Oil Removal From Oilfield-Produced Water By Cross Flow Ultrafiltration

Abstract

Abstract The technical and economic viability has been determined for crass flow ultrafiltration of oilfield-produced water with the goal of producing oil-free water for steam generation. Experiments with produced water supplied from various oil producing sites of western Canada have been performed with successful permeation rates and permeate quality. The resistance to membrane fouling and other factors that decrease permeation rate have been determined in long-term experiments. These results have been used to prepare a cost survey of the cross flow ultrafiltration process, and this has been compared to conventional treatment. The results indicate the level of performance that must be maintained in the ultrafiltration process to be economically advantageous. Introduction Enhanced oil recovery (EOR) operations in western Canada often employ stearn to stimulate reservoirs. These steam stimulation operations require large volumes of water for steam generation and injection into the oil-bearing reservoirs. They generate large volumes of produced water which is contaminated with oil, dissolved organic compounds, and inorganic material(1). Because many recovery operations in western Canada are located in water deficient or environmentally sensitive areas, several EOR recovery operations are recycling a portion of their produced water. Oilfield steam boilers that generate 80% quality steam (80% Vapour, 20% liquid) can tolerate minor amounts of total dissolved solids (TDS), hardness, and silica, but require total removal of the oil(2). Conventional treatment for the removal of the oil is limited by the presence of very stable oil-in-water (o/w) emulsions, stabilized by the presence of natural or added surfactants(3, 4). As well, subsequent operations of silica removal and softening perform best when the oil has been removed. Some EOR operations use large amounts of water, sometimes as much as 2 m3 to 10 m3 for every cubic metre of recovered oil. Some of this water may be present in the recovered oil as a water-in- oil emulsion while the balance may be present as a distinct water phase with an o/w emulsion. The produced fluid is generally treated in a free water knock out (FWKO) tank and a high-temperature separator (HTS) to separate as much of the oil and water as possible. Water from the FWKO and HTS is usually combined and sent to a skim tank where some residual oil is removed and recycled to the oil recovery part of the process. The remaining water, with some oil, is often disposed of by deep well injection (below the oil bearing formation) or it is sent to further treatment to prepare it for steam generation. Further treatment may consist of final oil removal by induced gas flotation (IGF) and filtration followed by softening operations such as hot lime softening and ion exchange. All of the oil removal operations with the exception of FWKO require large amounts of demulsifying chemicals to destabilize the emulsions. Occasional process upsets and changes in the nature of the emulsions can cause the failure of these units to remove oil from the produced water, and results in the passage of slugs of oil with catastrophic effects on the subsequent de-oiling and water softening operations.