In the July/August 2015 issue of Upstream Pumping, a detailed discussion was offered about the complexities involved in achieving effective solids control when deploying a dual-centrifuge barite recovery system (“The Uses & Limits of Decanter Centrifuges,” read part 1 of the article here). As noted in that article, when centrifuges are properly used, they enhance the drilling fluid properties, thereby improving rig performance. By maintaining the target properties of the drilling fluid, centrifuges also lower the volume of waste drilling fluid and reduce raw material additive costs.
Despite these achievements, the evolution of the drilling industry has resulted in the rapid deployment of centrifuge-based solids control systems that are unable to reach their full potential. This is predominantly because shale shakers—as the first line of defense—tend to allow far too many fine solids into the active mud system, and solids control centrifuges—the second line of defense—tend to process just 25 percent of the total mud system circulation rate.
Modern drilling rigs are continuing to set standards by drilling deeper, faster and longer. As such, the oil and gas industry has seen a substantial increase in the volume of waste solids and liquids being generated from the solids control system. There has never been a more critical time to manage drilling fluid and the associated wastes as an integral and inherently inseparable element of an effective solids control system. This is especially the case when a properly deployed vertical cuttings dryer (VCD) can significantly reduce waste disposal costs, dramatically lower whole mud losses within those wastes, and improve the overall quality of the drilling fluid by allowing the shale shakers and centrifuges to be used at their full operating potential. All of this can be done for about the same investment as a typical small-bowl decanter centrifuge system.
Applications of VCDs
Common flow-line shaker cuttings can maintain an oil-on-cuttings (OOC) or water-on-cuttings (WOC) moisture content as high as 25 percent. As such—and on a conservative basis—an average well will lose approximately 5 gallons per minute of drilling fluid with the discarded flow-line shaker cuttings. Over a 10-hour day, this would equate to 3,000 gallons (71 barrels).
VCDs are designed to recover the drilling fluids that are found on the drill cuttings discarded from the flow-line shakers. The intent is to have the VCDs installed in a manner that allows the flow-line shaker cuttings to be immediately injected to a VCD to recover the lost drilling fluids. The lower the cuttings’ “age” (the amount of time that the formation solids have been exposed to drilling fluid), the higher the performance that can be achieved by the VCD.
Decanter centrifuges and VCDs share several similarities; however, their objectives are quite different. Centrifuges are deployed to cut waste solids from the liquid stream. The cut is usually called the underflow (also solids discharge or cake), and the cleaned liquid stream (or centrate) is usually called the overflow.
Conversely, VCDs are deployed to cut (recover) the valuable drilling fluid from the waste solids. For VCDs, the cut drilling fluids are considered the valuable centrate (also called filtrate), and the waste solids are considered the underflow, or solids discharge. Similar to decanter centrifuges, the centrate contains most of the liquid and the finer solids. The solids discharge contains limited liquid and the coarser solids. Like decanter centrifuges, the goal of a VCD is to have the solids discharge as dry as possible. Ultimately, the drier the solids, the more effective the drilling fluid recovery.
Given the fact that the cuttings discharged from the flow-line shakers are surface wetted, those cuttings maintain the full range of solids, from colloidal to coarse. As such, when the cuttings are subjected to the VCD, approximately 90 percent of the surface wetting drilling fluid is recovered from the cuttings via centrifugal force.
It is not uncommon for VCDs to be able to reduce the OOC/WOC content from 25 percent by weight to 2.5 percent by weight. In doing so, the centrate slurry maintains a high volume of fines that must be further treated by a dedicated high-speed decanter centrifuge or through the rig’s existing dual centrifuge barite recovery system before reintroduction to the active mud system.
VCD Deployment Challenges
Given the immediate return on investment achieved through the successful deployment of VCDs, the fact that VCD technology has not become a standard practice on all drilling rigs is the direct result of four factors:
1. The WBM Challenge
Until 2014, VCD technology was not well adapted for water-based drilling fluids. However, new proprietary screen media technologies have been developed that allow VCDs to operate in both water-based mud (WBM) and oil-based mud (OBM) environments.
