DEOXYGENATION OF COAL SEAM GAS PRODUCED ASSOCIATED WATER FOR MANAGED AQUIFER RECHARGE UTILISING MEMBRANE TECHNOLOGY
A Practical Application of Gas-Transfer Membranes
B Murphy, K Yerrell
Publication Date (Web): 31 July 2017
Coal seam gas (CSG) is formed nearly entirely of methane (CH4) and can be gas and in a near-liquid state in the pores of coal and adsorbed onto the coal surface. The gas is secured in its location with the surrounding pressure of the reservoir and water. To capture the gas the water pressure within the coal seams must be released. Hydraulic fracturing is carried out to achieve this.
The volume of associated water involved in CSG extraction varies and is dependent on the hydrogeology, but recent estimates are in the order of up to 300 GL/Yr. This water requires treatment so it can be used for managed aquifer recharging (MAR). The receiving hydrogeological properties of the aquifer are very important. The compatibility of the injectate must be in-line with the receiving water so as to mitigate the risk of encrustation, precipitation, scaling and bio-fouling. This is achieved through conventional physico-chemico treatment and reverse osmosis filtration.
Redox, or the state of reduction – oxidation, can require an amendment to the dissolved oxygen content of the associated water. The amount of oxygen (O2) dissolved in liquid can be derived from Henry’s Law. This states that “at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid”. To define the partial pressure of a gas we look to Dalton’s Law, which states that “the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases”. At 25 degrees Celsius there are 8.52 parts oxygen per million parts water.
Gas transfer membranes differ from standard liquid-solid separation in that they do not operate at high differential pressures in the liquid phase. The driving force is instead instigated at the boundary wall of the membrane, which consist of hollow hydrophobic polypropylene fibres. This allows for mass transfer between the phases without one phase infiltrating another.
The MAR deoxygenation package was designed to reduce dissolved oxygen (DO) in the water prior to re-injection. The required reduction in DO was to achieve a discharge level of less than 0.5 mg.L-1 (500 ppb).
The deoxygenation plant consisted of several major components including eight 90 kW multi-stage centrifugal feed pumps, micron filters, HMI/PLC/DCS, analysis instrumentation , membrane contactors, air compressors, air dryers, air accumulator, PSA nitrogen generator, and a complete clean-in-place system.
The efficacy of utilising gas-transfer membrane contactors for dissolved oxygen removal was established during the course of the project. Overall the MAR deoxygenation package operated to design levels at a flowrate of 375 kL/hr against a pressure of 2100 kPa and a reduction in dissolved oxygen of 98.4 % to a value of 0.098 mg.L-1 of DO in the treated CSG associated water. Plant operating analysis elucidated on the importance of vacuum line integrity to the process and that the purity of the nitrogen sweep gas and vacuum pressure were vital to high-end performance.
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