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.

Often, casedhole petrophysics provide essential information for reservoir surveillance and monitoring in mature field reservoirs for recovery optimization. One of the vital casedhole data acquisitions involves a pulsed-neutron tool for formation evaluation. The service company's recently developed multidetector pulsed-neutron tool (MDPNT) with a long-spacing detector provides a unique measurement [sigma, carbon-oxygen, and saturation gate (SATG)], which significantly helps improve sensitivity for formation gas saturation. The multidetector pulsed-neutron tool (MDPNT) is the only casedhole formation evaluation tool within the industry to be used with a memory system simultaneously acquiring the inelastic carbon/oxygen (C/O) ratio and capture (sigma) logging modes. The high-power battery pack provides up to 11 hr of continuous logging with superior data quality. A memory-based logging system provides advantages in terms of a smaller footprint, improved operational safety, and easier logistics handling, which results in significant operation cost reductions. A successful application of the first MDPNT memory logging on slickline deployment in Field A of Peninsular Malaysia is discussed. Pulsed-neutron data were acquired in the well with the objectives of validating hydrocarbon fluid types for additional perforation operations and determining current hydrocarbon saturation to derisk openhole log interpretation uncertainties and fluid contact delineations. The targeted sand is located above the top packer section, and based on openhole logs alone, there is a high uncertainty of distinguishing between the presence of oil and water. The well consists of a complex completion string with 3.5-in. tubing inside a 7-in. casing along with a packer and sliding side door (SSD). Gas trapped under the packer is a common scenario and poses another challenge in saturation computation. Post-data acquisition results exhibited a positive indication of oil presence within the targeted reservoir. Gas-oil contact (GOC) was successfully determined within one of the reservoirs despite the thin oil rim presence, which could be easily overlooked. The discovery has led to the assessment of a nearby well to be the potential add-perf candidate within the same reservoir for quick gain monetization. The add-perf operation was conducted in two reservoirs and flowed successfully at 500 B/D instantaneous production. The effort was well paid in reactivating an almost four-year idle well status with sustained production and a 0.2 MMSTB reserves addition. In other words, the three-phase pulsed-neutron analysis consisting of the C/O, sigma, and SATG combination has convincingly generated a robust saturation computation covering all the zones of interest. The case study described in this paper emphasizes the benefits of the MDPNT memory logging, particularly within a challenging offshore environment with a limited footprint profile and logistics arrangement, and yet successfully unlocked the low-hanging fruits for production enhancement.