Produced water has not been considered a real issue in the oil&gas industry for long time, until hydrocarbon and metals pollution of sea water reported by fishermen caused restrictions to off-shore activities. These concerns were a major driver to start research activity on produced water treatment for sea water discharge of purified water.
At the same time, pure water availability and waste water disposal for on-shore activities in remote locations aroused more interest in this area too. It is important to notice that purified water reuse may be a relevant driver for on-shore location, especially where fresh water availability is a critical issue.
Cross-flow membrane separation technologies (ultrafiltration, nanofiltration, reverse osmosis) have been playing a major role for this application for different reasons:
- low energy consumption and low capital investment;
- high selectivity and possibility of water reuse;
- small space requirement
- continuos operation and no use of chemicals.
Sepra, a company with more that 20 years expertise in membrane separation technologies, participated in important research projects in the field of produced water treatment, starting from laboratory scale up to industrial installations. Original development involved the use of different technologies on small batches, namely 1000 liters.
After this a containerized system for field tests was built. This allowed continuos operation studies in different oil and gas fields. Each test was carried out over a 2-4 weeks period.
In order to consolidate field test results, and industrial scale installation was built and has now been operating for few years. These test activities allowed Sepra to develop specific expertise on produced water treatment with membrane technologies.
Based on above mentioned experience, we can offer solutions for the oil&gas industry to treat produced water according to different needs and using a wide range of technologies. The goal of these systems is to produce purified water that can be reused or discharged to the environment and to concentrate contaminants in small volumes, for further disposal.
The first need related to produced water purification and reuse is removal of suspended solids, including small metal oxides particles, formed mainly because of oxygenation and pH changes.
We can make available a wide range of pretreatment technologies, which include:
- cyclone separators;
- sand media filtration;
- catalytic media filtration.
A specific approach will be designed based on site requirements. Pretreatment is a very important step to allow further treatment with more sophisticated technologies.
Heavy hydrocarbons are among the most difficult contaminants to remove from produced waters.
Ultrafiltration is a membrane separation technology that allows separation of higher molecular weight substances, such as oil and even waxes.
In order to control membrane fouling, we use specialty polymeric membranes that incorporate a extremely hydrophilic surface; for more extreme applications we also use tubular ceramic membranes.
Media absorption is used to remove lower molecular weight hydrocarbon, often in combination with nanofiltration/reverse osmosis that are described in the following points.
Regular activated carbon vs special media can be selected according to the specific site requirements.
Nanofiltration is a membrane separation technology that allows separation of lower molecular weight substances, like smaller organics substances and metals, still allowing sodium chloride passage.
This way, we can achieve substantial COD reduction and produce clean salt water to be returned to the sea environment or to be injected in the well.
Nanofiltration also allows the removal of bivalent salts such as sulphates, reducing scaling potential of purified water.
Reverse osmosis is a membrane separation technology that allows complete water demineralization.
It is the most effective purification approach, but it is limited by osmotic pressure of the solution to be treated.
Normally, reverse osmosis is an effective approach for treatment of lower salinity produced water.
While approaching a new site, the first important goal is to characterize produced water, in terms of quantity, physical properties and chemical properties.
We recommend checking the following parameters:
- pH, temperature;
- suspended solids;
- conductivity, hardness,
- main anions (chlorides, sulphates, alcalinity);
- main cations (sodium, calcium, magnesium);
- other metals;
- oils and waxes.
After water analysis, it is possible to make a feasibility study, that includes a budget evaluation of capital cost and operating costs of a produced water treatment system.
In case the feasibility study leads to interesting results, we normally recommend to plan test work, that may be organized on a lab scale or in the field, the results of which are used for industrial scale-up.