The objective of this paper is to show the effectiveness of a modern integrated logging suite in well integrity diagnostics, or, particularly, in identification of channelling and location of potential leaks in casing and other downhole equipment in underground gas storage (UGS) reservoirs of different types: an aquifer, salt dome, or depleted gas reservoir.
To check the integrity of UGS wells, a conventional integrated downhole logging suite supplemented with High-Defenition Spectral Noise Logging Tool (SNL-HD) was run. The Spectral Noise Logging survey was performed using a new generation SNL-HD tool which is capable of recording acoustic data in a wide range of amplitudes (90 dB) and frequencies (8-58,600 Hz). In addition to the SNL-HD, High-Precision Temperature (HPT) survey was run to identify and study thermal anomalies caused by fluid flows in the reservoir and the wellbore. Pulsed Neutron logging (PNL) method was used to determine gas and water content in the reservoir and microannuli between casing and cement sheath.
The SNL-HD was found highly informative in assessing the integrity of operating wells in all UGS types, especially when used in combination with HPT tool and pressure gauges. For example, the SNL-HD successfully identified and distinguished noises generated by gas flows through the reservoir and behind casing in an UGS located in an aquifer. PNL tool detected gas in cement sheath fractures and microannuli. For the UGS located in a salt dome with a partially washed-out cavern, SNL-HD identified the cause of sustained annulus pressure (SAP). Pulsed electromagnetic defectoscope (EmPulse) detected leaks which coincided with casing collar locations. HPT survey proved to be uninformative because of low-rate casing and annular flows.
In the UGS located in a depleted gas field, the primary objective was to identify the source of increased water cut, which was successfully done using SNL-HD and then confirmed by other logging techniques. Based on the survey results, workovers were carried out in the surveyed wells.