Injection Well - Producer Well Combinations in THAI 'Toe-to-Heel Air Injection'

Abstract

Abstract THAI - ‘Toe-to-Heel Air Injection’ is a new, short-distance displacement process, that achieves high recovery efficiency by virtue of its stable operation and ability to produce mobilised oil directly into an active section of the horizontal producer well, just ahead of the combustion front. THAI, therefore, avoids the pitfalls inherent in most conventionally operated in situ combustion (ISC) processes, which employ vertical injection-vertical producer wells to achieve long-distance displacement. However, the problem of achieving efficient ignition and start-up still requires critical attention, in order to ensure optimal process operation. A series of 3-D tests on heavy Wolf Lake crude (10. 5 °API) and Athabasca Tar Sand Bitumen (8 °API) were made using well configurations: vertical (VI) or horizontal injection (HI) and horizontal producer (HP) wells in direct line drive (VIHP, HIHP); staggered line drive (VI2HP) and line drive (2VIHP). Experimentally, the horizontal injector configuration (HIHP) was found to be the most efficient for achieving rapid start-up, i. e. the shortest time to achieve stable combustion front propagation. However, injection of air via a horizontal well is not a very practical design for field operation. The single vexrtical injector configuration (VI2HP) was slow to achieve stable operation, due to the development of a much smaller ignition zone, initially, compared with the HIHP configuration. When hot air was used for ignition, the time delay for oil production is related to the reservoir temperature. When the initial temperature in the sandpack was 15°C, then a vertical injector, placed high in the sandpack, combined with a horizontal producer well (VIHP) achieved slower start-up than VI2HP for post-steam flood THAI, with the initial sandpack at 100°C. All of the tests achieved very high oil recoveries, averaging greater than 80% OOIP, except in the VIHP test. The recovery in the latter case was only 70% OOIP due to the loss of air injectivity during later stages of the combustion. Significantly, THAI also preserves the very substantial thermal upgrading which occurs in the mobile oil zone, averaging an increase of 6 to 8 API points. 1. Introduction The three natural oil recovery mechanisms include solution gas drive, gas cap expansion drive, and water drive. Water or gas injection drives are known as secondary recovery methods. Enhanced Oil Recovery (EOR) is usually considered to apply at the tertiary stage, but can also be applied as primary or secondary methods. For light oil reservoirs, the ultimate oil recovery by conventional methods can reach up to 50%, or more, using water injection. However, for highly viscous oils, comparable recoveries would only be about 5% to 15%, and essentially zero in the case of extremely viscous oils, or bitumen. The main objective of an EOR process is to achieve high oil recovery and high production rate. The low recovery for heavy oil is mainly due to its high viscosity, i. e. too viscous to flow to the producer wells at rates sufficient to support an economic operation. Thermal EOR methods are required for heavy oil production. Thermal EOR processes are achieved by injecting a hot fluid (steam), or air for combustion, with the aim of increasing the reservoir temperature to reduced the viscosity of the heavy oil1,2. Because of the dramatic effect of temperature on heavy oil viscosity, more oil is mobilised at a higher temperature and is capable of being displaced.