The impact of steam quality, circulation rate and pressure difference between the well pair during SAGD initialization using steam circulation was explored through the use of numerical simulations employing a discretized wellbore model. These operating parameters appear to affect uniformity of reservoir heating, occurrence of steam breakthrough and time required to establish communication between the well pair.
The simulation results indicate that, for the given tubing and liner sizes and reservoir properties, relatively lower circulation rates at high steam quality are more favourable for faster initialization and development of uniform temperature between the horizontal well pair. At lower steam qualities, however, higher circulation rates appear more favourable. The use of high steam quality in combination with high circulation rates leads to slower rates of initialization, less uniform heating along the length of the wells, and possibility of premature steam breakthrough at the heel. It was also found that having a higher steam quality in the lower well than in the upper well could lead to faster initialization and more uniform heating between the well pair. A small pressure difference, offsetting the natural hydraulic pressure (50kPa), appears to be more favourable for faster and more uniform initialization.
Start-up or initialization of a well pair can be a challenging part of SAGD operation but is one of the least studied. Start-up procedures may have a significant impact on the subsequent production history and, in the extreme, can result in damage and/or abandonment of the well pair. Producers use a variety of strategies that balance the need for rapid start-up to production against maximizing the chances of better long term SAGD performance. These start-up strategies are normally reservoir specific and not available through public database so detailed analysis of these strategies is not the objective of the current work. Rather, we seek to determine, for the commonly used steam circulation strategies, how a range of operating parameters might affect the generic start-up behaviour of SAGD well pairs.
Start-up of a gravity drainage process in a very viscous reservoir using a pair of parallel horizontal wells requires the establishment of oil mobility between the well pair. In the case of SAGD, this requires that the intervening fluid between the well pair be heated to a temperature sufficiently high to cause the oil to flow from upper well (injector) to a lower well (producer). This is normally achieved by initially circulating steam in each well. The horizontal portion of each well can consist of tubing and liner, which provides two possible channels for fluid flow. In the simplest case, hot fluid can be injected into a steel tubing at the heel, flow from the heel to toe through the tubing, and then out through the annular space between tubing and liner from the toe towards the heel. With such a flow arrangement combined with heat transfer, from fluid in the tubing to fluid in the annulus and from annulus to formation, the thermodynamics combined with hydrodynamics can be fairly complex to describe.
