Stability of In Situ Combustion Process to the Air Injection Stoppage

Abstract Unlike steam injection for heavy oil recovery, in situ combustion (ISC) process requires an extra-step, namely initiation of the process, the so called ignition operation. During ignition, an ISC front is generated near the air injection well and, thereafter, this front is propagated towards production wells. Air injection interruptions can be un-intentional or intentional, including one-day-per-month scheduled interruptions for cleaning oil from air lines exiting the compressors. Regarding air injection interruptions a question arises: can the process be resumed without initiating a new ignition operation? The paper attempts to provide clear answers to this question. The paper reviews information on more than 6 cases of air injection stoppage in the field, involving well instrumented dry ISC projects, conducted either in patterns or in a line drive system. Most of the cases are from Suplacu de Barcau (Romania) project, which is the world’s biggest ISC commercial project. The duration of air injection interruptions ranged from a few hours to 36 days. Performance before and after interruptions are analyzed in terms of changes in air injectivity, effluent gas composition and oil production. To support various field observations, essential results from simulation of the dry ISC process are also included; Temperature distributions during air injection stoppage were determined analytically for a linear system and numerically, for a radial system. In both cases a specific characteristic of the temperature re-distribution was noted, namely the move/shift of the peak temperature inside the former burned out zone. Some additional conclusions regarding the stability of the process as function of elapsed time since the initiation of the process were also formulated. Stability of the process was investigated for air injection interruptions of up to 90 days. Finally, a case is presented involving a pressure cycling ISC process; stability performance of the Morgan ISC project (Alberta, Canada) during its 6-year field piloting is discussed. In this project, interruptions of air injection of up to 21 months occurred. Inherent limitations in evaluating the stability performance are reviewed given the very complicated nature of this dry ISC process in which reservoir pressure was cyclically increased and oil was allowed to flow through the burned out zones during the air injection