A systems approach to coring improves operations
As a critical component of the formation evaluation process, coring allows operators to acquire definitive data sets, which may be used to calibrate other forms of reservoir data. Without adequate planning, coring can pose operational risks and lead to unbudgeted costs, which causes some operators to consider it an optional service. The application of thorough planning activities is a critical element of a systems approach to combating core jamming and unbudgeted coring costs. This includes the use of software modeling to determine lithology characteristics and coring assembly behavior; the validation or adjustment of the operational plan using downhole coring data loggers; and the application of core jam mitigating technologies.
SOFTWARE APPLICATIONS/CORING DATA LOGGERS
The coring service provider may apply a detailed plan to ensure safety, efficiency and delivery of high-quality, fit-for-purpose core samples. NOV has developed a suite of new coring technologies and services that addresses the operational risks of a coring program. Software designed for coring operations allows operators to improve planning by processing offset well information, to provide knowledge of expected lithology and rock strengths over the planned coring interval. This knowledge can then be applied to select the appropriate core head design and cutter type to complete the coring operation as efficiently as possible, considering requirements for high ROP, durability to complete the section, and stability of the coring bottomhole assembly (BHA).
Complementary to understanding the characteristics of the formations to be cored, is understanding the behavior of the coring BHA during the core acquisition process. Coring design software may be used to plan the coring BHA and define the applied operating parameters, such as weight-on-bit (WOB) and rotary speed (RPM), to ensure that the coring assembly will be run smoothly and without high levels of vibration that can initiate core jamming incidents, Fig. 1. To demonstrate the value of coring design software, an operator in Chile recently planned a BHA based on RPM and WOB parameters recommended by the software. The operator avoided a core jam and achieved 100% core recovery, rare from offset performances.
State-of-the-art, high-frequency, downhole drilling dynamics recorders identify vibration modes that may be experienced during coring operations, and that could be responsible for initiating core breakage and jamming incidents. Their unique design allows this family of data recorders to be run in multiple locations within the coring BHA and throughout the drillstring.
Post-run analysis of coring data provides knowledge of the downhole environment, helping to reduce BHA vibration during coring and optimize the next core run. This insight may be used to recommend changes to the coring equipment proposed; the re-configuration of the coring BHA; changes to the operational procedures applied; or the coring parameters selected. Implementation of these changes to subsequent core runs results in fewer core jamming incidents, longer cores with improved core recovery, and reduced operating costs. While these coring data loggers are presently only available in memory mode, the current technology is the precursor to the next phase of coring development from NOV, which will include the ability to provide this data in real time, via mud pulse or wired-pipe telemetry.
CORE JAM MITIGATION TECHNOLOGY
A core becomes jammed inside a coring assembly when the frictional forces, between the core and the internals of the coring assembly, become so great that the inner tube can no longer pass over the core column, as efforts continue to core ahead and deepen the hole. High levels of downhole vibration in the coring BHA can initiate a core jamming incident by causing breakages within the core column, and contiguous sections of core to move apart and become mechanically wedged inside the inner tube assembly.
Core jamming may occur while coring across interbedded formations in a deviated well, or during coring of fractured formations, where the fractures may become mobile as the overburden is removed. Reactive shales or clays may swell and pack off inside the assembly, if cored with inappropriate water-based mud (WBM). Core jamming is also a risk in formations with clasts that can become disassociated with the matrix, or in formations containing zones of low compressive strength that may fail under load and pack off inside the assembly. The use of high-friction muds for coring, such as silicates, may also lead to core jamming.
Whatever initiates the core jamming incident in a conventional coring application, the outcome is the end of the coring run. An experienced coring technician will identify the development of the core jam from surface parameters, and will call for the coring assembly to be tripped out of hole (TOOH), to minimize rig costs and protect the core that has already been captured downhole.
In some operations, the surface indications of a downhole core jam may be masked or missed, causing operators to continue coring with the jammed assembly. With the core firmly jammed inside the coring assembly, there is no potential to acquire additional core, and the assembly will mill away the formation below it until it is pulled to surface. This is the most serious consequence of a jammed core, since that interval of formation—which the operator needed to core for critical data—has been milled away and can only be captured by coring in a sidetrack or new well.
Oilfield technology companies have introduced coring technologies to provide additional assurance against core jamming, such as the JamTeQ active core jam mitigation system from NOV, Fig. 2. This technology improves conventional coring efficiency and core recovery by overcoming core jams, and allowing the coring operation to continue unaffected, without tripping the BHA to surface.
The system is comprised of a frangible liner that is run inside the slick aluminium or fibreglass inner barrel, which is attached to the coring upper shoe, above the core catcher system. The frangible liner extends for 26.24 ft upward, into the lowest inner tube section, and contains engineered weak points at approximately 3.2-ft intervals over its length. These weakened points enable the liner to separate into sections when subjected to axial or torsional forces, and decrease in strength from the base upwards.
If a core jam occurs inside the frangible liner, the applied coring WOB transfers to the jam point within the liner and causes the liner to separate at a weakened point below the jam location, allowing the liner to contain the jammed core and move up inside the inner barrel as the coring operation continues. The frangible liner sections below the activation point remain in place and are available to address any further core jams that may occur in that location. The activations happen automatically, if a core jam occurs and requires no operator intervention by the coring technician.
During system set-up, the frangible sections can be adjusted to suit the specific rock strengths of the planned coring interval, to ensure activation if the core were to jam. The coring assembly requires no changes to run the frangible liner technology, and the system does not require any special core head, inner tubes or handling tools to operate.
