Core solution

March 2, 2011

Coring practices and equipment used in the Gulf of Mexico's Lower Tertiary needed to adapt to formation conditions to improve on past recovery success. Petrobras' James Meyer opens OE's drilling & completion technology two-parter with a review of some redesigns that improved core recovery from the Lower Tertiary.

The Lower Tertiary in the Gulf of Mexico is regarded as a potentially huge petroleum resource with limited known reservoir characteristics or production ability. Core is essential in determining reservoir parameters and producibility. Since the Tertiary interval is quite thick (1000-3000ft of gross interval), a significant amount of core is usually requested for study.

Core recovery in the Lower Tertiary has been poor to below average for most operations in the Gulf. With the Tertiary being in deep and ultra-deep waters of the and at depths of 25,000-30,000ft, operations are time consuming and costly. When coring operations fail to go as planned, it can lead to significant expenditures. The Tertiary trend of interest in the Gulf of Mexico is deep, and is usually below 25,000ft. The sands are laminated and have a high compressive strength with variability ranging from 5000psi to 25,000psi (Figure 1). Pressures at this depth are high, normally exceeding 18,000psi.

Core data
Coring has usually been a directive from the G&G or reservoir groups. These groups usually establish criteria such as core point, size of core required, footage to be cut, and minimum amount of core to be recovered. Additions to these requirements may be other special needs, such as core tracer, tripping protocol, core processing, and testing at the rig site. All of these requirements add to the time needed for the operations and tax operational capabilities.

With high rig day rates, the coring operation can be extremely expensive. A time breakdown for coring operations can be:

The resulting time for this operation is four to six days with a cost implication of $4-$6 million without any NPT or dependence on recovery.

Core recovery
Typical core recovery in the Lower Tertiary is 72%. The average may be somewhat misleading from the data as shown in Figure 2.

A database of 63 wells that have been cored in the Lower Tertiary was reviewed and the data categorized to determine if any pertinent advantage of the coring equipment characteristics existed over the other. The three different characteristics investigated were barrel length, bit type, and hole size.

Review of the database showed no particular parameter had an advantage for recovery success. In a review of the data, the shorter core barrels tended to have a higher recovery regardless of bit type or hole size. The bit type had no discernable effect. The 8 1/2 in hole size has a higher recovery with barrels being 180ft or shorter. The 12 1/4 in hole size was above average, using barrels longer than 180ft.

With this bit of information, it would make sense to have a 120ft long barrel in an 8 1/2 in hole. However, when the core requirement is on the order of 500ft or more, the option to make two runs instead of five runs make economic sense. The hole size may also be limited to well design parameters, such as casing size that exists above the reservoir.

Review of a planned coring operation that was completed on a Tertiary field showed the coring success was in line with the previous statistics: poor. Eight runs were made with core recovery being 35% with 180ft core barrels.

After researching well files and coring reports, an in-depth look into the recorded operational data followed. A trend became apparent when the recorded data was placed against the log/formation data. Jamming at the bed boundaries or immediately after crossing the bed boundaries was evident. Available wireline logs on the wellbore geometry also confirmed that the wellbore axis had shifted through the interval adjacent to the bed boundary.

An issue with being able to maintain rate of penetration (ROP) resulted in a reactionary response to increase the weight on bit (WOB). Jamming would occur shortly after this reaction, or coring would continue, with the result of grinding the core with no recovery.

Coring parameter requirements
During coring operations, it is desirable to maintain a light and constant WOB of 5000lbs to 15,000lbs. The ability to maintain this weight is difficult at depths of 25,000ft+, when the drill string weighs from 800,000lbs to 1 million lbs, which can lead to having a weight applied in excess of what is required.

BHA analysis
After reviewing the data, a BHA design analysis was performed to determine what effect was happening to the various coring BHAs. What was observed was a definite bit tilt when WOB exceeded certain thresholds. After coring is initiated, and ROP begins to slow, additional weight is usually added, causing the neutral point to move up the core barrel and creating a tangency point or bend between the stabilization points. The excess weight in combination with dipping beds and laminated formations provided additional potential for jams and minimal recovery.

