Steam Injection Pressure and the SAGD Ramp-Up Process

Abstract

Abstract Most SAGD projects require about one to two years for ramp-up. Over this period of time, oil rate will be below peak oil rate and SOR will be higher than long-term steady-state SOR. This paper discusses the effect of steam injection pressure on SAGD ramp-up time, the associated geomechanical effects and optimization of the ramp-up phase of SAGD. Different steam injection pressures induce different reservoir geomechanical behaviour in oil sands. Higher steam injection pressure is capable of inducing more favourable reservoir geomechanical effects (such as shear dilation and isotropic unloading), improving the reservoir permeability, and subsequently, benefiting the long-term SAGD operation. This paper indicates that the ramp-up time can be reduced due to the favourable geomechanical effects. A coupled reservoir geomechanical simulation technique was applied for this investigation. In addition, cap rock integrity concerns when applying high injection pressure are also addressed. It is recommended that during or following the ramp-up phase, the injection pressure be lowered to a safe operating pressure to ensure cap rock integrity. The effects of low and high steam chamber pressures on SAGD oil rate are also discussed. Introduction The steam-assisted gravity drainage (SAGD) process has been proven to be the most promising technology for developing the Athabasca oil sands reserves in northern Alberta. In 2001, only four commercial SAGD projects were operating in the field(1). By March 2006, the number of active SAGD projects increased to 15(2). Field SAGD operation experiences and numerical simulation show that the steam injection pressure plays an important role in SAGD production performance. In general, higher steam injection pressure helps lift fluid from downhole to the surface, increases the oil production rate, reduces the overall well life and improves the ultimate oil recovery. Some of these enhancements could result from geomechanical effects induced by higher steam chamber pressure. For the unconsolidated oil sands reservoir under certain in situ stress conditions, higher steam injection pressure tends to induce larger volumetric strain associated with shear dilation, thermal expansion, and even, tensile failure. As a result, reservoir permeability can be improved and oil recovery will be accelerated. Higher steam injection pressure, however, can also have undesirable effects. It may cause the reservoir cap rock to be breached due to geomechanical behaviour, and then result in a very high steam-oil ratio (SOR). Therefore, the time for applying high steam injection pressure needs to be optimized for maximum geomechanical benefits without causing cap rock failure. From the beginning of steam injection to the time when the partially drained zone approaches the cap rock, higher injection pressure may be safely applied. This strategy will accelerate the oil production rate for the ramp-up phase and will attain the peak SAGD oil rate sooner than otherwise possible. This paper applies a coupled reservoir geomechanical simulation (called "coupled simulation") technique(3) to investigate the relationship between the steam injection pressure and the SAGD ramp-up process.