Unique Offshore Recovery System Dramatically Reduces Environmental Impact by Recycling Stimulation Proppant
- Martin Slater (BP plc) | Daniel Perez (Schlumberger Oilfield Services) | Chris Teesdale (BP plc) | Nicholas Hilbig (M-I L.L.C.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2003
- Document Type
- Journal Paper
- 5 - 12
- 2003. Society of Petroleum Engineers
- 1.13 Drilling Automation, 2.5.2 Fracturing Materials (Fluids, Proppant), 7.2.1 Risk, Uncertainty and Risk Assessment, 2.4.5 Gravel pack design & evaluation, 2.5.1 Fracture design and containment, 1.2.7 Geosteering / reservoir navigation, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 2.2.2 Perforating
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During the past 4 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 pumping 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 with coiled tubing. Because the material contains a resin coating to facilitate adhesion in the reservoir, disposal is restricted.
This waste previously had to be collected offshore, placed in large bags, containerized, and shipped onshore 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 recycling and use in future operations with minimal impact on fracture performance. While reusing 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 immediately reused offshore using specialized vacuum equipment.
Approximately 150,000 lb of proppant was successfully recovered and recycled from 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 health, safety, and environment (HSE) impact.
Located in the Norwegian sector of the North Sea, the Valhall field is an Upper Cretaceous, asymmetric, chalk anticline that forms an overpressured and undersaturated oil reservoir. Owing to poor chalk stability that results in formation influx and tubular collapse in both the reservoir and the overburden, the completion methodology used throughout the producing life of the field has proven to be a continually developing strategy. In 1982, the completion strategy centered around indirect proppant-fractured vertical wells, and, by 1986, had evolved to gravel-packed direct proppant fractured wells. Initial production following the introduction of horizontal blanket-perforated wells in 1992 was encouraging, but 2 years later, the field production had dropped by 20%. This was primarily a result of high incidences of formation influx and terminal liner collapse.
In 1995, multiple proppant-fractured completions were initiated in the flank areas of the field to couple the high productivity experienced with the horizontal wellbores and the longevity of the proppant fracture completions.1
Fracture design development in this field has centered on maximizing fracture conductivity while optimizing fracture dimension and spacing along the horizontal wellbore. This has been accomplished through laboratory, computational, and empirical methods.
Continued production gains have driven all subsequent wells in the field flanks to be completed in this manner. To reach reservoir targets, measured depths (MD) vary from 3400 to 7200 m. In a typical 1000-m horizontal section, as many as seven proppant fracture zones, spaced 165 m apart, yield production rates of 4,000 to 6,000 STB/day. Each fracture consists of a portion of ceramic uncoated material along with a resin-coated portion to provide wellbore stability. This is designed to prevent proppant flowback.
Aggressive tip screen-out designs continue to be performed because well productivity dominates economic optimization. This leads to an average of 12% excess proppant left in the well following screen-outs. This material was initially returned onshore to await disposal after being lifted from the wellbore with coiled tubing. The condition of the returned proppant varies depending on the pressure drop required on the surface choke to maintain balanced reservoir conditions during the wellbore cleanout procedure. Tests determined that on average, the returned proppant exhibited a conductivity of 3495 mD/ft, which corresponds to 33% of the retained conductivity of new material (Table 1).
Throughout the life of the well, production simulations indicated that recycling the proppant would provide additional net present value when compared to a transport and surface disposal option, which is both undesirable and costly.
Initially, proppant cleaned out of the well was shipped onshore for disposal. Since 1997, the proppant cleaned out of the wellbore following each fracturing treatment was recycled for use on specifically designed fracture treatments. Excess proppant was collected in 2,000-lb bags, then shipped onshore. It was stored here until it could be loaded into the stimulation vessel before a new fracturing campaign. The bags were lifted by crane one at a time and emptied into the silo onboard the stimulation vessel. A metal grid was used to separate any large contaminants that may have gotten into the bags at the rig site or during storage. Clearly, a step change in performance could be achieved if the proppant could be returned to the stimulation vessel immediately and reused in the next zone.
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