The tendency for the core barrel to bend is a function of the distance between stabilization points and the mass of the BHA or the wall thickness of the barrel.

The wall thickness on smaller systems is 0.6875in, and 0.9375in on the large system, (Baker Inteq Coring Systems HT – 30 and 40, HT – 60 respectively). The available weight that can be applied to the coring bit is a function of core barrel size and length. If drilling jars are used, it is possible to transfer sufficient weight down to activate the jars when smaller core barrels are used.

Stabilization points for the coring systems on the core barrels are located at intervals of 29ft with a stabilizer blade gauge length of 11.8in.

Bit analysis
In reviewing the core photos of the previously mentioned coring operation, in most instances, the sands exhibited a necked down feature.

This feature may be easily attributable to core whirl, but this should also occur in the sand-shale interfaces, but does not. After reviewing several bits, a design feature incorporated into the bit was an ‘anti-whirl' feature, which has a tendency to rotate off center to achieve a higher ROP.

Further review of available bits revealed the lack of use of available PDC cutter technology or PDC cutters in use on drill bits. The cutters for the majority of the coring bits were 3/4in cutters. The available gauge pads on the majority of coring bits in use were limited to 1-2in. The coring bits for the hard formations were increased to 3-4in of gauge protection.

Tertiary redesign
Discussions with the coring operator led to fit-for-purpose modifications of the BHA and bit. The coring bit design changes were made based on drill bit performance in the Lower Tertiary.

The significant changes made to the bit were:

  • Reduce the cutter size to 13mm versus the 3/4in presently in use.
  • Add new cutter technology for heat and impact resistance.
  • Add limited depth of cut to minimize aggressiveness and any stick-slip that may occur.
  • Seven blade design.
  • Add additional gauge length for stabilization at the bit.
  • The significant changes to the BHA were:
  • Stabilizers to be 1/32 under gauge for all planned systems.
  • The stabilizer distance was to be reduced through the addition of pup joints or recut barrels (only for smaller hole size system).

One of the bigger issues mentioned earlier is the ability to maintain a constant and accurate WOB. Downhole real time monitoring was added to understand the downhole WOB and torque being experienced.

The analysis and redesign of the coring equipment was incorporated into the planned operation. The coring operation called for 2ft-360ft cores with a minimum recovery requirement of 70% or 504ft of recovered core. A 12 1/4in coring system was used as per well design on hole size requirement.

On Run 1, a 360ft core barrel was run. The coring operation was stopped after coring 97ft. The parameters exhibited were stalling and high torque without any further progression. During this weights as high as 35,000lbs were introduced to the coring system. The core barrel was pulled and found to have encountered tar. 85.1ft of core was recovered.

The wellbore was sidetracked to avoid the tar. A second core barrel of 180ft in length was then run. The core barrel length was reduced to minimize handling time at the surface in case tar was encountered again. Coring was initiated with a weight on bit of 10,000lbs to 12,000lbs and an RPM of 80. The parameters were held constant at an ROP of 41ft/hr over the entire interval. A total of 180ft was cored with 180ft of core recovered for 100% recovery.

The well was then drilled to the next core point and coring operations for the third core began. A 360ft core barrel was run due to the success of the previous coring run. The coring operation was initiated with 10,000lbs to 15,000lbs weight on bit and 80 RPM. The parameters varied slightly over the interval at an ROP of 29.5ft/hr average. A total of 360ft of interval was cored and 360ft of core was recovered for 100% recovery.

Overall a total of 625.1ft of core was recovered. The coring operation was considered a success in comparison to previous core run averages and offset attempts in the Lower Tertiary.

Some of the unexpected benefits were minimal time spent washing and reaming through the cored intervals. The wellbore was smooth and in-gauge.

The core was nearly full gauge and pristine over most of the core. The core was intact with no significant inertial damage or necking observed. However, some washing of the core was apparent in some intervals. OE

James Meyer has worked in the oil industry for 25 years as a petroleum engineer, primarily as a drilling engineer, in the US and around the world. He is a graduate of Texas A&M with degrees in geology and petroleum engineering. He is a registered professional engineer and member of SPE, AAPG, and SPL WA.

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