Whether it is for disposal or reuse, produced water treatment has become a major focus in both the water and the oil and gas industries during the past decade. Ceramic membranes, which were developed more than 30 years ago, are becoming an important technology for produced water treatment. In fact, it is becoming evident that these membranes offer some of the best values in the emerging areas of water management for the upstream oil and gas industry, as well as in refineries.
The key advantage of ceramic membranes over their polymeric counterparts is their ability to tolerate elevated levels of oil and suspended solids originating from extracted petroleum, and heir chemical and thermal robustness allowing them to be cleaned with harsh chemistries and high temperatures.
Water treatment in the oil and gas industry
Water treatment features prominently in every aspect of upstream, midstream, and downstream oil and gas operations. In midstream and downstream operations, particularly at upgraders and refineries, the requirement for water management is somewhat similar to other industrial problems.
In E&P operations, treatment of “produced water” – water that is co-produced when oil is extracted from the wells – is an area where water treatment is strongly integrated.
Whether it is for disposal or reuse, produced water treatment has become a major focus in the oil and gas industry during the past decade. A key enabling factor in reuse or recycle of produced water is the availability of suitable technology that allows treatment of the water in an economically viable manner.
Enhanced oil recovery (EOR) operations including water flooding, polymer or alkali-surfactant-polymer (ASP) flooding, thermal EOR (e.g., steam flooding), or hydraulic fracturing use water as the base fluid for reservoir stimulation. The water used in these EOR applications often needs to be treated to specific quality standards.
Generally, the water should typically be free of suspended particles, free oil and bacteria, and in many cases it should have low sulfate, boron, hardness, and silica content. Many EOR applications require “designer water” to optimally enhance oil production.
These EOR applications are actually changing the paradigm of water management in the upstream oil and gas industry. Such processes are now looking into utilizing the produced water as a source of injection water. This requires on-site treatment of the produced water to provide the designed characteristics that would allow the water to be used as the base matrix for formulation of the injection fluid.
In some segments of the EOR industry, such as hydraulic fracturing and steam flooding, the produced water treatment and recycle are so integral to the E&P operations, that these plants have sometimes been referred to as “water treatment plants that produce oil as a byproduct.”
Modular and mobile treatment technologies have become a highly attractive option in such produced water treatment scenarios.
Conventional produced water treatment in the oil and gas industry predominantly involves use of gravity based separation to remove oil and suspended solids. A variety of separation processes, such as free water knock out (FWKO), skim tanks, induced gas flotation, and media filters are mainstays of the deoiling and solid removal technologies in onshore applications. In offshore platforms, use of hydrocyclones followed by induced gas flotation is more common. In onshore operations, the produced water is often disposed through deep-well injection.
As disposal regulations become more stringent, or as newer reuse and recycle scenarios are being explored, these conventional treatment technologies seem to fall short in delivering the treated water to meet the level of purity mandated or required.
Removing the FOG and suspended solids from the water is rarely adequate for reuse. This necessitates incorporation of secondary, tertiary and advanced (desalination) water treatment technologies to adequately purify the water.
Ceramic membrane filtration
Ceramic membranes are made from layers of sintered metals, metal oxide and metal nitride materials. Individual modules typically comprise tubular monolithic elements with multiple feed channels running through each element. The flow enters and runs laterally along the multiple parallel feed channels at an elevated pressure. A portion of the feed fluid permeates from inside the feed channels through the porous walls and monolithic tubular support structure to the outside of the tubular element. There, filtered water is collected through ports that keep it isolated from the concentrate water (that which was fed into the element, but did not permeate). Figure 1 provides a few images of ceramic membrane modules.
A key issue in upstream operations arises from the occasional surges in oil concentration (sometimes referred to as gulps and burps). Higher concentrations of oil in the influent of conventional produced water treatment equipment translate into surges in the effluent oil concentrations from these operations. Membrane systems, however, owing to the inherent nature of barrier filtration, prevent any oil concentration surge in the filtrate even if the influent experiences intermittent surges in FOG concentration.
Numerous pilot studies have been conducted on ceramic membrane based filtration of produced water. These studies indicate that the ceramic membranes can be applied to feed streams containing 20 to 100 ppm free oil and gas, provided the filtration systems are designed to automatically manage the extent of membrane fouling and membrane cleaning.
Ceramic membranes have proven to be highly successful in filtration of bacteria and suspended solids in hydraulic fracturing flowback and produced water treatment. Tight ceramic membrane based nanofiltration is also suitable for removal of divalent cations and anions from the produced water to enable its use for production of low salinity flooding matrices, as well as feedwater for steam generators.
Ceramic membrane filtration systems can be designed to accept influent water originating from multiple sources in an onshore E&P operation, including the water phase effluent from the FWKO, water streams from treaters and skim tanks, or effluents from induced gas flotation vessels.
Properly designed ceramic membrane systems can potentially replace a combination of gravity based deoiling steps including the induced gas flotation and the oil removal filtration processes. They can also act as a polishing step for the produced water following primary deoiling by replacing the conventional media filters (such as nut-shell filters). Furthermore, the high level of polishing of the filtrate achieved in ceramic membranes can make the product water suitable for direct feed to a final water purification step such as demineralization (weak acid cation exchanger) or desalination (reverse osmosis) as depicted in Figure 2.
Advancements in technology
The “Achilles heel” of membrane technology is fouling. High fouling and variable influent water qualities are common sources of membrane system failures in oilfield and other challenging water treatment applications. Fouling clogs membrane pores, increasing the energy required to drive the process as well as the cleaning frequency, process downtime, membrane replacement and operating costs. Fouling also increases capital cost by requiring low flux operation (requiring more membrane area) and expensive pre-treatment (physical and/or chemical conditioning).
Fortunately, new water treatment solutions have been developed to alleviate this problem. For example, Water Planet, Inc. offers their IMS-5000 Integrated Produced Water Treatment Solution. Highly effective, affordable and reliable, the IMS-5000 system features a pre-integrated mechanical oil-water separator, robust ceramic ultrafiltration membranes with a unique hydraulic design, and automated self-adaptive process controls through Water Planet’s IntelliFlux Intelligent Flux Management Control System.
Up-front mechanical oil-water separation and slop oil dewatering protect downstream ceramic MF/UF membranes while enabling recovery of black oil. The “brains of the system,” IntelliFlux adapts and adjusts operating conditions and membrane cleaning protocols if the rate of membrane fouling increases or decreases. This automated, self-adaptive flux maintenance allows the system to function in the most severe fouling conditions.