Figure 1. Wireline tool string.
HYDRAULIC FRACTURING PROCESS
Again, let's first establish the situation here. Let's continue to examine a typical horizontal shale well. At this point, the well has been drilled to its entirety and has had production casing installed. Remember that this production casing is steel pipe that is installed the entire length of the well. Even more important to note, this production casing has cement all around it. Thus, in order to hydraulically fracture the rock, communication with the formation must be established through the casing. The production casing runs all the way to surface and will be the conduit for the fracturing fluids to be injected into the formation. The production casing will also be the conduit by which hydrocarbons will travel to the surface and be produced. Finally, remember that this shale well has a horizontal lateral that is thousands of feet long. Thus, we cannot hydraulically fracture all the rock at the same time. Because of this, the process must be done repeatedly - over and over, starting at the end (toe) of the lateral and working back towards the surface (the heel of the lateral). Each of these separated zones is called a "stage." These treatment zones are separated by plugs that are set in the casing after the treatment is over for that stage. We need to isolate and hydraulically fracture the rock into smaller sections because of the amount of pressure and water rate that is needed to fracture the rock and create a dense fracture network. If we tried to treat the lateral all at once, the entire treatment would go to the weakest part and not treat the rest of the lateral.
Now that you understand the situation more thoroughly, we can now begin to talk through the process. Remember that there are long papers and books written entirely on the process and design of hydraulic fracturing. Here, I will try to keep it basic, but try to ensure that you are quite informed on the subject.
Depending on how the well was left from the drilling rig, the first stage is going to be executed slightly differently. Thus, in order to not get too confusing or difficult, we will discuss the process that is used for the other stages.
A typical "stage" will start with the wireline crew. The wireline crew will run tools on a wireline into the well slightly further than the section that is "up next" to be fraced. The first thing they will do is place a plug into the wellbore just behind the section that was fraced last. This will isolate the stage that is about to be fraced from the stage that was just fraced. Once that is done, they will begin pulling the tools back out of the well. However, on their way out, they will fire perforation guns which will literally perforate (put holes into) the casing. When the casing is perforated, communication with the formation can now be established. These tools on the wireline are activated from the surface via electronic signals sent through the wireline down to the tools. Multiple precautions are taken to ensure that this is done safely and that the guns are only fired when intended (i.e., not at the surface!).
Figure 1. Wireline tool string.
Figure 2. Standard Composite Bridge Plug (Frac Plug).
Wirerline perforating guns
Once the wireline portion of the stage is finished, the casing has been perforated, the tools have been removed from the well, and the wireline lubricator (what holds the tools at the surface) has been disconnected from the well, the actual hydraulic fracturing process can begin. Once all the pump trucks and pipes are connected to the well, water and a small volume of acid is pumped. This small volume of acid (usually 3-15% HCl) will clean around the perforations and will allow for a lower surface-treating pressure. After the acid is pumped, a couple hundred barrels of water is pumped. This is called the "pad." When high surface pressure is applied and the pad is being pumped, the formation rock around the wellbore will be hydraulically fractured. Once the rocks begin fracturing, the volume of the pad will then create the fracture network. Then, sand (or proppant) is introduced. The purpose of the sand (or proppant) is to literally keep the formation propped open after the treatment is over and the well is produced. It wouldn't do any good to spend millions of dollars to frac the well, and then allow the formation to close back up once the treatment is over. Remember, the fracture network is needed to allow the trapped hydrocarbons to flow into the wellbore. When the sand is introduced into the water being pumped into the well, it is done slowly and at increasing concentrations in order to prevent plugging off the perforations and losing the ability to pump into the formation. Also, sometimes a cross-linker fluid system is used instead of or alongside the standard slickwater system. Cross-linker allows greater ease of placing proppant into the formation. However, many considerations must be made in the determination of whether or not to use cross-linker or gel.
Once the amount of desired sand has been placed, the wellbore is then flushed with water and the pumping is stopped. This "flush" is done to ensure that the wellbore is cleaned of sand so that when the wireline unit goes back into the well to set their isolation plug and perforate the next section, they should not get stuck in the sand.
This is the overall process in a small nutshell, but keep in mind that nearly every part of the process can be modified. Different types of proppant, fluid systems, plugs, perforating guns, etc. exist and can be utilized based on what the operating company believes will be the most economical for their well. Also, some options such as using "sliding sleeves" throughout the lateral can be used in order to remove the need for perforating. What I described above, however, is the most typical process.
Thus far in the lesson, I have used "water" as a very general term. Water used during the frac can be sourced from freshwater surface bodies, water wells, or can be reused brine (produced water from other oil & gas wells). Regardless of the source, the water will be treated on location to the specifications for use in the frac. The water is commonly treated with KCl, friction reducers, biocide, scale inhibitor, surfactants, and sometimes linear gel or cross-linker. However, a vast majority of the frac fluid is water and sand. These chemicals make up just a slight percentage (if that) of the overall fluids being used. More about these chemicals are below:
A standard horizontal shale well in any of the U.S. basins could cost roughly anywhere between another two to eight million dollars to complete. The cost will depend on the lateral length, efficiency of the operator, the pricing environment, and much more.
Belyadi, Hoss, et al. Hydraulic fracturing in unconventional reservoirs: theories, operations, and economic analysis. Gulf Professional Publishing, 2017.