Abstract
Saudi Aramco has been producing from the Manifa offshore field in the Arabian Gulf since 2011. By drilling mainly lateral wells, Saudi Aramco has targeted the main reservoir that belongs to the early Jurassic period. The pay zone is overlaid with weaker interbedded sand/shale formations and carbonate formations of the upper and lower cretaceous period.
The main challenges that were encountered while drilling these wells included total/partial mud losses and severe wellbore instability-related issues across the weaker formations overlaying the reservoir section (pay zone). An advanced geomechanical study was crucial for understanding the nature of the existing problems faced during drilling and applying the study's findings for drilling future extended reach (ERD) wells successfully.
A comprehensive geomechanical study of the field was conducted by constructing a calibrated post-drill 1-D Mechanical Earth Model (1D-MEM) for selected vertical wells. The study helped in providing reliable rock mechanical properties and fracture gradients across the problematic formations, a lack of which was a limiting factor in understanding the nature of the problems while drilling.
For the current study, an integrated workflow was implemented mainly to perform a root cause analysis of mud losses and to define well inclination (deviation) limits to drill through the problematic formations to minimize wellbore instability-related issues. Open hole log data was used to construct the 1D-MEMs. The developed models were further calibrated using all the available data (drilling observations, results of well testing, core mechanical testing, etc.). Image data was also analyzed to capture the presence of fractures/faults or related features along the wellbore.
Based on the analysis results, it was concluded that the mud losses in the overlaid carbonate formations were attributed to the presence of localized open natural fractures or caverns, or vugs. It was also concluded that most of the wellbore instability-related issues (hole pack-offs, stuck pipe incidents, etc.) occurred due to insufficient drilling mud weights that were not able to support the rock on the wellbore wall. The present study helped in better characterization of the problematic formations and also provided useful information for planning future extended reach (ERD) wells. Some of the provided recommendations included acquisition of micro resistivity image logs (to identify nature of fracture), implementation of real-time geomechanics, and optimization of mud rheology, mud weight program, and casing design.
