Behind Casing Gas Identification Using Ultrasonic Wireline Logs: An Overview of Multiwell Field Plug and Abandonment Study, Offshore Malaysia

Abstract

During the plug and abandonment (P&A) of a gas field, offshore Malaysia, detailed cement evaluation logs were performed using a circumferential ultrasonic cement evaluation tool in combination with a traditional cement bond log (CBL) tool. The tools were combined in a single string and conveyed on wireline monocable. A total of eleven wells are included in a multiwell analysis across the field, which resulted in the successful identification of gas sources behind casing and the detection of buildups of migrated gas behind casing. An ultrasonic scanning tool operating with a rotating transducer (which transmits high frequency acoustic waves) measures the acoustic impedance of the material behind the casing wall at more than 50 azimuthal points. In combination with a CBL tool, these are used to simultaneously evaluate the cement bond quality and integrity between the 7-in. production casing and the formation to help ensure adequate isolation to continue with abandonment plan. Analysis of this data revealed consistent low impedance gas anomalies across specific permeable formations. These formations were subsequently interpreted as possible gas sources contributing to sustained annulus pressure and were identified without the requirement to perform dedicated pulsed neutron logs. Correlations were identified between sustained annulus pressure, historical petrophysical and diagnostic logs, and modern-day cement evaluation logs. Formation evaluation logs revealed parts of the identified formation were likely gas-bearing. Results from the ultrasonic data showed that the gas now occupied areas of the annular space behind the production casing, and the position of the gas within the annular space could have been affected by depletion. In several logs, it was observed that low impedance measurements continued to shallower depths above the major producing formation, signifying a gas migration pathway. In cases within the field, the shallowest hundred meters of the data were dominated by low impedance fluid measurements, correlating with sustained annulus pressure. The results were corroborated with separate acoustic-based diagnostic measurements in the same field, which also indicated a probable source of gas migration from the same identified formation into the cemented annulus. This paper highlights a case study whereby ultrasonic cement evaluation logs run on monocable wireline were able to identify trapped gas within the 7- × 10 ¾-in. annulus, as well as pointing the probable source of gas bearing formation contributing to gas migration into the annular space. This real-time additive information allowed the operator to help ensure proper planning for gas evacuation, zonal isolation, and cement remedial operations. By analyzing multiple offset wells, it also allows for predictions to be made for future wells in the field that suffer from sustained annulus pressure.