Growth in the CSG industry has created significant expansion opportunities for the pipeline industry that covers both gas and water and deals with pressures ranging up to 15.3 MPa. Gas is extracted from a large number of wells at low pressure and this gas is saturated with and accompanied by water for most of the pipeline network systems upstream of the processing facilities.
Water management in these upstream gas pipeline networks represents a significant element of the day-to-day operations within the CSG industry.
Processing plant feeder lines typically operate at inlet pressures in excess of 2,000 kPa. These pipelines receive gas from field compressor coolers which has been cooled down to less than 55ËšC. This gas then enters the feeder lines as gas saturated with water at temperatures up to 55ËšC, then cools down towards ground temperature as it travels along the pipeline. The gas then condenses out water to form a two-phase fluid of gas and free water in the pipeline.
The water condensation rates can be as high as 125 L/TJ of gas transported, which, for an 80 TJ/d pipeline, equates to 10,000 L/d of water. The presence of water can create a number of issues, including loss of capacity due to increased pressure drop in the pipeline, flooding of downstream facilities during pigging or ramp up, corrosion of the inside of the pipeline, and excessively high levels of water can render pigging impossible.
Understanding gas sweeping and its principles
A number of options for managing water currently exist, including regular pigging, automated sphere and intermittent batch pigging, low-point drainage, liquid removal at field compressors, and gas sweeping.
Gas sweeping is possible where the system is designed and operated in a manner that ensures the gas velocity is sufficiently high for the gas to cause the free liquid to be carried along with the flowing gas, in effect sweeping out the liquids.
To appreciate the issues with high-velocity
gas sweeping, it is first important to understand a typical well production profile. The production flow typically commences from zero gas flow, which then rapidly ramps up to maximum capacity and tapers off as the production declines.
The following principles are considered to be fundamental when considering using designs that would use high-velocity gas sweeping for liquids management:
1. At any time, a pipeline should be able to be pigged without the risk of inundating downstream facilities with excess liquid. In other words, liquids should not build up beyond those levels which cannot be handled during any known operational activity. To ensure this, the amount of liquid held up in any pipeline should not exceed the working capacity of the end-of-line liquid-handling facilities.
2. Each pipeline has a critical flow rate (CFR). The CFR is defined as the minimum flow rate required to ensure the liquid hold-up in the pipeline is less than that of the liquid handling facility (LHF) working capacity. If for any reason the volume of liquid held up in a pipeline exceeds the capacity of the LHF, liquids can be removed by steadily increasing the flow velocity, provided the modelled liquid removal rate plus the combined liquid removal rates of the other pipelines that feed into the LHF is less than the emptying rate of the LHF and does not cause the LHF vessel to be over-filled.
3. Pipelines are generally sized to ensure the pipeline flows exceed the CFR for the majority of the life of each pipeline.
4. The amount of liquid hold-up in a pipeline is maintained below the level which could impact on the ability of the pipeline to be pigged taking into account upstream driving pressure, topography, static head, etc.
Investigative modelling
Two-phase steady-state modelling is conducted to prove that pipelines can be sized to give sufficiently high-flow velocities to keep the held-up liquid volumes less than the working capacities of the LHFs for the majority of the life of the pipeline and sufficiently low levels of velocity to not cause wall thickness reduction through erosion/corrosion mechanisms. Transient modelling is then used to cross-check the steady-state modelling results.
Modelling has determined for a typical range of pipelines varying between 300 mm and 600 mm in diameter, and lengths from 50 km to less than 1 km, the required velocities were typically in the 10-25 m per second (m/s) range.
Issues to consider
In addition to the usual control room functions, including managing the system to receive and deliver required nominations, monitoring and responding to alarms, etc., the operator would need to monitor the flow velocities in each pipeline to ensure the flow velocities are kept above the CFRs.
To provide the necessary operational guidelines, modelling would be required to determine the quantity of liquid build-up for various flows and times for each pipeline.
Having quantified the magnitude of liquid build-up, and on the assumption that the controlled ramp up of velocities option is available, the operator then needs to know what the liquid removal rates are for various flow velocities and durations, which would again require modelling.
High velocities
Velocities of up to 25 m/s required for liquid sweeping are well above traditional sales gas transmission pipeline velocities which are typically in the order of 5-10 m/s. Given that these higher velocities would
be required for using gas sweeping as a method of managing liquids, WorleyParsons confirmed that an investigation was required to establish if such velocities would be detrimental from a corrosion/erosion perspective.
The company’s investigation involved a review of available literature, practices used in hydrocarbon industries, an analysis of the reasoning behind standards like API 14E, and drew on experience from experts from within the worldwide WorleyParsons organisation.
A key outcome from this investigation determined that for non-solids service, the industry has adopted a variety of guidelines for limiting velocity, with the most common being API RP 14E. Based on the densities, pressures and temperatures likely to be encountered, API RP 14E allows velocities up 32 m/s without an inhibitor. It was noted that API RP 14E assumes continuous removal of the corrosion product films. Metal-loss mechanisms, including electrochemical corrosion and mechanical removal of metal with mechanisms considered included the following:
- Non-corrosive, solids-free fluids and gases – metal loss from liquid and/or gas bubble impingement (cavitation);
- Non-corrosive, solids-laden fluids and gases – metal loss from solid particle impingement;
- Erosion-corrosion, solids-free fluids and gases – metal loss from liquid and/or gas bubble impingement coupled with corrosion due to corrosive media; and,
- Erosion-corrosion, solids-laden fluids and gases – metal loss from solid particle impingement coupled with corrosion due to corrosive media.
Important matters for consideration are assumed corrosion rates, corrosion allowances, inhibitor availability, etc. It should be noted that as the higher gas velocities will impart higher shear stresses on the inhibitor, inhibitor shear limits will be an important criterion for consideration during inhibitor selection.
Noise associated with higher velocities also needs to be considered for the small sections of above-ground piping associated with the pipelines. This can be mitigated for these short sections by increasing diameter and corresponding reduction in velocity, or by lagging – externally lining pipe with a material to insulate against noise – if it is determined that the noise levels are an issue.
Gas sweeping is a technically feasible option for managing liquids in pipelines. Although control room management would need to be significantly more sophisticated to ensure liquids are managed effectively, there is the potential to save on capital expenditure and ongoing operational expenditure through the adoption of this option. A backup method for removing liquids may be required for cases of abnormal operating conditions.