Once a well has been installed, the next phase involves stimulating the surrounding shale formation for the production of hydrocarbons through the process of hydraulic fracturing. Remember the shale has a very low permeability (measured in nanodarcies, it's less permeable than concrete!), therefore fractures need to be opened up in the shale to connect pores containing oil and/or gas to provide pathways for the hydrocarbons to flow through and toward the production well. In order to hydraulically fracture the shale the following steps occur:
- A hydraulic fracturing fleet is mobilized to the site which provides the necessary equipment to hydraulically fracture the shale. The figure below shows the various types of equipment that is brought in and how it is arranged on the well pad. Water is pumped from frac tanks or impoundments (more on that later), which is then mixed with chemicals (from chem storage) and sand (from sand trailers) in the blender, which is then routed through the pump trucks to increase the pressure of the fluids high enough to fracture the shale and then inject them into the wellhead and ultimately the shale. This is all monitored from the control van to make sure the fracturing occurs according to plan.
- Before the shale can be fractured the initial phase includes perforating the production casing with shaped charges with a perforating gun that is sent down the well to install dime-size holes through the production casing and cement. These perforations allow communication between the well and the shale formation for hydraulic fracturing fluids to be injected into the reservoir rock surrounding the lateral at high pressure. The perforating is done in relatively short segments of the well, typically 150-250 feet long, which are known as stages. The first stage is typically at the end of the well which is known as the toe of the well. For example, a well with a lateral that is 5,000 feet long may have 25 200-foot stages in order to fracture the entire horizontal interval of the well.
- Once a stage is perforated or "perfed", that stage of the well is isolated with a special seal known as a packer or plug, and hydraulically fractured ("fraced" or "fracked") using the equipment described above. Hydraulic fracturing involves injecting a mixture of fluids as shown below at high pressure (~8,000 to >12,000 PSI) into the stage interval in the lateral, in order to fracture the rock surrounding the well adjacent to that stage. The pressurized fluid causes fractures to open, most likely pre-existing joints or bedding planes. The fractures must be "propped" open to allow hydrocarbons to flow more easily to the well. "Proppants", such as fine-grained quartz sand or ceramics, are added to the hydraulic fracture fluid as it is pumped down the well and are forced into the fractures to act as small wedges to prop the fracture open just a few sand grains wide, thus allowing hydrocarbons to flow through the now open fractures. The fracturing fluid mixture is carefully formulated in order to optimize the production from the well. The chart below shows the typical mixture of fracturing fluids, note that most of the fluid consists of water, with a small percent of a variety of compounds.Source: FracFocus
- Once a stage is fractured, it is sealed off with a plug and the next stage is perfed, fractured, and plugged. This process is repeated until all intervals of interest are fractured. Each stage may take about 4 hours to fracture. Moving equipment up and down the hole takes time and the actual time to inject fluids into the shale is 2-3 hours. The injection rate is typically measured in barrels per minute (BPM). There are 42 gallons in a barrel and the injection rate may be 80-100 BPM, so a stage may take approximately 300,000 to 400,000 gallons of fluid or about 1,500 gallons of fluids per foot of shale being fractured. In addition to that, about 1-2 pounds of proppant is used per gallon of water, therefore upwards of a ton of sand may be injected into each foot of the shale, which is hard to imagine but that's what keeps the fractures open!
- It may take a week or more to fracture a well, which means a 24/7 operation in all kinds of weather, it's definitely hard work! Once all zones have been fractured, it is time to drill out the plugs that isolate each stage. The plugs are commonly made out of a composite plastic material, which can be drilled out with a drill bit attached to a wireline unit. A wireline unit is basically a large spool of steel cable on the back of a truck that can have a variety of tools added to the end of the wireline. In this case, a bit capable of drilling out the plugs is installed to remove the plugs, which relieves the pressure on the zones of the well that have been fractured. This means any fluids in or near the wellbore will flow back out of the well at a significant rate and need to be captured in tanks. These fluids are known as flowback fluids and contain fracturing fluid additives along with naturally occurring metals, salts, and radionuclides from the shale. For Marcellus shale wells, only about 5-10% of the volume of water that is pumped down the hole returns to the surface. The shale is relatively dry so has the ability to absorb the fracturing fluids and then release them, along with the gas, during the productive life of a well, which are known as production fluids. The longer the fluids have been in the shale the higher the concentrations of salts and other contaminants. Salts, in particular, reach high levels, upwards of 350,000 mg/L, which is 10 times saltier than the ocean so it is critical to properly manage these fluids. We will cover fluids management in a few lessons.
- Once the fracturing is completed and some of the injected fluids have flowed out of the wellbore, then well will begin to produce oil, natural, gas and/or natural gas liquids and can be "turned inline", which means commercially producing hydrocarbons. For natural gas, there will be a gas/water separator near the well, which sends gas into a pipeline and produced fluids into a tank for later reuse or disposal. Oil and liquids may be captured into tanks, water and brine are separated and ultimately trucked or piped to a refinery.
This video shows the hydraulic fracturing to help illustrate what we've just described.
Video: Producing Natural Gas From Shale (3:09)
The next section will discuss well production once a well is fractured.