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Improving Unconventional Completion Efficiency with Optimal Dissolvability & Structure Design for Dissolvable Frac Plugs

BackgroundIn various dissolvable plug applications, VERTECHS has been working with since 2016, statistics have shown that in China, Saudi and Argentina, casing deformities and slow mill-outs are common issues which greatly reduce the efficiency of completion operation, increasing overall costs and delaying wells from producing. Based on a study carried out by a major Chinese shale operator, in 2018, nearly 35% of completion NPT in the south-west region was caused by casing deformity. Before dissolvable technology was introduced and implemented in plug and perf operations, operators heavily relied on cast iron and composite frac plugs. These tools are extremely difficult to remove from the well, depending on the downhole conditions and many other factors. You might ask why composites are difficult to remove? Aren't they designed to be milled out within 3 – 5 minutes, like many suppliers claimed? I shall address this further down the page.


                                                                          


Casing Deformity Issues


A lot of factors can cause casing deformity that prevents a smooth plug & perf run, such as formation stress change during fracking, poor cementing quality, mechanical damage during running, and more. Most of the time, we don't know if there is a downhole restriction, until we pumped down a plug that hits something and gets the BHA stuck. It is only later that we spent hours and days getting the BHA unstuck, flushing the well and running coiled tubing to remove the plug and to confirm the severity of the restriction. It normally takes up to 1 ~ 2 days to retrieve the stuck BHA, but if the wireline gets snapped from the BHA while trying to pull out, then a fishing operation must be arranged, exposing the operation to uncontrollable risks, delaying wells from production. Calling in another wireline crew to work on other wells on the pad will increase the overall completion cost, also facing the uncertainties of operation risk, such decisions are not to be made lightly.


Mill-Out Run Limitations



Based on statistics in the above regions mentioned, it normally takes up to 20~60 minutes to mill out a composite frac plug. The reasons for such a strenuous mill-out are caused by many factors. Firstly, the size of coiled tubing used is only 2", that is due to the transportation limitation for oversized vehicles in mountainous terrain. Restricting the reel size and weight mounted on the truck that is allowed to drive on the road, 2" coiled tubing is as good as we can get here. Secondly, limited by size, the choices for aggressive motors and mill shoes are also limited. High downhole temperature and chloride concentration impairs the viscosity of gel sweep, affecting cutting carrying ability, also the circulating rate will be relatively lower. Thirdly, the length of these frac plugs are quite long (sometimes close to 20"), this means a lot of cuttings are produced during mill-up. So the coiled tubing crew is required to make short trips after milling every 3 ~ 5 plugs, to assist getting the cuttings out of the well and to prevent cuttings from filling up the annulus. Combining all the above factors, the overall mill-out efficiency is greatly reduced. Now imaging doing all that with multiple downhole restrictions in long horizontal laterals, to make matters worse, what if these plugs aren't composites at all? You get the picture.


For the past few years, operators in these regions have desperately turned to dissolvable technology to resolve these challenges. Dissolvable frac plugs can be quickly removed from the well after getting stuck due to downhole restriction. By injecting catalysts, the dissolving process is accelerated, causing the plug to dissolve and lose its integrity. The operator can then simply flush the well at high displacement to pump the plug further down the well, without the need to call in a coiled tubing unit.


Once the well is completed, a mill out run is no longer necessary, whereas the plugs will dissolve in the wellbore fluid, leaving a full ID producing well over time. However, if the operator wishes to speed up production, an easier version of mill out run – clean out run, is performed with coiled tubing. 




As time goes by, dissolvable frac plugs have replaced the majority portion of composites in the market, reducing mechanical intervention hours and costs, bringing more wells into production at an unseen rate.




Pursuit for Efficiency



As I mentioned in another article – Overcoming Learning Curve Leads to Successful Deployment of Dissolvable Plugs in Chinese Shale: balancing the scale of mechanical properties and dissolvability is the key to construct a qualified dissolvable frac plug.


Although these dissolvable frac plugs have successfully addressed the above challenges, there is still a lot of room for improvement. Since some dissolvable frac plugs still inherit the same traditional structure design of cast iron and composite frac plugs, they tend to be long and made up of many parts. So how can we make them more efficient?


To improve efficiency, we intend to improve the dissolvability of these new plugs, which we call Dissolvable Frac Seat. By improving dissolvability, it simply means that: Less materials used to construct the plug, less materials awaits to be dissolved downhole. Larger the surface contact area with wellbore fluids, more thoroughly the dissolution takes place.


Moreover, the “blend” for material composition is another key factor. Different application environment requires different material composition, and knowing the right blend requires massive experimental and operation data, as well as expertise.  


In comparison to a traditional frac plug, the newly designed structure has 60% shorter length with 50% larger ID once it set inside the casing, without compromising its anchoring or sealing abilities, the pressure rating of 10K psi is still a certain feature. In 2019, this 10k psi tailored dissolvable plug tests are conducted and broadcasted through our eLAB system for various clients. By compacting the structure, parts such as upper slips, upper cone, and back rings are removed, thus, reducing the length of the mandrel that is required to support them. If we run them separately and compare the results (old structure VS new structure), assuming both plugs are constructed with the same dissolvable material composition for a specified downhole condition, the dissolution rate will be identical. But the new structure features will greatly improve the dissolvability by reducing total body mass and increasing surface contact area with wellbore fluids, resulting in a much shorter total dissolution process, leaving zero debris downhole.


In early 2019, WIZARD Dissolvable Frac Seats were deployed in several major shales. With 10Kpsi pressure rating and improved dissolvability, it ensured successful frac jobs and smooth clean out runs all the way to TD in multiple high-temperature applications, reducing completion time and overall costs. Furthermore, it was also implemented in north-west China, where downhole temperature of 100degF can be extremely challenging for dissolvables. With field-proven reliability and dissolvability, it was quickly implemented in large quantities in these regions.