Start-up of SAGD Wells: History Match, Wellbore Design and Operation

Abstract

Abstract A smooth start-up is crucial to the successful operation of a SAGD process. A simulation study was conducted to analyze the start-up or circulation period of SAGD well pairs. A coupled reservoir/wellbore model was developed and used to history match field data obtained from wells with extensive instrumentation at Petro-Canada's MacKay River development. The history matched model was then used to conduct a sensitivity study on some of the parameters that affect the circulation of a well pair. First, the steam-to-toe time was examined for the two typical wellbore completions that Petro-Canada employs at MacKay River. Various flow rates were tested for the two completions to see how the steam injection rate affected the steam-to-toe time. Next, the circulation pressure along with the injector/producer pressure gradient was investigated. Various pressure gradients were applied between the injector and producer to examine the effects on the circulation duration and the formation of steam coning. Finally, the distance between the injector and producer was studied. The purpose was to explore the effect of the vertical separation between the injector and producer wells on conversion time. Introduction The Steam-Assisted Gravity Drainage (SAGD) process has become the leading in situ bitumen recovery technique in the Athabasca oil sands deposit. In the SAGD process, two horizontal wellbores are drilled approximately 5 m apart vertically. The upper well is used as an injector. High quality steam is injected into the reservoir, heating the bitumen and reducing its viscosity. The bitumen then flows, by gravity, to the lower producer well, and is then produced to the surface. Before the SAGD process can be initiated, the horizontal well pair must be heated by circulating steam in both the injector and producer. The purpose of the circulation process is to heat the near wellbore region and to establish flow communication between the two wells. Two common completions of SAGD horizontal wellbores are shown in Figures 1 and 2. In the injector completion, there is a single long tubing string landed at the toe. The long tubing string is placed within the casing of the well. During circulation, steam is injected down the long tubing string and exits into the annulus at the toe of the well. The steam then travels back to the surface through the annulus of the well. In the producer completion, there are two tubing strings; a long tubing string landed at the toe and a short tubing string landed at the heel. Steam is again circulated down the long tubing string. It then exits into the annulus at the toe and travels to the heel. At the heel, the steam travels back to surface via the short tubing string, versus the annulus in the injector completion. Petro-Canada is operating the MacKay River SAGD commercial project and is preparing for a project expansion. At MacKay River, the initialization of SAGD included three stages(1). The first stage involved getting steam to the toe. To do this, steam is circulated into the long tubing of both the injector and producer at equal pressure.