Optimized Drilling and Logging Practices Across Depleted Reservoirs – A Case Study in ADNOC Onshore Wells Based On – A Comparative Analysis
Authors: Bethancourt Roswall , Aguilar Victor , Mubarak Al Braiki Ali , Dua Ajay , Sarhan Mohammed , Al Blooshi Nouf , El Wazeer Fathy , Khalife Bassam , Propper Maarten 
Published: November 2020

Abstract
As mature reservoirs continue to be produced, drilling activities become more and more challenging. Risks are mainly posed by large variations in pressure gradient, while having intercalated high- and low-pressure zones adds to the complexity of the operation. High mud weigh is often used for well control and hole stability; however, it may result in lost circulation, differential sticking or wellbore collapse. On the other hand, increasing depletion may further cause compaction, and therefore instability. Wellbore trajectory and deviation may be extra factors increasing the hazard likelihood. In such circumstances, stuck pipe, which is one of the main drilling problems worldwide, seems like an imminent risk that generally is addressed on a reactive basis, amounting to 25% - 40% of the well budget, according to industry literature. Furthermore, the presence of radioactive sources, necessary for measuring and recording density / neutron petrophysical data, in the LWD (Logging While Drilling) string while drilling across such reservoirs augments the severity of the potential risks. This paper aims to present the significant gains obtained by incorporating the Logging While Tripping (LWT) technology in the standard operational practices, while meeting the drilling, formation evaluation and data acquisition requirements.
Data from three sample wells were analyzed: In Well A, the 8.5" section was drilled in two runs: - Run-1 - RSS (Rotary Steerable System) + Triple Combo LWD above the high overbalance zone X. Run-2 - RSS only till section TD (Total Depth). In Well B, the 8.5" section drilling was attempted in a single run, leading to stuck BHA (Bottom Hole Assembly) with expensive LWD tools and radioactive source, and eventually leaving the total BHA in the hole and sidetracking the well. In Well C, the 8.5" section drilling was again attempted in a single run, but only with GR-RSS for landing inside the reservoir. Formation evaluation data was acquired using LWT technique, which requires negligible additional rig time, and virtually eliminates the risk of losing expensive LWD tools and radioactive sources in the well, as the logging tools are protected inside the pipe and retrievable at any time.
Rig time, cost and risk were evaluated and compared for the three cases. Results show 1.85 days rig time reduction and 23% cost savings in Well C compared to Well A, while statistics of Well B showcase the risk magnitude, which can be effectively diminished by implementing the methodology used in Well C. A quantified matrix has been utilized to contrast the approximative cumulative risk per well. The use of LWT in 8.5" deviated hole sections has become part of the best operational standard practices for the operator while drilling across depleted and deeper reservoirs, as it leads to optimal time - cost - risk balance.