Modeling of Steam-Liquid Flow Inside and Around SAGD Wells During Startup Stage

摘要

Canada's oil sands deposits in northern Alberta are estimated to contain more than 1.35 trillion barrels (～215 billion m3) of bitumen. Such a large resource base constitutes the world's second largest oil reserves (behind only Saudi Arabia's), even allowing for (relatively) low recovery factors. In-situ recovery technologies for these deposits, in view of the extremely viscous bitumen typically existing in them, commonly require thermal heating and/or solvent dilution to mobilize the bitumen and enable it to be produced. SAGD and CSS are currently two technologies accounting for the bulk of in-situ bitumen production in Canada. The SAGD process, utilizing two long parallel horizontal wells (upper well for steam injection, lower well for production), involves typically three sequential stages: Start-Up (Circulation), (proper) SAGD, and Wind-Down (or Blow-Down). In the Start-Up stage, steam is circulated through the two horizontal wellbores for sufficient time duration to create communication between them, prior to converting to SAGD operation. During this stage, complex heat and mass transfer phenomena occur in counter-current flow in the tubing and annulus (or tubing), and between the wells and adjacent reservoir. This paper first reviews briefly the common approaches currently utilized to model the Start-Up stage for a SAGD well pair. Next, it presents simulation results of transient steam-liquid flow inside SAGD wells, and temperature changes at the inter-wells mid- point reservoir, for a case typical of Athabasca SAGD project. Discussion – based on the distribution of steam quality, pressure and temperature losses along the axial well length – is provided, as is the estimation of the duration for effective SAGD circulation for the cited example.