Open hole logging is challenging in Tengiz reservoir where lost circulation events and abundance of natural fractures lead to wireline tool conveyance issues. In order to address conveyance issues, several alternative methods were reviewed and logging while drilling in washdown mode was selected as the most appropriate technique for given conditions.
Alternative solutions such as hole finders, pipe conveyed logging, through bit logging were rejected due to inability to provide required results within the specified time constraints or due to well control risks. Conventional logging while drilling method was declared not feasible due to excessive shocks and vibration under managed pressure drilling technique that is currently being used in Tengiz for drilling through fractured reservoir with significant fluid losses. Therefore, logging while drilling equipment, together with the newly developed procedures on preparing hole conditions, pumping rates and logging speeds were utilized to log reservoir section in washdown mode.
Prior to commencing of logging in a fractured zone with significant losses, extensive trial was conducted on two stable wells to develop and refine logging procedures. In order to assure quality log data acquisition, full suite of wireline open hole logs was also acquired from these two wells. Further comparison and calibration of logging while drilling data vs. wireline log data allowed to achieve required accuracy and repeatability. Procedures developed on stable wells then applied to wells with significant losses considering specific pump rates to achieve constant stream of log data to surface. It is critical for Tengiz operations to establish and maintain continuous log telemetry and stream data in real time for efficient rig time utilization. The challenge was to sustain positive pipe pressure in a well with total losses to enable mud pulse telemetry. This issue was successfully resolved by increasing pump rates and use of flow restrictor sub as a part of logging tool string.
The novel approach of the described logging method enables log data acquisition on a well that experienced total losses and had significant obstructions for conveyance of wireline tools. Method allows to efficiently collect critical log data required for well completion and reservoir characterization.