Plug Configurations

There are several different configurations that plugs can be run, all of which have to do with when pressure can be applied to the plug. Over time, the preferred configuration has changed. The current preference is the ball in place or ball on seat method of pumping down the ball with the plug. The goal of this analysis is to discuss the different configurations, their pros & cons, and whether or not pumping the ball down with the plug actually saves water/time.

Solid Plugs

The first plugs were designed to do just that, plug the well to stop flow in or out of the well at the position the plug is set. Originally made of cast iron and then innovated to composite, these plugs had solid mandrels that enabled the plug to hold pressure from both sides.

Once a solid plug is set, the well will be “plugged”. Pressure can be applied to the plug. The benefit of running a solid plug is that once it is set you can pressure test the plug to ensure it is working, prior to perforating the zone. Some operators like this ability to determine that the plug is functioning prior to perforating and then facing the zone.

The challenge with solid plugs is, if the guns do not function correctly then the well will be a closed system, and you will be unable to pump the next set of guns down the perforate the zone. This would result in having to use a wireline tractor or coiled tubing to carry the guns down to location, which would incur additional cost and time. Another downfall is that in the event of a screen out, where the frac sand locks up in the well and cannot be pumped into the formation, the operator cannot take advantage of pressure from below to help clean out the well.

From a design perspective, the solid part of the mandrel is located above the upper slips. This is done so that during milling the solid part is milled up prior to the slips. This allows the pressure from below to equalize prior to milling through the slips, reducing the risk of the pressure acting against the entire area of the plug during milling.

Ball Drop

A ball drop plug has a hollow mandrel with a clear passage through it. With this configuration, the operator will have to drop a ball onto the plug once the wireline BHA has been removed from the well to complete the isolation. From the beginning of plug & perf origins to mid-2019 this was the main configuration used for composite frac plugs. The challenge for this configuration is that you cannot pressure test the plug until you are performing the frac. Also, this requires someone to walk up to the wellhead and physically drop the ball, some companies are concerned about the safety implications of this action.

Ball in Place

For ball in place, the operator will run the ball with the plug. With traditional plugs, the ball will be tucked up inside the wireline adapter kit and will fall out near the plug after setting. When the frac starts, the ball will just roll up to and engage the plug, just as it would if it were dropped from surface. The benefit of this configuration is that the operator does not have to drop the ball from surface. This eliminates the time required to wait for the ball to get to the plug, but also eliminates the need for a person to get near the wellhead to drop the ball. Having the ball separate from the plug allows the operator to produce the ball out of the well if necessary.

Some newer plug designs have a wireline adapter kit that connects to the bottom of the plug. These “bottom set” plugs make it difficult to just tuck the ball inside the wireline adapter kit to achieve a ball-in-place configuration. These providers have gotten creative with how to achieve the ball-in-place configuration. These solutions depend on the provider and design of the plug.

Internal/Captured Ball

This configuration has the ball captured inside of the plug. This will typically be done with a smaller ball that is inside the mandrel and lands on a smaller seat. The ball will be captured inside the mandrel with a pin or some sort of mechanical lock. The benefit of this design is that you can pressure test right after setting and it will allow for some production if flow back is needed. The downfall again is if the guns do not go off, you cannot pump guns back into place. Also, the flow through the plug is reduced in the event of a screen out.

A note on Wireline Switches

In a wireline BHA there may be up to 15 distinct guns that the operator will use to perforate the zone to be treated. Each gun will be activated individually so that the perforations can be spread out across the entire zone. These guns are activated with an electrical signal down the wireline to a switch at each gun.

With a traditional switch system, each switch is actuated using the same signal. The first switch is waiting for a signal to actuate, once it is actuated the pressure signature from the actuation of the guns arms the next switch. This switch can then be actuated with the same signal. This sequence continues until all the guns are actuated. If any of the switches has an issue, the rest of the switches will be rendered useless because there is no way to arm them. If this happens with the first switch, none of the guns will operate.

The newer switch systems are called addressable switches. These switches are individually activated using a specific signal, allowing them to be controlled separately. Any issue associated with one switch will not affect the performance of the rest of the switches. This significantly reduced the risk associated with guns not performing. One operator told me that they perforated almost 5,000 stages without a single stage that required pumping the guns back down after an issue.

This higher reliability from addressable switches has made running ball in place or internal ball plugs more acceptable, by significantly reducing the risk of requiring an intervention.

Surface Operation

On the surface, during a frac job, a crane lifts a wireline lubricator onto the frac wellhead. The lubricator is a chamber that allows the wireline BHA (guns, setting tool, and Plug) to maintain pressure integrity in the well during run-in. The main valve on the wellhead will be closed and the BHA will be lowered into the lubricator. Once in place, the lubricator will seal around the wireline. The main valve on the wellhead can now be opened without exposing the pressure of the well to the atmosphere. Once sealed, the wellhead valve will be opened and the wireline BHA can be lowered into the well. When the BHA reaches to kick-off point, where the well starts to become horizontal, pumps will be used to pump tools to the desired location. This is done with relatively fresh water on location.

The wireline operator will then set the plug and start to pull out of hole (POOH) and perforate the STAGE based on the frac design. After perforation, the wireline BHA is removed from the well. At this point, the well is filled with pump-down water. The wireline lubricator is removed from the wellhead and replaced with the frac manifold that is connected to the frac trucks on location. Once connected the frac treatment will be started. The first part of this treatment will include acid to help break down the stage. This is then followed by frac fluid and varying concentrations of proppant (sand), until the stage has been stimulated per the frac plan.

How are the balls handled?

Each configuration has a different method of handling the ball that completes the isolation on the set plug.

Ball Drop Plugs

Once the lubricator has been removed from the well a ball will be placed on top of the wellhead valve. Or, some wireline operators offer a system that will drop the ball automatically. When the frac manifold is set up and the frac operation is ready, the valve will be opened. This releases the ball into the well. The operator can wait for the ball to fall for a bit and then start the frac operation. Just prior to the ball landing on the plug, the operator will slow down to allow the ball to land on the plug at 10-15 bbl/min. Once landed the fluid in the well will be directed into the stage above the plug. With ball drop plugs, most of the fluid in the well when the ball is released will be directed through the plug and into the stage below.

Internal Ball/Ball In Place/Solid Plug

No need to drop a ball because the ball is already in the well when the plug is set. When the frac is set up and the operations start, the ball will land on the seat of the plug and create isolation. The fluid in the well at the start of the frac will be directed into the stage currently being stimulated. Typically this takes a higher pressure than with the ball drop plugs because the stage hasn’t been broken down. Once the acid hits the stage, the normal operation commences.

Does any configuration save the operator time and money?

With any of the configurations, the fluid left in the well after the wireline operation does have to be pumped into the formation before the acid and frac fluid reach the stage. The configuration just determines if the fluid goes into the current zone or the zone below. With a ball drop, there may be some additional time, waiting for the ball to fall in the well prior to pumping. And, in a world where frac crews and wireline crews are completing 20 stages in a day any little bit of time can affect the operation.

With the improvement in wireline switches, drive for efficiencies, and the overall convenience of knowing the ball is delivered with wireline, the ball in place configuration will continue to be the configuration of choice. Any technologies developed that require a ball drop will have to be compelling enough to overcome the costs of dropping the ball.

STAGES Trends: Does plug configuration save Time and Money?