2. The Myth of the Colloidal Solids Monster
A great deal of stigma had been perpetuated about the development of colloidal solids via the use of VCD technology. Historically, there was potential foundation for this theory, since older VCD systems operated with a “cake-wall,” in which a multitude of flites would carve away at the solids as they accumulated on the interior screen surface. However, the modern systems do not use a cake-wall. Instead, the flites maintain a tight tolerance to the screen interface, which sweeps the solids from the screen with each pass.
3. Potentially Counterintuitive Business for Major Service Provider Incumbents
The recovery of drilling fluids by VCDs will reduce the volume of lost drilling fluids, meaning that the deployment of such technology will lower mud bills. It is not practical to expect the same service companies that are selling drilling fluids to recommend a technology that lowers their revenue base by as much as 5 percent.
4. Lack of Education
Despite a deployment history that dates back more than 20 years and the fact that more than a quarter of the world’s rigs are already successfully using VCD technology, there has not been a great deal of formal education presented to the market.
Overcoming Common Solids Control Failures Using VCDs
Drilling fluid conditions will inevitably degrade over the course of a well’s life. Theoretically, there are means to keep this from happening. However, many of today’s solids control techniques make this hard to avoid. This is driven by two common solids control errors: coarse flow-line shaker screens, and 25 percent centrifuge slip stream treatment.
Most drilling rigs operate their primary flow-line shakers with screens that are too coarse (screens rated at American Petroleum Institute [API] 140 or lower). To cut costs, rig operators use coarse screens that last longer.
Despite the fact that most rig drilling fluid circulation rates operate between 800 and 1,200 gallons per minute (gpm), the largest centrifuge applications treat 100 to 300 gpm. Most centrifuge applications treat only one out of four parts (25 percent) of the drilling fluid during each pass. The challenge is that modern drilling techniques constantly generate colloidal and ultrafine solids, through the natural degradation cycle, faster than can be removed by the solids control system.
The combination of these factors results in drilling rigs operating with primary flow-line shakers that are too coarse to sufficiently support the goals of the solids control program and using insufficient centrifuge capacity to make up for the poor performance of the shaker systems. Despite the fact that drilling fluid operations have become significantly more advanced in the last 20 years, most rig operations continue to use 14-inch solids control centrifuges. Though these centrifuges may have been well sized for systems a decade or two ago, this is no longer the case.
Conclusion
The most cost-effective, field-proven and efficient means of deploying highly effective waste management at the rig site is through the deployment of VCDs. The common flow-line shaker cuttings discharge may maintain an OOC or WOC moisture content as high as 25 percent. As such, the average well loses approximately 3,000 gallons (71 barrels) of drilling fluid during a 10-hour day. If the makeup cost of this drilling fluid were just $50 per barrel, more than $3,500 of drilling fluid is lost per day.
A VCD operating at a 90 percent recovery rate will return $3,200 (64 barrels) worth of this drilling fluid to the active mud system. If there were only 10 drilling days per month, a drilling contractor could pay for an entire VCD system—including dryer package, telescoping stand, control panel, cuttings feed system and cuttings collection system—in less than one year.
However, the recovered drilling fluid savings are just one component of the big picture. By recovering 64 barrels per day of drilling fluids, the waste disposal volumes are reduced by as much as 27,000 pounds (assuming the drilling fluid maintains a weight of 10 pounds per gallon).
In many cases, this means one less truckload and one less landfill disposal fee per day, further increasing the potential daily savings—not to mention the fact that this presents a smaller environmental footprint. Even if the wastes are being submitted for further treatment, such as thermal desorption, the significantly drier material will dramatically lower the energy consumption and therefore the costs required for further processing.
More importantly, when properly integrated with a solids control system, VCD technology allows shale shakers to be fitted with finer screens. This lowers the volume of drilled solids entering the active mud system. The lower volume places less stress on the centrifuge system, improving its performance.
The combined effect of improved shaker and centrifuge performance ultimately results in higher drilling fluid quality. This improves the drilling rates of penetration and reduces the damaging effects of accelerated wear on bits, mud pumps and related equipment. In essence, when properly used, VCDs can help improve the performance of the solids control system, enhancing the drilling fluid properties, thereby improving rig performance with increased rates of penetration, improved cake-wall stability, reduced bit torque and reduced pipe drag.
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