Operational Data From the World's First SAGD Facilities Using Evaporators to Treat Produced Water for Boiler Feedwater
Authors: Heins W.F. 
Published: March 2010

Many new Steam-Assisted Gravity Drainage (SAGD) heavy oil recovery facilities have recently shifted from the use of warm or hot lime softening, filtration and weak acid cation (WAC) ion exchange to pretreat de-oiled produced water to an approach using falling film, mechanical vapour recompression evaporation to produce steam generator feedwater. This method of SAGD-produced water treatment is much simpler to operate, is more cost effective and results in significant increases in equipment reliability, on-stream availability and, ultimately, oil production. There are currently about 14 such evaporators operating, under construction, or in various stages of delivery in Alberta and overseas. Several of these evaporators produce feedwater for standard drum-type boilers rather than the traditional Once-Through Steam Generators (OTSG) due to the high level of water purity obtainable with the evaporative approach.

This paper provides data from operational facilities, including evaporator distillate quality, heat transfer information, fouling rates, cleaning frequencies, energy and chemical consumption, and other technical and operational data.

In conjunction with this evaporative produced water treatment process, some facilities have taken the additional step of recovering all liquid waste streams for re-use in the plant, resulting in zero liquid discharge (ZLD). Designing the facility for ZLD eliminates the need for deep well injection, minimizes make-up water requirements and simplifies the permitting process.

Introduction

The recovery of heavy oil from oil sands formations requires large volumes of water; sometimes three times the amount of water compared to the oil recovered. In steam-assisted gravity drainage (SAGD) facilities, 100% quality steam is injected into the well to heat up the formation and get the heavy oil to flow. Oil and condensed steam are brought to the surface where the oil is separated and the condensate, or produced water, is treated and recycled to produce the steam. This huge water demand and ultimate conversion to steam requires the maximum amount of recycle potential from the water.

Produced water derived from oil recovery processes (SAGD and non-SAGD) can be characterized as predominantly sodium chloride brine with high silica and minimal amounts of hardness. High alkalinity is present as well. Overall concentration of the water can range from about 1,000 mg per litre TDS to over 10,000 mg per litre, and there is always some amount of organic material present. The method of converting this water to boiler feedwater must be carefully evaluated, both technically and economically.

This paper discusses an evaporative method of deriving high quality boiler feedwater from produced water. A comparison is made to the 'traditional' method of warm (or hot) lime softening combined with weak acid cation exchange (WLS/WAC). A complete evaluation of these two approaches must also include the steam generation equipment. Having high quality boiler feedwater, as derived from an evaporative process, allows the use of drum boilers in lieu of once-through steam generators (OTSG). Since the WLS/WAC system does not produce a high quality water, OTSG must be used as these boilers are more tolerant of poorer quality feedwater.