5.5.2: Fracking and Production Phase
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Once the drilling of the well and emplacement of the casing is completed, the next phase aims to initiate and enhance the production of hydrocarbons from the well. This phase includes perforation of the production casing and hydraulic fracturing of the reservoir rock surrounding the lateral. Initially, the lateral production casing is perforated by spaced charges sent down on a wireline (a "perf gun"). The small explosions penetrate the steel casing, providing an avenue for the later high-pressure fluids that will help open fractures in the reservoir rock to allow hydrocarbons to flow into the production casing and into the well to be produced at the surface. The perforation is done in relatively short segments, as is the hydraulic fracturing that follows; these are called "stages." Laterals can exceed 2 miles in length, but cannot cross under a lease held by another company. The longest lateral to date in the Appalachian Basin was completed in the Utica Shale, which underlies the Marcellus Shale by several thousand feet, in Ohio by Eclipse Resources and is 18,544 feet long consisting of 124 stages!
Once all stages are "perfed" the well must be hydraulically fractured ("fraced" or "fracked"). Fracking involves pushing fluids at high pressure into the lateral, stage by stage. Deeply buried shale formations are dense and nearly impermeable to flow of fluids and gases. The hydrocarbons that occur in the shale are trapped in tiny "pore" spaces that are generally isolated from one another in the rock matrix. These hydrocarbon-bearing pores must be connected in some way in order to allow the hydrocarbons to flow to the well. This is where fracturing comes into play.
In Marcellus Shale operations the fluid used for hydraulic fracturing is usually water containing a variety of additives. Water is rather incompressible, so that it can be pumped from the surface by large volume pumps to depths of thousands of feet while maintaining pressure great enough to open fractures in the rock around the lateral (connecting those pore spaces), with the hydraulic fracturing fluid penetrating the casing where perforations were made earlier. The pressurized fluid locally counteracts the great pressures exerted on rock at depth by the overburden and causes fractures to open, most likely those pre-existing joints that are oriented perpendicular to the lateral. However, once the fluid pressure drops, the fractures will close again because of the overburden pressure, so the fractures must be "propped" open somehow to keep them open and 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 end up being forced into the fractures that form to act as small wedges to hold the fracture open on a microscopic scale. The hydraulic fracturing fluid additives mentioned before give the water both a viscosity to carry the sand particles into the well and the formation while minimizing frictional resistance to flow. Thus, the typical Marcellus Shale fracs are called "slick-water fracs." One may actually see the nature of the additives used in individual wells by visiting "FracFocus.com." Until recently, this site was favored as a data repository. Note that Pennsylvania DEP is currently developing its own site and requirements (mid-2016), partly because of the difficulty of data searches and incomplete reporting on the industry-supported FracFocus platform.
There are significant safety considerations during a hydraulic fracturing job because of the high pressures being applied. The volume of water required for fracking is also significant, with an average of about 4.5 million gallons used per well through 2014 for Marcellus Shale wells. Hydraulic fracturing initially required low-salinity water because the chemical additives would not work effectively with salinities over about 20,000 ppm (2%). So most of the water was fresh water pumped from streams and rivers. Industry developed large impoundments to store this water strategically for frac jobs in areas of rapid well drilling. However, over the past several years, additives have been modified to allow use of water of higher salinity, which means that wastewater can be mixed with fresh water leading to less freshwater withdrawal. Huge volumes of water must still be stored on site. Also, after fracking there is a period of "flowback" during which excess water returns to the surface. Interestingly, for Marcellus Shale wells, only about 15 to 30% of the volume of water that is pumped down the hole returns to the surface. It appears that the shale "imbibes" the water. In addition, the water returning to the surface is far more saline than that used in fracking. Flowback can occur over a period of a year after completion of a well and the late-stage water returning can have a salinity as great as 10 times that of seawater. The flowback water can contain toxic substances, including radium (a radioactive element) and various trace metals, so treatment and proper disposal are important considerations. This aspect of water handling is detailed in a previous "module" in this course. During the flowback phase any natural gas coming to the surface is typically "flared" to the atmosphere (methane is combusted to prevent dangerous buildups). These flares can be seen at night.
Once the fracking is completed, wells can be put online, as long as there is pipeline access (see earlier "module" on pipelines) and gathering lines are built to the wellsite. Pressure is controlled at the wellhead so that gas can flow into the lines. There is a water separator as well, and any water produced with the gas is siphoned off and held in a tank on the restored pad. These tanks must be emptied periodically by truck. The gas produced by each well is monitored by gage in order to determine royalties to be paid to leaseholders. Effectively, an area of one square mile (640 acres) can be drained by six wells on one pad in a pitchfork layout. This is far better than conventional well production (vertical only) for which there would be one pad and well for each 16 to 32 acre plot with much more surface impact for infrastructure.