improvements, rise of steam chamber to the top occurred much earlier, and a favorable tilt-forward-angle of the thermal front was quickly obtained. TTH steamflooding with solvent proved superior to the TTH steamflooding, as the channeling of steam through the horizontal well was much better controlled, and the oil recovery was considerably faster. With these improvements, the oil recovery increased from 50-54% to 75- 77%, and the operation became smoother. Presently, the process can be considered only for reservoirs where oil has some mobility under reservoir conditions.
