Composite Plugs: The Difference can be Material
For a product group labeled Composite Plugs, there sure are a lot of different materials used in the construction of Frac plugs. Even in the category of composite material there are different types of manufacturing that can affect the performance of the product. Fundamentally, there are three components of the plug that have differen materials, the body& components and the slips. Understanding the design and construction of the products you select for your well can help in determining how the frac plug will pump down, set & anchor and then mill up.
Composite Material
The definition of a composite is something that is made up of more than one material. For our purposes, composite refers to fiberglass. All composite plugs are primarily made up of fiberglass material, which is a combination of glass fibers and a resin material. The glass fibers are very thin, 2-10 times smaller than a human hair, and are either continuous and wound/woven into the resin or chopped and molded into the resin. The resin material is what binds the glass together, enabling it to take shape. Fundamentally, glass fibers and resin are combined then cured into a solid. From there, the solid is machined into a shape that can be used. There are several ways to combine the resin and the glass to achieve the desired goal. Some of the composite manufacturing techniques used in the construction of composite plugs are filament wound, convolute wrap, and resin transfer composites. Each of these type combine the resin and glass in ways to achieve different properties.
Filament Wound
With filament wound composite, continuous glass fibers are pulled through a liquid resin to coat them. The fibers are then wound around a metal mandrel to create a tube of composite. Once the desired outside diameter (OD) of composite is achieved, the composite tube & metal mandrel are removed from the winding machine and cured within an oven to create a solid composite. After curing, the metal mandrel is removed and the remaining composite tube can be machined into different components.
Filament wound composite is very good for tubular components. They can be highly engineered with specific glass types, resin types, and the wind pattern of the glass fibers. These variables can be altered to achieve different goals including higher collapse, higher tensile, higher temperature rating, easier milling, etc. All of this benefits the production of composite frac plugs because we’re operating within a tube and have to set within a tube (casing).
Also, the filament winding machines can wind up to 30’ tubes of composite, some of which can wind 6 of these tubes at a time. It is easy to produce volumes of filament wound composite with low amounts of labor. This lends itself to producing volumes of product at a lower cost.
Convolute
While filament wound machines use long continuous glass fibers to wrap resin soaked glass into tubes, convolute composite is made using a woven glass fabric that is already impregnated with resin. This “pre-preg” cloth is wound around a mandrel to create a tube, and is then cured to harden into the composite. The benefit of using a fabric made of glass, rather than continuous strands, is that you get the strength of glass in two directions. This adds additional strength to the composite for tensile and compressive applications.
Resin Transfer
With transfer molding the glass fabric is stacked or formed within a mold into a specific shape. The fabric is then impregnated with the resin through a transfer process. The resin is held at a specific temperature in a vessel and the glass fabric is held within a vacuum. The resin is then released into the vacuum environment of the glass, forcing the resin into the voids between the glass fibers within the fabric. The composite is then cured and machined to create the final part.
Molded Composite
Molded composites utilize Bulk Moulding Compounds (BMC) to form composite shapes using either injection or compression molding. BMC is either glass fabric or chopped fibers that are mixed with a resin. These compounds are either placed or injected into a mold and then thermoset or cured under temperature and pressure. The benefit of molded composite is the ability to quickly generate complex shapes in volumes.
There are many ways of combining the resin with the glass and these are just a few of the techniques used in the production of composite frac plugs. What is important is the composite is easily millable into small pieces. Also, the combination of glass and resin results in a specific gravity of 1.8-1.9 creating pieces that are easily lifted from the well during the milling process.
Slip Material
When setting a composite plug the tool is anchored in the well with sets of “slips”. Fundamentally, there is a cone paired with a wedge. The wedge will have sharp hardened areas that when forced up the cone will “bite” into the casing, creating an anchor capable of locking the plug in place and withstanding forces in excess of 200,000 lbs. For the slip to “bite” into the casing the hardened areas or material must be harder than the casing itself, which is typically ~30 HRC.
Full Cast Iron
For most packers in the industry, and for a large portion of the composite plugs run, the slips are made out of cast iron. Typically, a ring of cast iron is machined to include a “wicker” profile that once set in the casing holds the slip from moving back towards the direction of the set. The wicker profiles are heat treated to increase the hardness of the sharp points without hardening the rest of the slip body. This provides better mill up characteristics.
Cast iron is over 3x heavier than composite
The difficulty with cast iron is that it is heavy and more difficult to mill when compared to composite materials. As stated above, composite has an SG of 1.8 while Cast iron is around 7, over three times more dense. The result is less of the slip material is lifted from the well, increasing the chances of getting hung up during the milling operation.
Composite Body Slips with Inserts
The second most widely used configuration of a slip is a composite body with hardened buttons to provide the anchoring.
Metallic Buttons
Some plugs have buttons made of metal, either fully cast iron or powdered metals. Powder metal buttons are made from sintering metallic powder into the shape needed from the button. While powder metal sounds like it would be easier to grind/mill up it all depends on the metal powder, heat treatment, and manufacturing process.
Ceramic Buttons
Some composite plugs utilize a composite slip with ceramic buttons to provide the bite into the casing. While ceramic material is very hard, it is also very brittle. This allows the ceramic buttons to break up better during milling when compared to a metallic button. Ceramic has an SG between 5-6, making them slightly easier to remove during milling that their metal counterparts.
Slip Millability
So much focus is placed on the milling times for a composite plug that the actual goal of milling the plugs can sometimes be forgotten. The ultimate goal of the mill up operation is to remove the plugs from the well. Yes, it's important to get it done quickly and for the pieces to be small. However if you tear through the plug quickly and even get small cuttings, but you don't remove the debris from the well the goal hasn't been achieved. Choosing a plug with metallic slips or buttons will make it harder to remove all of the debris from the plugs just due to the specific gravity of the material.
A note on setting
All composite plugs are set using a setting tool that anchors them in the wellbore. The composite plug is connected to the setting tool using a shear mechanism. This shear mechanism is typically a shear ring or shear screws designed to withstand the amount of force needed to fully set the plug. Once the plug is fully set, the shear mechanism with shear and release the setting tool from the plug.
These shear mechanisms are metal, made of different alloys of brass or steel. All plugs contain these parts in various sizes, either less larger pieces or more smaller pieces. Most providers do not consider this part of their “plug” and will not count it in their calculation for % of metal content.
The difference can be material
The selection of materials by a provider are driven by several variables including cost, performance, ease of design, and availability. Each material selected has pluses and minuses for each of these variables. The goal of a completion engineer is to select the composite plug that brings the best mixture of performance and price. For performance the plug must deploy reliably, isolate completely, and be removed easily. For ease of removal the plug that can accomplish the first two challenges while containing the least amount of heavy material, overall material, the breaks up efficiently will provide the best performance.