Abstract
Rotary coring has become significantly more important as an alternative to conventional coring due to the high total cost of conventional coring operations. Coring wells below 30,000 ft encounter significant rig costs, operational challenges, and a high amount of time involved, even for wireline operations. Formation pressures can exceed 30,000 psi, and differential pressures over 5,000 psi also exceed the capabilities of traditional rotary coring tools.
New technology has been introduced to enhance the recovery of rotary sidewall cores to improve operations and capabilities on these challenging wells.
The improvements include:
- 1.5-in. diameter core samples, on a 35,000-psi tool
- New high-powered coring tools with significantly more power for cutting Lower Tertiary well cemented rock (Wilcox, etc.)
- Higher torque and horsepower at the bit
- High-powered surface systems and high-strength wireline cables
- New drill bits and catcher rings to utilize higher power and operate in harsh coring environments
- Cutting and handling advancements for soft and hard formations
- Combinability upgrades to reduce wireline trips
- Dual-coring tools with the ability to have different catcher rings and bits downhole simultaneously on a single run
- Combination of rotary coring and formation sampling to obtain formation pressures, fluid samples, and cores on a single run
- Full downhole monitoring of the coring operation, which includes the drilling functions like torque, bit force, penetration rate, and depth of the core drilling, along with tool orientation
Core recovery information to enable 100% core detection downhole so extra cores are not cut when not needed during the job, since the individual core plugs are measured downhole for recovery
- A unique method to seal the cores in a pressure-compensated coring tube downhole to capture all the formation fluids in the rock in the borehole environment
- Complete rotary coring downhole operations can be monitored remotely for offsite interaction during the coring operation
In addition, we will include some historical information on rotary sidewall coring dating back to the 1980s and earlier to highlight the improvements and challenges that have been resolved with new technology.
We will provide examples of the different applications above and where they have been significantly beneficial for deepwater and other operations.
Rotary coring has become significantly more important as an alternative to conventional coring due to the high total cost of conventional coring operations. The acquisition of 1.5-in. diameter rotary sidewall core samples is a key component of a quality formation evaluation program. When the cores are targeted to critical specific depths, they provide specific tangible data to validate log-based formation evaluation. Coring as a key component of formation evaluation dates from the origin of the oil industry in 1859, and it continues to be an invaluable source of subsurface information.
