Water is considered a byproduct of oil and gas production and must be carefully managed. This water is usually referred to as ‘produced water’ and is the largest volume waste stream associated with oil and gas production. Because produced water was viewed as a waste by product historically, the most commonly practiced management strategies were aimed at disposal, rather than beneficial use.
Produced water is generated from most of the nearly one million actively producing oil and gas wells in the United States. The cost of managing such a large volume of water (21 billion barrels in 2012) is a key consideration for oil and gas producers; however it is also important to consider produced water management strategies in the context of overall water supply and economics for the benefit of all. Most oil and gas producers have installed, or are installing, pipeline infrastructure to gather produced water from wells. Since these pipelines contain produced water (which is hazardous to the environment), the ability to prevent and detect pipeline leaks is critical.
The pipeline infrastructure gathers produced water from temporary storage available at each well pad site and transfers (using automatic pumping) the produced water to temporary storage at either a disposal well location or at a central treatment facility. This pipeline infrastructure typically has some unique characteristics:
- Gathering network with many inlets (well pads) and many outlets (injection or treatment sites);
- Typically monitored using a slow update time in SCADA (minutes at best);
- Typically only monitored using flow and pressure measurements at inlets and outlets;
- Quality (i.e. specific gravity) is variable, for individual well, well to well, field to field, region to region; however quality measurements are typically not available for monitoring in SCADA for each well;
- Intermittent flow or no continuous flow that could create air pockets in the network;
- Bi-directional flow dependent on supply/demand in the gathering network.
Preventing a leak before it happens is always more beneficial than reacting to a leak. A produced water network is typically a small diameter pipeline network, which means cleaning the inside of the pipeline is problematic and pipelines could therefore become clogged with deposits. This will have an impact on pressures in the pipeline. Accurately monitoring these pipeline pressures for unexplained pressure changes is a good way to operationally prevent leaks in the produced water network. This can be done using Supervisory Control and Data Acquisition (SCADA) monitoring methods or by utilizing a Real-Time Transient Model (RTTM).
Density of water unknown
Since the produced water varies from well to well, and it gets mixed within the pipeline network, the density of the produced water is essentially unknown in the pipeline since the density is typically not measured at each inlet. However, the hydraulics associated with this mixed produced water still behave similar to other liquids. This means that by assuming the produced water in the pipeline is not compressible and by using an average density for the produced water, it is possible to create a Real-Time Transient Model. This model can be used for pressure monitoring to detect increases in pressure in the pipeline at locations where there are no measurements, providing a chance to prevent a leak before it occurs.
Detecting a leak in a produced water pipeline network can be done using internal or external leak detection methods. External leak detection methods attempt to detect the leak on the outside of the pipeline using techniques that are:
Installing sensing-based or imaging based equipment for external leak detection along a pipeline network for gathering produced water quickly becomes expensive, although the resulting capability might meet the oil and gas producer’s leak detection requirements.
By comparison, internal leak detection methods, also known as Computational Pipeline Monitoring (CPM), detects leaks by relying on instrumentation on the pipeline. When the standard CPM methodologies for leak detection outlined in American Petroleum Institute (API) Recommended Practice (RP) 1130 are evaluated against the characteristics of the pipeline network for gathering produced water as outlined in this article, the difficulty of attempting to do leak detection on a produced water gathering pipeline network. It should, however, be clarified that if the produced water is transported in a single pipeline, this would resolve many of the issues identified above. Similarly, if the infrastructure associated with the produced water network is updated to allow faster scan rates for measurements to SCADA, then better leak detection could be achieved. It should be also be noted that, in general, more pressure readings in the pipeline network adds greater benefit than better pressure readings.
Detection in pipelines difficult
Leak detection on produced water pipeline networks is also difficult because most leak detection applications look for the smallest leak in the shortest amount of time with the inherent allowance of false alarms. By comparison, if the focus is on getting a highly certain indication that uniquely occurs when there has been a large volume rupture or high rate commodity release, the problem become less insurmountable. Including a focus to look for ruptures as well as smaller leaks provides oil and gas companies extra insurance, as they would have a primary and a secondary (multi-tiered) leak detection system to rely upon.
In many ways the produced water pipeline network corresponds to the water distribution networks in topology, the difference being that the water is contaminated. Water distribution companies use techniques that create flow control zones and then monitor for leaks/ruptures within any of the zones before giving an alarm. Similar techniques could also be useful on produced water networks, especially where instrumentation is scarce.
In today’s volatile world, oil and gas producers can benefit from looking at produced water as something more than a waste. If produced water is managed efficiently, it could extend the life of the hydrocarbon well from a pure commercial operation point of view.
When oil and gas producers transport produced water by pipeline, it is contaminated water and as such must meet the regulatory requirements for hazardous liquids. Prevention and detection of commodity releases, even if the commodity is produced water, is still important.
Well density and growing produced water volumes continue to increase; a produced water pipeline network could provide sustainable and efficient produced water gathering and delivery solutions for oil and gas producers.
It should also be noted that pipeline leaks have occurred and have been mishandled, in spite of the installed leak detection system functioning as designed. Humans are making operational decisions based on the information they are presented, along with their knowledge of the current operational circumstances. In order to improve the likelihood of success for leak detection on a produced water network, a thorough analysis should be performed prior to installation.