Optimisation of Plug and Abandonment Process Utilising Nuclear Technology for through Tubing Cement Evaluation

One of the major factors in the well plug and abandonment (P&A) process is to provide a proper isolation in aging wells which requires effective placement of the cement plug in the most suitable location in the well. Identifying cement placement is usually achieved by running cement evaluation logging to define the quality of cement and top of cement depth behind and in between the casing annuli. However, this comes with significant costs due to tubulars or casings removal requirement prior to logging run in order to conduct a proper evaluation. This is necessary since acoustic and ultrasonic based cement evaluation technologies will not be able to determine cement quality behind several casing layers if the job is done through tubing. The cost involved is substantial especially in offshore operation in which the daily operating rate is significantly higher compared to an onshore operation. A new approach to cement evaluation has been tested during the well P&A campaign in one of the aging oil fields in offshore, Peninsular Malaysia. A nuclear based technology comprised of Gamma-Gamma, Neutron-Neutron and Neutron-Gamma measurements were utilized to evaluate cement integrity behind production casing and between production casing as well as intermediate casing while logging run was deployed through tubing in memory mode. Log data was compared with acoustic and ultrasonic based cement evaluation technology that was deployed after the tubing was pulled out in one of the wells. Results had shown a consistent finding with the conventional ultrasonic based cement evaluation data. Based on the logging results, cement placement design and depth was finalized and the cement plug was successfully tested as outlined in the well P&A guideline. Findings from this logging run had provided useful insight to the operator to validate the nuclear based thru tubing cement evaluation technology for wells P&A application. Huge cost saving could be captured through this application as a result of eliminating total rig days via offline cement evaluation logging and based on the results obtained planning for the exact well P&A design requirement prior to the rig arrival. This paper will outline the method, tools used to acquire the cement evaluation data and its operational advantages. Acquired data will be presented and discussed along with the methodology used to determine cement volume and top of cement depth behind and in between the casings.