Numerical Investigation of SAGD Process Using a Single Horizontal Well

Abstract

Abstract Recently several articles have been published regarding the innovation and field applications of the Steam-Assisted-Gravity-Drainage (SAGD) process using a single horizontal well for the recovery of heavy oil. In the process, an insulated concentric coiled tubing, inserted into the horizontal wellbore, injects high quality steam to the toe of the well. The steam condensate and reservoir fluids enter the wellbore and flow back along the annulus to the heel to be produced. There remain some arguments on whether the SAGD mechanism is working in field operations and on what oil rates can be achieved in the long run. In an attempt to answer these questions, numerical simulation studies were pursued in the present work. The studies focused on three major aspects:the possible counter-current fluid exchange pattern between the reservoir and the well;the means that could be exploited to promote the initiation of a steam chamber; andthe potential oil rates that could be expected under possible field operating conditions. The results showed that the presence of small capillary pressure near a horizontal wellbore is a physical constraint preventing counter-current exchange of steam and oil. It was also demonstrated that vertical undulation in well-profile or a certain level of formation dilation around the wellbore could potentially overcome the capillary pressure constraint and promote the initiation of a steam chamber under field operating conditions. Introduction After over 20 years of extensive laboratory studies and field trials, the concept of Steam-Assisted-Gravity-Drainage (SAGD)(1) has evolved into a commercially viable thermal oil recovery process for certain heavy oil reservoirs(2,3). In the conventional SAGD process(4), a pair of vertically stacked horizontal wells is used; in which the top well is used for steam injection and the bottom well is used for fluid production. Even with the latest drilling/completion technologies, it may still be difficult to properly place the well pair into a formation(2). In order to reduce the high cost associated with the drilling and completion of two horizontal wells, industry has sought alternative well configurations since the first field trial(5–9). One of the approaches is to utilize only one horizontal well for both steam injection and fluid production(5,8). As early as 1978, Imperial Oil conducted the pioneering horizontal well field pilot (HWP-1) to test the SAGD process(10). In the pilot, the single-well concept was tested. The steam was injected through an annulus to the heel of the well; the steam condensate and oil were produced through a nitrogen gas insulated FIGURE 1: The well profile of Imperial Oil's Horizontal Well Pilot 1 (HWP-1) (a vertical well for observation). (From Hawkins, 1979.) (Available in full paper) tubing inside the wellbore. Figure 1 shows the well profile. The vertical well shown in the figure was an observation well. During the 94-day test period over 3,500 m3 of cold water equivalent (CWE) dry steam was injected into the horizontal well, but only 9 M3 of bitumen was produced from a reservoir with high viscosity bitumen (200 Pa ·s).