Acoustic Emission and Guided Wave condition monitoring techniques promising candidates for casing integrity assessment

Comparative Assessment of Casing integrity using Acoustic Emission and Guided Waves condition monitoring techniques

Casings are generally large diameter pipes, which are inserted in a drilled section within a monitoring well. The purpose of the casing is to protect and support the well stream. Due to the extreme environments and due to the chemical substances, which are released in the subsurface, casings suffer from different failure modes such as corrosion, erosion or cracks. For that reason, there is a need to monitor the integrity of the casings to be able to identify any failure well in advance before it propagates along the whole structure.

Currently the techniques, which are used for well integrity monitoring, are the following:Mechanical Integrity testing (Test to verify that there is no leakage and that the mechanical components of the well are functional)

  • Pressure and Temperature monitoring
  • Underground water sampling
  • Distributed temperature sensing

Mechanical integrity testing is associated with leakages and determination of the movement of the fluid through the different casings. Pressure and temperature monitoringis related more to the assessment of the production performance and optimization of the production rates.Underground water samplinginvestigates the groundwater quality with a purpose to monitor the aquifer within a borehole. Distributed temperature sensing makes use of optoelectronic devices that measure temperatures with the use of optical fibres targeting the enhancement of oil and gas production. All the above mentioned methods are used either for monitoring of a specific part of a borehole or for generic monitoring of the wells with a view to ensure safe operating conditions and enhance the final production.

TWI Ltd within the S4CE collaborative project would like to develop a solution for examining the integrity of the casings of wells with the use of one of the most suitable condition monitoring techniques namely acoustic emission (AE) and guided waves (GW). One of those techniques or a combination, is planned to be used to monitor the integrity of the casings specifically, and identify any corrosion or cracks developed during the operation of the well.

Casing monitoring is extremely useful due to the fact that it offers information regarding the well safety and performance, regarding any potential leakage or corrosion that could lead to contaminated water of the aquifer and provide early alarms on induced seismicity.

In this project, TWI aims to provide complete monitoring solutions so that costs related to inspection and maintenance are going to be reduced. Structural health monitoring is associated with the overall structure of assets and involves the observation of a complete structure over time. With the use of condition monitoring, the different components are examined on an individual basis with the use of different techniques such as Acoustic Emission or Guided Waves. Acoustic emission is the release of elastic energy due to growing damage under the effect of a stimulus. Acoustic emission can be used to test the structure of interest at any stage of life cycle, provides real time evaluation and the ability to discern between developing and stagnant defects. The technique of Guided Waves is one of the latest methods in the field of non-destructive testing evaluation. It employs mechanical stress waves that propagate along an elongated structure guided by the boundaries of the structure. This allows the waves to travel long distances without significant loss of energy. The advantages of Guided Waves technique is the detection of external or internal metal loss, fully recorded data and reduced costs of gaining access due to the fact that inspection can take place at elevated locations with minimal need for scaffolding.

TWI is currently testing the potential of the aforementioned condition monitoring techniques with a view to be able to conclude regarding the following:

  • Identification of appropriate sensors for casing monitoring
  • Necessary data processing of results acquired during field testing
  • Limitations of the relevant instrumentation keeping in mind the field complications as well as the environmental conditions.

For that purpose, testing of a typical production steel pipe used in monitoring wells has been tested by means of acoustic emission. Acoustic emission sensors were placed on the structure of interest (2 meters pipe) with a view to calculate the rate at which the acquired signal decreases in intensity due to long distance travelling across the structure (attenuation rate). To identify that, a pencil lead breakage experiment was carried out, as it is show in Figure 5. During a pencil lead breakage experiment an intense acoustic signal is generated, quite similar to a natural acoustic emission source, which the sensors detect as a strong burst.

Figure 1: Typical production steel pipe used in monitoring wells. The four acoustic emission sensors can be seen along the pipe acquiring data during the application of the pencil lead breakage experiment.

Based on the first experimental results the attenuation rate for that pipe was equal to 4.34 dB/m revealing that there is not significant change of the amplitude as the wave travels across the pipe. This is something promising in terms of real time monitoring and the necessary number of sensors to check that structure.

Next steps towards the casing integrity monitoring are to run finite element simulations to assess the applicability of Guided Waves technique for this particular application. Apart from that, TWI would like to test the Acoustic Emission technique in real field conditions. A typical monitoring well consists of different layers of steel casings surrounded by cement. Cement acts usually as an attenuation factor in both Acoustic Emission and Guided Waves. For that purpose Acoustic Emission experiments will be carried out on a pipe surrounded by concrete to identify its effect
on the acquired signal. The pipe that is going to be tested is shown in Figure 2.

Figure 2: Steel casing surrounded by concrete

Casing integrity monitoring is an extremely challenging opportunity to identify the most suitable technique that could be applied for that purpose. S4CE project, offers the chance to test the final monitoring system in real site conditions in geothermal wells, which are currently drilledin the United Kingdom.

Authors: Xiaofei Cui, Sofia Sampethai, Jialin Tang