Fast SAGD and Geomechanical Mechanisms

Abstract This paper outlines a new variation of the SAGD process, called Fast SAGD. Numerical simulation of the Fast SAGD process using a thermal reservoir simulation software package is performed in order to investigate the operating strategy. A geomechanical model is used to analyze the impact of geomechanics on the recovery process. It is concluded from this study that, besides gravity, steam drive, shear failure, and pore volume deformation are the additional mechanisms in this recovery process. Clearly, Fast SAGD is a process with high productivity and low cumulative steam-oil ratio. Introduction Cyclic steam stimulation (CSS) was first discovered in Western Venezuela in 1959 and developed in the field. In Alberta, injection of any fluid into the oil sands is problematic because of the very low mobility of the in situ bitumen. As a result, the injection pressure is increased to a level to part the formation. In Cold Lake1, an injection pressure of 12 MPa is reached and steam is introduced into the formation at high rates of about 200 m3/day. However, the major problem with CSS as it is practiced in Alberta is that typically 15 to 20% of the bitumen is recovered from the resource, even in a pattern comprised of closely spaced vertical wells. For a reservoir with gas cap or bottom water, the parting pressure would probably leak more heat into the over/under burden by heat convection and decrease heat efficiency. The Steam-Assisted Gravity Drainage (SAGD) process was developed in the 1980s, and currently several pilot projects are operated in Alberta. In this process, steam is injected continuously into a horizontal well, located parallel to, and closely above, a horizontal production well at the base of the reservoir. Heated oil drains by gravity and steam fills the vacated pore volume. It has been reported that SAGD is an attractive recovery process, which results in low steam-oil ratios (SOR). However, a challenge for the SAGD is to try to promote the lateral and downward expansion of the steam chamber2. A new process, called Fast SAGD, which combines SAGD and CSS, was recently3,4 proposed. Reservoir simulation of the process has shown that Fast SAGD is a process with relatively high productivity and low operating pressure. FAST SAGD PROCESS Fast SAGD is a combination of both SAGD and CSS, as can be seen in Figure 1. After the SAGD steam chamber is developed, steam is injected into a single horizontal well, called offset well, 50 m away from the SAGD well pair, in a cyclic mode in order to help propagate the steam chamber expansion down the reservoir. The first cycle lasts one year, nine months of steam injection followed by three months of production. In the second cycle, after six months of steam injection, the offset well is converted to a production well for the remainder of time. Meanwhile, extra steam is injected into the SAGD injector to maintain and expand the combined steam chamber that is generated by the offset well. Two-week soak is considered in the cyclic steaming of the offset well.

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