A Case Study on Enhanced Top of Cement Predictive Modelling Validated Against Bond Logs

Abstract To comply with requirements of local regulations, challenges can arise when cement operations do not go as planned and cement does not circulate to surface. A coal seam gas (CSG) operator in East Australia requested validation of the top of cement (TOC) in three wells (Wells A, B, and C). This was attributed to losses during cementing with uncertainty related to wellbore geometry or a combination of the two. Having the correct TOC estimated with pressure map correlation could help ensure an effective barrier was in place. Post-job data was gathered and collated as inputs for each well for software simulation. Data included wellbore parameters from drilling data, fluid properties, volumes and rates of fluids pumped, and returns measured in trip tanks and on job pump data acquired for pressure mapping. Iterations were simulated based on the cases until accurate pressure mapping was observed. For Well A, pressure matching calculations between the design and execution phases had iterations in calliper hole size, expected loss zones, and loss rate/volume based on what was observed on the surface. For wells B and C, no losses were encountered. No open hole calliper data was available; hence, the uncertainty with regard to the open hole size and washouts were the key variables to iterate. Software analysis predictions with post-job field data provided the operator with surface pressure comparisons and predicted TOC for these wells. The accuracy of the simulations were within ± 50 m, later confirmed by cement bond logs (CBLs) conducted post-job. Data screening for the inputs, results analysis, iterative requirements, and comparison of simulation results with log data recorded are discussed. For well A, deviation between the prediction and the actual log result was 3 m.For well B, deviation between the prediction and the actual log result was 20 m.For well C, deviation between the prediction and the actual log result was 58 m. Log validation of software simulation predictions of the TOC provide operators confidence in the accuracy of predictions. This potentially reduces the need for CBL logs and can result in cost savings for operators in addition to increased trust in available cement tools.