Lost circulation generates significant risks and associated costs, particularly in an offshore drilling environment. In an ideal application, prevention is the best remedy, but this is not always possible. A material that can both help prevent losses and aid in mitigating losses when they occur is ideal.

STEELSEAL resilient graphitic carbon (RGC) lost circulation material (LCM) from Halliburton Baroid helps prevent lost circulation by sealing porous and fractured formations. It is designed with a unique resilient characteristic that allows it to compress within a fracture as downhole pressures increase during fracture closure stress, then rebound if pressures decrease, such as during tripping.

Tests available from a joint industry project in 2000 indicate that LCM type, concentration and particle size distribution are important factors for lost circulation control. Of these parameters, particle type seemed to be the most important variable for obtaining a fracture sealing response in the test apparatus. Repeated fracture sealing responses were seen in tests using STEELSEAL RGC. Further, combinations of this material were effective when used in conjunction with other materials that were ineffective on their own.

Manufacturing Process

Resiliency is a compressive property allowing the material to mold itself into a fracture tip, promoting screenout. If pressure is released, the material rebounds, thus continuing to plug the fracture. The RGC materials are manufactured in a proprietary electric furnace process that produces a material with a high resiliency plus other desirable characteristics. The LCM is a purified resilient calcined petroleum coke that has been processed and structurally modified by exposure to high temperatures. The precursor, calcined petroleum coke, is processed in a continuous electrothermal purification furnace. The furnace operates in an inert environment at temperatures reaching 2,400 C (4,352 F). The resulting thermally purified calcined coke is uniquely modified to offer high carbon purity (99.5% C) and a more ordered crystalline structure with high resiliency.

Resiliency Testing

Since resiliency is not a general characteristic of most other LCMs, special test equipment and protocol must be used to measure this characteristic precisely. A special cell along with a compressive device to provide a measurable force on the test material is required and shows that RGC materials possess a unique spring-back effect greater than all other graphite-based materials. There are a number of unique attributes that lend to their solution as a wellbore strengthening and loss circulation management additive, including resiliency, morphology, true density and lubricity.

A hydraulic press that can apply load in the range of 10,000 psi was used to determine the resiliency. Steps followed for determining the resiliency were based on the standard operating procedure of Superior Graphite. Determination of resiliency requires two measurements to be made, height of the compacted sample when load has been applied and height of the rebounded sample when load has been removed. To accomplish these measurements, certain modifications in the Superior Graphite operating procedure were done. Full details of the testing method are covered in the SPE-133484-MS technical paper “Wellbore Strengthening: The Lessstudied Properties of Lost-circulation Materials.”

Characteristics

Morphology. Irregular shape and open pores aid in locking the particle in place (Figure 1).

True Density. Lower true density with respect to conventional graphite and carbon materials makes it possible to achieve higher loading values pounds per barrel and reduction of particle settling in the drilling fluid.

Lubricity. The LCM is unique in the fact that the surface is partially graphitized during the purification process and offers only enough lubrication to optimize interparticle locking and packing. A solid lubricant physically separates surfaces; a liquid lubricant cannot. Moreover, a solid lubricant is particularly effective in reducing hardfaced tool joint wear and liner running and aids in casing rotation and placement.

Nonmagnetic. The material is also not magnetic, as it is sometimes mistakenly perceived. Refer to SPE-153154 paper for more details on this perspective.

Resistance to size degradation. The benefit of resiliency is that it imparts significant resistance to size degradation. This is particularly important when it is used in loss prevention modes as a background material in the drilling fluid. STEELSEAL LCM was measured to have the highest resistance to size degradation (under similar test conditions) when compared to LCMs like ground marble and other regular conventional graphites. These results have been reported by several investigators and can be found in publications such as SPE-133484, AADE-12- FTCE-27, SPE-151227, SPE-165150 and SPE-163512.

Worldwide Field Applications

The versatility of the RGC has been proven in many field applications. Examples range from use in moderate to high concentrations to combinations of materials.

In the Middle East in an application where a well control event and subsequent lost circulation occurred, 20 pounds per barrel of sized RGC 100 and RGC 400 LCMs were incorporated into the active system through drilling operations, gamma ray, resistivity and sonic LWD tools. The well was killed successfully, and shut-in casing pressure was reduced from 900 psi to 0 psi. The INNOVERT fluid oil-water ratio changed from 80/20 to 60/40 with no loss of stability.

Additionally, an operator in North America that was drilling with a low solids nondispersed fluid experienced severe lost circulation, and about 1,500 bbl of fluid was lost to the formation. Historically, losses were previously minimized to less than 10 bbl/hr, and full returns were never regained. In more severe cases, cement plugs had to be placed to continue drilling operations. Several 50-bbl pills with 100 pounds per barrel of STEELSEAL LCM were mixed and spotted on bottom through the troublesome intervals. After spotting the pills the operator applied a gentle squeeze to the formation. After applying each squeeze, it was noted that the formation integrity was increased by more than 1 pound per gallon (ppg), and full returns were regained.

STEELSEAL has demonstrated benefits in deepwater environments as well, including a case where an operator encountered lost circulation at a rate of 15 bbl/ hr. The formation integrity test (FIT) had not yet been conducted. The drilling fluid in use was a 14.2-ppg synthetic- based mud. The density was decreased to 13.8 ppg, but losses continued at 2 bbl/hr. To drill ahead, the operator needed to achieve a FIT equivalent to 14.6 ppg. This problem had not been encountered on other wells in this field. Halliburton developed a customized high-concentration LCM pill prepared on the surface to be ready for deployment. The 100-bbl pill consisted of the STEELSEAL engineered composite solution and BARACARB sized ground marble.

The 70-pounds-per-barrel pill was pumped through a 105⁄8-in. by 11¾-in. drilling bottomhole assembly that had 8 12⁄13-in. bit nozzles and included downhole tools such as pressure- while-drilling, resistivity, gamma ray, sonic, density and neutron services. After spotting in the open hole, small hesitation squeezes were performed in 2-bbl increments. The operator was able to increase the wellbore stress by 915 psi. Then 3 m (10 ft) of new formation was drilled and the required FIT of 14.6 ppg was achieved. The production interval was drilled successfully to the target depth with no additional losses.

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