Over the past four years, the use of multiple-zone proppant fractures has significantly improved production and the ultimate productive life of horizontal wells in the Valhall chalk field. A typical well is designed with up to 10 proppant-fractured zones. The stimulation technique involves the pumping of as much as 300,000 lb of proppant into each zone and during this process up to 70,000 lb of excess proppant may remain in the wellbore, which is cleaned out using coiled tubing. Since the material contains a resin coating to facilitate adhesion in the reservoir, disposal is restricted.
Previously, this waste had to be collected offshore, placed in large bags, containerized and shipped to shore for incineration - a practice that was costly, wasteful and environmentally suspect. In 1997, engineering studies revealed that the properties of the waste material made it suitable for re-cycling and use in future operations with minimal impact on fracture performance. While re-using proppant has become an accepted practice with no noticeable effect on well productivity, logistically it had some limitations. The material still had to be collected offshore and transported onshore where it was stored for several months before being reloaded into the stimulation vessel for reuse in the next fracture treatment. This represents storage problems and environmental exposure of these materials. Furthermore, with no unforeseen delays, this was a 24-hour two-way trip.
This paper describes the development and successful application of a unique system that allows the excess proppant to be collected, treated and recycled on location. The authors will discuss the evolution of the recycling technique and discuss the step-change improvement that allows the material to be reused immediately offshore utilizing specialized vacuum equipment.
Some 150,000 lb of proppant was successfully recovered and recycled on the first well. The technique is still evolving and is likely to become a standard operation on Valhall wells. The offshore collection and recycling technique has eliminated the logistical problems associated with the previous method and will in time reduce the total proppant costs, along with minimizing the associated HSE impact.