The MaxCOR service was the first rotary sidewall coring tool on the market capable of acquiring larger, higher quality samples while operating at the highest temperature and pressure ratings in the industry. The service can reliably retrieve 60 samples during a single trip in formations that range from soft to hard lithology, in highly overbalanced formations, or in environments of up to 25,000 psi and 400°F (204°C). Its use improves the accuracy of reservoir rock and fluid analyses, such as porosity, relative permeability, capillary pressure, water saturation, geomechanical, and other special core analysis (SCAL) properties.
“The step change in this technology gives you a bigger core sample, for more accurate lab measurements,” said Gigi Zhang, region geoscience manager at Baker Hughes.
Technology’s Development
The path to the tool’s development was twofold. First, with the goal of providing a faster and more reliable coring service, Baker Hughes engineers revamped the legacy sidewall coring tool to develop the PowerCOR rotary sidewall coring tool, which retrieves the industry standard of 1-in. diameter core samples. At this stage of the design, engineers focused on improving the device to deliver better performance. Many of the old generation tool’s complex components were simplified, such as a new core separator design with fewer moving parts to enhance the reliability of the operation.
To increase reliability, engineers de-signed the MaxCOR bit control mechanism on a piece of steel, which is then machined to reduce the parts count of the tool and to provide greater strength. The rigidity of the one-piece bit drive mechanism also contributed to an improved core break function by providing a more positive break when extracting the sample.
Since the legacy coring tool was hydraulically powered, it was saddled with unprotected tubing running over the outer surface. The new design eliminated the hydraulic motor and hydraulic lines and uses gun-drilled holes for the wires. This gives the tool a cleaner profile and makes it less prone to failure as a result of being exposed to the downhole environment.
The coring bit is the heart of the operation, and engineers determined that by replacing the hydraulic-driven powering mechanism with a direct current driven motor, the bit would rotate more than three times faster. The new motor design significantly reduces the time in coring each sample, from an average of 9 or 10 minutes to 4 or 5 minutes, thus saving valuable rig time.
The direct-drive electric motor is controlled by a sophisticated downhole power management system that ensures maximum power transfer efficiency under all load and borehole temperature conditions. The bit is a patented design that matches the greater rotational speed of the electric motor. The new bit design also allows cuttings to be cleared more easily, enhancing the tool’s performance in high overbalance conditions.
The second stage of the tool’s development focused on a simple compatibility design with the PowerCOR coring tool, allowing the mandrel section of the PowerCOR tool to be swapped out for a MaxCOR tool mandrel when a larger core sample is needed.
“The design philosophy was to share as much as possible between the two services, which allowed us to cut down manufacturing costs and pass on the savings to customers,” Zhang said. “It also offers our geomarkets the flexibility to run either service, and it saves logistics costs because instead of ordering two complete strings, they can just order one string and an additional mandrel.”
In addition to the tool’s enhanced downhole performance, engineers developed a much improved graphical user interface to provide better monitoring capabilities. The new interface uses multiple windows to control the operation, from torque and rotational speed to a real-time video feed, thereby giving the surface operator a better feel of the process during the coring and providing a new level of control for better decision making.
Results in the Field
The need for a coring technology that delivers larger samples at a faster rate was first driven by an operator that had made a significant discovery in offshore Brazil. The formation was highly complex and called for SCAL-size or larger-sized core samples to provide a more accurate reservoir characterization.
In March 2010, the first successful MaxCOR service field tests in a deepwater Brazil field were conducted. The operator requested 90 cores in the Campos Basin, primarily in reservoir rock, and the service recovered 94 cores. The tool drilled each core in an average of 4 min. and 36 sec., with maximum lengths of 2.5 in. (6.4 cm) and an average core length of 2 in. (5.1 cm). During a subsequent field test in the basin, the service successfully completed a 52-core program in two runs. Several more jobs have been run for the same operator, with an overall core recovery efficiency of 97%.
In addition to the deepwater work, the service has been used successfully in a number of United States shale plays, including the Eagle Ford, Marcellus, Niobrara, and Bakken.
“The service is geared toward complex reservoirs such as shale reservoirs,” Zhang said. “For those types of reservoirs, we are not just determining the conventional petrophysical properties, but also some special ones like total organic content,” she said. “In addition, shale core samples require different lab protocols than regular samples, and many measurements end up with the core being unusable for follow-on tests. Getting more sample volume in relatively the same amount of rig time affords operators the chance to conduct more destructive types of lab analyses. We actually see the most applications in the various shale basins for both US and international operations.”