Achieving Zero Liquid Discharge in SAGD Heavy Oil Recovery

Abstract

Abstract Designing a plant for maximum water recycle and reuse (i.e., zero liquid discharge) is not the mystery it once was. Planning to implement zero liquid discharge right from the start wins faster community acceptance, streamlines the permitting process, eliminates the need for deep well injection or other disposal methods, and minimizes make-up water requirements. Over 100 mechanical zero liquid discharge systems are now in operation worldwide using Ionics RCC Brine Concentrator and Crystallizer technologies, including two SAGD heavy oil recovery projects currently underway in Alberta. Wastewater is converted by the Brine Concentrator to extremely pure distilled water for reuse in the steam generator or other process applications. The waste from the Brine Concentrator is reduced to dry solids in the Crystallizer, while recovering the remaining wastewater for reuse. This paper will discuss the various applications of Brine Concentrators and Crystallizers as they apply to the treatment of SAGD heavy oil recovery produced water. Specific examples will be used to illustrate the wastewater recycling process and demonstrate how the zero liquid discharge system is integrated into the SAGD heavy oil recovery process. Introduction Traditional Heavy Oil Recovery Process Water treatment is a necessary operation in the heavy oil recovery process. In order to recover heavy oil from certain geologic formations, steam is required to improve the mobility of the oil. Traditionally, "once-through" steam generators have been used to produce 80﹪ quality steam (80﹪ vapour, 20﹪ liquid) for injection into the well to fluidize the heavy oil and allow the oil/water mixture to be 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. The produced water, which must typically be < 8,000 mg/l of total dissolved solids (TDS) as well as meeting other specific constituent requirements, is typically pre-treated using hot or warm lime softening, a weak acid cation system, and other processes prior to use in the steam generator The SAGD Heavy Oil Process A relatively new heavy oil recovery process, referred to as SAGD (Steam Assisted Gravity Drainage), requires 100﹪ quality steam to be injected into the well (i.e., no liquid water). To produce 100﹪ quality steam using once-through steam generators, aeries of vapour-liquid separators are required to separate the liquid water from the steam. The 100﹪ quality steam is then injected into the well. The separated water is then either disposed of via deep-well injection or, if deep well injection is not possible, the separated water may be taken to Zero Liquid Discharge (ZLD) using a Brine Concentrator and/or a salt Crystallizer. Brine Concentrators and Crystallizers Prior to addressing the details of applying Brine Concentrators and Crystallizers to the SAGD process, a brief history of evaporation and a technical explanation of the Brine Concentration and Crystallization process is presented. Following these technical descriptions, specific examples of Brine Concentration and Crystallization in the SAGD industry are presented.