Plug & Perf: Pump Down Operations
By some estimates over a million stages of frac will be pumped in North America in 2018. Most of these stages will be completed with the help of a composite plug. There are three phases to the operation that the composite plug is designed to endure. First the plug is pumped down at high speeds to its desired location without causing issues. Second, it must set, anchor, and seal to withstand the pressures of the frac, and then be quickly & efficiently milled up so the well can be placed on production without interruption. Each of these phases in the operation come with their own challenges that require careful attention in the design and deployment of the plugs. The pump phase is critical to the success of the overall operation and delivery of wells. A snag in the pump down on one well can, in some cases, affect the planning, commitments, and completion of several wells. Also, the costs associated with water and time used during the pump down operation can be a consideration as well.
I want to start by considering the dynamics, forces, and details associated with the pump down operation. First, perforating guns, a wireline setting tool, and a composite plug are connected to the wireline. This entire bottom hole assembly (BHA) can be up to 40’ long and weigh close to 1,000 lbs. In the entire BHA the plug will have the largest outside diameter (OD). This BHA will be pulled into a lubricator connected to the top of the well. When ready, the well will be opened and the wireline BHA will be lowered into the well. For the vertical section, the BHA will be lowered to the kick off point of the horizontal section. As the well becomes more horizontal the pumps will start to pump the wireline BHA along the horizontal section of the well to the specified location.
Let’s consider, for example, the dimensions of a 5.5” 20# pump down operation:
The plugs are being pumped down with less than ¼” clearance on each side of the plug. Considering the dynamics of the fluid velocity around the plug, they can experience some extreme forces. Most plug providers provide guidance on the balance of pump rate and line rate. Their goal is to manage the flow bypassing the plug. Because the bypass around the plug is so much smaller than that of the casing, the velocity of the fluid around the plug is going to accelerate.
If you run the numbers, pumping 15 bbl/min into 5.5” 20# casing will result in a fluid velocity of 675 ft/min. If, at this flow rate, the plug is moving at 400 ft/min, the velocity passing the plug is 1,711 ft/min. As you can see, the velocity of the fluid passed the plug is much higher because the bypass is only ~10% of that in the entire casing ID. If the pump rate holds steady and the speed of the plug slows by 1 ft/min the velocity passed the plug increases by over 6 ft/min. These higher velocities will create a low pressure around the plug. With lower pressures on the OD of the plug if there are any material, such as the rubber or elastomers used in the seal, they can respond to the lower pressure causing them to swab or swell. This swelling of the elements can cause them to contact the casing and cause a snag in the operation.
This becomes a larger concern as operators want to push the boundaries of plug speed. As plugs are traveling faster with higher flow rates, relative changes in plug velocity have a bigger impact on fluid velocity around the plug. The result is greater risk of issues as you increase the velocity of the plug.
In 5.5” 20# the RzrFRAC has a larger OD than most of the competition. This acts to provide a more efficient pump down, but could cause an issue because the fluid rate around the plug will be higher. Taking this into consideration, the RzrFRAC construction does not include any elastomers that would respond to a low-pressure zone like on a traditional plug. This reduces risk of the operation as well as the risk of higher plug velocities. We've experienced customers running plugs as high as 1,000 ft/min without issue.
Most of the composite plugs run today require dropping a ball to land on the plug to create isolation. The ball dropped from surface has to be pumped down to the plug. The RzrFRAC utilizes a much larger ball than the competition, 3.313” vs. 2”, which also provides for a more efficient pump down.
Understanding the dynamics of the operation and how different plug designs can affect pump down efficiencies, risk level, speed, and overall success is important for making the right decisions for your well. If you'd like to know more about anything regarding completions please contact me.