Geomechanics of thermal oil production from carbonate reservoirs

Over two trillion barrels of viscous oil (heavy oil, extra heavy oil, and bitumen) are reported in naturally fractured carbonate rocks. Only primary cold production and CO₂ flooding with very low recovery factors have achieved some commercial success in these reservoirs. Steam injection in viscous oil carbonates is being piloted in the Middle East and in Canada. This article presents the definitions, geology and origins, geographical distribution, and world endowment of viscous oil in fractured carbonates, then some approaches and physical mechanisms involved in thermal viscous oil production processes are described. High T and p production processes have a profound impact on geomechanical behavior of viscous oil carbonate reservoirs. Thermal-stress- pressure effects on natural fractures can generate changes in flow capacity of several orders of magnitude from wedging to shear dilation around the thermally stimulated zone. Approaches to calculating thermally induced stresses are described. Available experimental data and field evidence of the thermal, physical, and geomechanical behavior of carbonate rocks under elevated temperature and pressure are reviewed, and some of the practical consequences are discussed. Most important, thermal production changes reservoir behavior, generally leading to production enhancement, although thermal stimulation can generate operational issues such as CO₂ production, induced casing shear, and seal breaching.