Ultrasonic processor reduces drill-cuttings size and eliminates subsea mounds

Dean E. Gaddy
Drilling Editor

Nelson-Farrar Cost Indexes [76,405 bytes]

Drill cuttings size reduction using ultrasonics eliminated a subsea clean-up and significantly reduced the environmental impact in a North Sea drilling project.

Reduction in cuttings size allows for a wider areal dispersion when released into the ocean because they are held in suspension longer than larger sizes. Thus, ocean currents carry the smaller cuttings farther away from the well template, leaving a much wider footprint than larger cuttings sizes. This eliminates the pile-up of cuttings that otherwise would contaminate and harm the marine habitat.

The U.K. government recently enacted strict legislation that limits drill cuttings disposal into the marine habitat to no more than 1% oil content. Agip UK Ltd. used ultrasonic disintegration introduced by Expro North Sea Ltd. to economically achieve these standards.

Because of its highly desirable rheologic properties, Agip uses pseudo oil-based muds (POBM) as a drilling fluid. However, POBM's have a propensity to permeate cuttings with oil. By reducing particle size, Agip was able to liberate the impregnated cuttings of oil.

According to Nigel Avern, technical support engineer for Expro, the process breaks the rock particles down to their constituent components. For instance, a well-indurated sandstone is broken down into sand grains, destroying the cement and the oil-bearing matrix in the process.

Ultrasonic destruction

Transferring cuttings in a liquefied slurry is a necessary component of the ultrasonic particle-size reduction process.

The process works by producing masses of air bubbles. This creates high and low-pressure regions as acoustic waves propagate through the liquid medium. As the bubbles travel through the liquid, localized areas enlarge, then implode, creating tremendous heat while adjacent areas remain near ambient temperature.

When solid surfaces like cuttings are subjected to the ultrasonic process in a liquid, asymmetric cavities are formed on the surface of the solid. When these cavities implode, the cuttings are subjected to a jetting action which impacts the surface at speeds up to 400 km/min.1

The jetting action, along with the imploding acoustic shock, induces high stress levels in the solid particles and results in the break-up and de-agglomeration of the solid components in the liquid. The disintegration process is complimented by high friction and particles collisions.

Ultrasonic processing

Expro's processor is a modular system of meandering pipe, 2 m long, 1 m wide, and 1.5 m high (Fig. 1 [45,210 bytes]). Drill cuttings are pumped through a series of processors at an optimum rate to maximize retention while maintaining particle suspension. Forty piezoelectric-crystals, located radially around the external surface of the processor, resonate through the chamber at an oscillating frequency of 38-40 khz.

The waves create a cavitational energy and increase in velocity as they reach the drill cuttings. This process causes high-energy collisions between the sonic emissions and the solid particles, resulting in particle size reduction.

Avern said that particle size reduction is proportional to the time that the cuttings are subjected to sonification. The longer the cuttings are subjected to the process, the smaller they become, until they ultimately reach a constituent particle size. The Agip field trials found that 90% of the cuttings were reduced to less than 250 microns over a period of 35 sec.

Before the cuttings are pumped through the processors, they are transported from the shale shakers to a buffer tank via conveyor. The cuttings are then mixed and agitated with seawater to form a slurry mix. The slurry is then circulated through the processors.

The ratio of cuttings to water can be adjusted to suit the ROP. Avern said that three parts water to one part cuttings (3:1) was optimal for Agip's North Sea wells.

Sonification time can be varied by adding additional processors or by changing pump speed. Cuttings handling capacity is a function of the rate-of-penetration (ROP). For instance, for an 81/2-in. hole, if the ROP is 220 ft/hr, four processors can adequately handle a 3:1 slurry at 60 bbl/hr; however, if the ROP is 435 ft/hr, it would take eight processors to handle a 3:1 slurry at 120 bbl/hr (Fig. 2 [58,243 bytes]).

Avern explained that existing methods for cuttings disposal, including natural dispersion, well bore reinjection, and ship-to-shore, will benefit from the process. He foresees that the industry is turning more towards well bore reinjection as the solution for cuttings disposal. This is perceived as the most efficient and effective environmental method throughout the industry.

Reference

Avern, N.R, Stables, R., and Simpson, D.T., "New Developments of Ultrasonic Processors and Applications for Environmental Improvement," SPE paper No. 37863, 1977.