Up to eight jamming incidents per core run can be overcome with each frangible liner system, and use of the system can result in significant time and cost-savings to the core acquisition process. There is no upper limitation on the length of the coring assembly that can use the frangible liner technology, which maintains full core diameter in all sizes. Cores of 5¼-in., 4-in. and 3-in. diameter in nominal 12¼-in., 8½-in. and 6-in. hole sizes, respectively, ensure that more uninvaded core sample is available for analysis, further enhancing the quality of the core analysis data generated from the samples.
Several commercial coring operations with the jam mitigation liner system have been successfully completed in the U.S. and Russia, and several more are upcoming in the immediate term, including offshore operations where this technology has the most benefit, due to the high cost of offshore rigs.
CASE STUDY: CORE JAM MITIGATION OVERCOMES HIGH CHERT SECTION
In first-quarter 2016, an Oklahoma operator had an objective to acquire 765 ft of continuous core with a high-chert-content section of 200 ft in the middle, between 7,550 ft and 7,750 ft. A 7-in. DuraTeQ conventional core barrel assembly, equipped with an 8.75-in. X 4 CDPHI713 core bit, achieved excellent performance, averaging 3-4 ft/hr through the chert-laden section.
The core jam mitigation liner system was used on the second and third runs within the chert zone. The system activated on both runs, achieving the longest core runs within this problematic interval, resulting in an average ROP of 7 ft/hr, and saving the operator two round trips. The solution cut 697 ft of core and recovered 696.05 ft, a 99.9% core recovery. Use of the core jam mitigation liner system significantly contributed to an overall nine days of saved rig time on the operation, as well as the recovery of more core than had been obtained on the previous offset well.
DUAL CORE CATCHER SYSTEM
Dual core catcher systems with full closure capability, such as NOV’s DuraClose core catcher, replace the standard core catcher that is situated at the bottom of the coring inner tube in a conventional core barrel assembly. The dual core catcher system is comprised of a conventional spring catcher and a clamshell-type full closure catcher. Both catchers are concealed behind a sleeve during the coring operation, ensuring that core entry to the inner assembly is slick, with no potential for the core to foul against the catcher and initiate a core jam—which may occur with a standard catcher system. This can be a particular benefit when coring certain problematic formation types, such as highly fractured carbonates or interbedded clastics.
While the conventional spring catcher is designed primarily to capture and retain cemented and consolidated formations, the full closure catcher captures poorly cemented or unconsolidated formations found in many shallow and geologically young reservoirs. When the downhole coring operation is complete, the dual core catcher system is activated, releasing both catchers to engage with, and capture, the core. The conventional spring catcher retains consolidated core, while the clamshell catchers shear through weak core and seal off the bottom of the inner tube, preventing loss of loose sediments, as the coring assembly is TOOH.
The dual core catcher system also delivers full-diameter cores, and has now been used successfully by many operators across several regions globally, both onshore and offshore.
The frangible liner technology is combined with dual core catcher technology to achieve a combination of active core jam mitigation with slick core entry, to prevent and overcome core jamming at both core catcher and inner tube level. Dual core catcher systems with full closure capability have contributed to successful core recovery.
CASE STUDY: MALAYSIA
An operator in Malaysia was challenged with obtaining high-quality 5¼-in. core samples from a formation consisting of claystone and sandstone in a medium inclined well, with maximum core recovery and optimum ROP. The system successfully cored the formation in three 30-ft conventional runs, achieving 100% recovery and cutting a total of 87 ft of core.
CASE STUDY: THAILAND
An operator in Thailand had an objective to obtain 354.33 ft of 4-in. core from a low-angle, deviated well, with maximum core recovery, optimum ROP and maximum core barrel length. Using a 7-in. conventional core barrel, dressed with a dual core catcher system with full closure, the operator cored 354.33 ft in four conventional runs within low-strength lacustrine/fluvial formations at shallow depths around 5,577 ft. The solution achieved 100% core recovery, full closure of inner barrel on each run, and allowed unrestricted core entry, reducing jam potential.
CASE STUDY: TEXAS
An operator in Navaosta, Texas, had an objective to core multiple levels of unconsolidated formations. A 9½-in. conventional core barrel, equipped with a dual core catcher system, provided 5¼-in. cores in a 12¼-in. hole section through numerous coring intervals. The system cut 234 ft of core and recovered 227 ft of core, a 97% recovery rate. The system achieved an average ROP of 16 ft/hr.
NEW LOW-FRICTION TECHNOLOGY
The vendor has applied new low-friction treatment for aluminium inner core barrels and liners, which passively combats core jamming incidents by reducing the coefficient of friction between the core and inner assembly interface. The treatment modifies the surface of the aluminium, creating a highly wear-resistant, composite oxide/polymer surface with coefficient of friction reduced to 0.12, from the untreated aluminium friction coefficient of 0.45. This significant reduction in friction at the core/inner assembly interface resists the initiation of core jams and enables longer core runs to be achieved in problematic formations.
Field results, where low-friction inner barrels and liners have been used, show a reduction in core jamming incidents, improved coring efficiency and recovery, with a consequent reduction in overall coring costs. Although primarily developed for use in aluminium inner assemblies, low friction treatments also can be applied to disposable, steel inner assemblies for high-temperature coring applications in excess of 392°F.
The low-friction treatment also may be run in conjunction with the other anti-jamming developments, creating a triple-combo system of core jam mitigation technologies to both actively and passively address the issue of core jamming at the core catcher level, as well as inside the inner assembly, once the core is captured.
SUMMARY
New coring technologies and services address the operational risks of a coring program, helping operators avoid core jamming and unbudgeted coring costs. By using software to determine lithology characteristics and coring assembly behaviour; downhole coring data loggers; and core jam mitigation technology; operators can overcome obstacles and achieve more successful coring operations. 
