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ESG: Microseismic monitoring

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LNG Industry,

Around the world, access to secure and uninterrupted natural gas supplies has become a geopolitical priority. Thousands of commercial and government owned sites for storage of natural gas, LNG, LPG, and crude oil are in operation, or are under development. Domestic access to gas storage can help countries better match supply and demand, guarantee energy security in case of supply disruption and/or unexpected increases in demand, and optimise gas infrastructure.

In all cases, operators need to ensure that the gas sequestered will remain contained and not pose any risks to the safety of nearby residents or damage to the surrounding environment. Microseismic monitoring is a tool that has been used extensively in the mining and geotechnical industries for over 30 years, and has gained incredible traction in the last 15 years with the growth of unconventional oil and gas production. This monitoring technique can be easily applied to a variety of hydrocarbon storage applications including observing the interaction of LNG storage tanks with nearby geological structures and faults, and trigger alarms when the storage facilities are exposed to vibrations that may compromise its structural integrity.

Microseismic monitoring

Many industries use microseismic monitoring to evaluate changing stress conditions and geomechanical deformation within rock. Common applications include mine monitoring, oil and gas production, hydraulic fracturing, enhanced geothermal, waste injection, carbon sequestration and gas storage. Often dams, bridges and storage tanks are exposed to vibration levels that exceed design thresholds, from activities such as blasting or earthquakes. Seismic monitoring systems can detect these exceedances and prompt the evaluation of the structural health of the infrastructure by sending out alerts to operators.

Seismic monitoring of human-induced microseismicity has been employed commercially for over 20 years. Early microseismic monitoring activities were focused on rockburst and roof fall prediction in underground mines, while the first investigations into larger magnitude induced seismicity were performed at the Rocky Mountain Arsenal waste injection site in Colorado in the 1960s. The techniques for evaluating induced microseismicity have also been successfully applied to hydrofracturing processes in oil and gas or enhanced geothermal operations, evaluation of slope stability in open-pit mines, and structural health monitoring of dams, bridges and hydrocarbon storage facilities.

An array of sensors including geophones and accelerometers passively listen to the sound of rock as it shifts or breaks in response to changing stress conditions within a rock mass. These arrays monitor microseismic activity occurring near the structure, feeding information in real-time so that these events can be mapped and visualised in space. The occurrence of microseismic activity can be used to trigger alarms that notify operators of abnormal behaviour, or can be used to interpret stability and pinpoint any potential risks associated with operation.

Unusual clustering of microseismic events, or the observation of larger magnitude seismicity (magnitudes above 0), consistent with slip along a fault may indicate the activation of a geological structure and warrant further investigation using advanced microseismic techniques.

Typically, induced seismicity is measured on micro-scale at levels, equivalent to very small earthquakes, measuring from -3 to +1 on the magnitude scale. More recently, “hybrid” microseismic systems combined information from surface-based regional seismic sensors to incorporate data from larger magnitude seismicity (up to +4Mw), effectively widening the range of detectable seismic signals.

Identifying the location and size of these seismic “events” and evaluating seismicity temporally and spatially as it relates to hydrocarbon storage operations serves as an essential tool to quantify and understand stress-induced rock mass behaviour.

Sensors are connected to 32-bit Paladin™ data loggers that receive and digitise seismic information relating to individual microseismic events. The Paladin data acquisition units are housed in junction boxes within stand-alone stations complete with power supply (i.e. solar) and GPS for time synchronisation of the signals. Each Paladin™ unit is a web-enabled device (for remote access/calibration) and is capable of providing continuous and/or trigger-based data acquisition. Data is then transmitted to a central acquisition workstation via wireless radio or wired communication, where seismic events are triggered in real-time and events are located in 3D (Figure 1).

Monitoring LNG storage terminals

LNG arriving at receiving and gasification terminals is pumped from specialised tankers into customised storage tanks designed to keep the gas cooled to liquid form at temperatures of approximately -260°F. These tanks are heavily insulated to keep the LNG cooled and contained until it is returned to its original gaseous state for distribution to customers via pipeline. The tanks consist of an inner shell, an interstitial space filled with insulation, followed by an external outer shell of concrete and a roof made of reinforced concrete with a carbon steel liner. The outer shell is designed to hold the entire contents of the inner tank and allow for controlled venting of vapours.

To effectively evaluate storage tank structural integrity, operators need to be aware of the impact of a nearby earthquake on the structures. Vibrations associated with strong magnitude events such as earthquakes could potentially impact the integrity of the structure, causing cracks in foundations or walls, or sudden settlement or subsidence beneath the structure. According to building codes, the construction design must withstand a certain threshold of vibration, which is often characterised by levels of peak-particle velocity (PPV) or acceleration (PPA). It is essential that operators have the ability to monitor generated frequencies to help mitigate risk to nearby structures caused by their operations. Using seismic monitoring equipment, PPV and PPA values associated with any seismicity near the storage tanks are measured, and the system will trigger an alarm if the signals exceed thresholds defined as critical to the structural integrity. Alarms can be configured to activate relays, lights and audible warning devices, or to send emails or text messages to appropriate personnel.

ESG Solutions was commissioned to monitor seismicity associated with earthquakes at an LNG receiving and regasification terminal which is equipped with state-of-the-art storage tanks and gasification facilities and is located along the Atlantic coast of North America. ESG designed and installed a system that consisted of two triaxial force-balanced accelerometers to accurately evaluate the PPA levels at the facilities. Sensors were installed at two indoor locations, one near the LNG storage tanks and another inside a control room. The sensors were equipped with a stainless steel base plate and bolted to a flat surface such as a cement platform to secure them in place. Sensors are also commonly installed outdoors, either in shallow pits with a cement base, or secured to a cement platform on the ground surface and surrounded by an enclosure to shield the sensor from excessive surface noise (Figure 2). The sensors are connected to a single junction box containing a Paladin™ digital seismic recorder, where seismic events are triggered and processed automatically to determine measured PPA.

In the event of a large seismic event for which and vibration thresholds of the LNG tank structures are exceeded, a web relay is used to activate an alarm warning for various threshold settings. If the PPA values measure 1 m/s2, 1.8 m/s2 or 2.5 m/s2, the alarms will indicate the vibration as Moderate, Severe or Excessive respectively, and the operators can, in turn, initiate a visual inspection of the tank structures as required.

Ensuring containment integrity for stored hydrocarbons is essential to protect the health and safety of workers and nearby residents, as well as the surrounding environment. Methods such as microseismic monitoring present an opportunity to observe the state of the surrounding rock mass and storage structure, allowing operators to benefit from early warning of abnormal conditions that may impact containment integrity at their LNG terminals.

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