Use of Evaporation for Heavy Oil Produced Water Treatment

Abstract

Abstract Heavy oil recovery processes have traditionally used "once through" steam generators (OTSG) to produce high pressure steam for injection into geological formations containing heavy oil. The heat given up by the condensing steam fluidizes the heavy oil and allows the oil/water mixture to be brought to the surface. The oil is recovered as product and the water, referred to as produced water, is de-oiled and treated for feedwater to the OTSG. The typical treatment method for produced water is warm or hot lime softening (WLS or HLS), filtration, and weak acid cation exchange (WAC). An alternative method of produced water treatment is vertical tube, falling film, vapour compression evaporation. This method:eliminates physical-chemical produced water treatment;results in lower life cycle costs;does not produce any softener sludge for disposal;minimizes the number and volume of waste streams requiring disposal;requires fewer maintenance materials and less maintenance labour;reduces the required amount of produced water de-oiling equipment;dramatically increases OTSG feed water quality, improving OTSG reliability; and,provides increased system availability and reliability. For steam assisted gravity drainage (SAGD) processes, which require 100% quality steam, it has the added advantage of producing water of sufficient quality for use in standard packaged boilers in lieu of OTSG. Packaged boilers are less expensive than OTSG, produce a much smaller liquid blowdown stream, and result in a boiler feed system which is 20% smaller than that of an OTSG. Introduction Heavy oil recovery requires large volumes of water commensurate in volume to the production of oil that it yields. Water is used in the form of steam to heat the geological formations that hold the oil. The oil is fluidized by the condensing steam and the oil/water mixture is pumped to the surface. The oil and water are separated. The oil is recovered as product and the water, referred to as produced water, is de-oiled and treated for reuse in the steam generator. Traditionally, "once-through" steam generators (OTSG) have been used to produce 80% quality steam (80% vapour, 20% liquid) for injection into the well. A relatively new heavy oil recovery process, steam assisted gravity drainage (SAGD), requires 100% quality steam to be injected into the well (i.e., no liquid water). To produce 100% quality steam using OTSG, a series of vapour-liquid separators are required to separate the liquid water from the steam. The 100% quality steam is then injected into the well. For both SAGD and non-SAGD applications, the produced water can generally be characterized as predominantly sodium chloride brine with high silica and minimal calcium and magnesium. High alkalinity, or carbonate, is present as well. The produced water generally contains about 4,000 mg/l total dissolved solids but can be much higher depending on the geological contribution. Dissolved and emulsified organics (oil) are present at a variety of levels, depending on the oil separation processes used.