In California, most oil and natural gas reservoirs are “conventional.” That is, the reservoirs are found in layers of underground rock (“reservoir rock”) beneath a layer of less permeable rock (“cap rock”). Over millennia, this less permeable cap rock trapped the oil and natural gas in the reservoir rock; without the cap rock, the oil and natural gas likely would have seeped to the surface long ago. These conventional reservoirs typically were under pressure. When they were first tapped, many would have had a natural “artesian” flow to the surface through the wells. Some would even have appeared as “gushers.” Today, after recovery of some of the reservoirs’ hydrocarbons, most of California’s oil and gas reservoirs require some form of stimulation to flow.
One way to stimulate flow is to fracture the rocks in the reservoir, creating channels through which the oil and/or natural gas can reach the well. The fluids are injected into the reservoir at high enough pressures to cause breaks in the reservoir rock. This type of hydraulic fracturing is conducted below the pressure at which the cap rock would fracture. This practice not only complies with CalGEM regulations to protect groundwater and public health and safety, but is also common-sense practice for the oil producer. No producer wants to take a chance on breaking the cap rock because doing so can cause a loss of production capacity from the reservoir.
In some other parts of the United States, natural gas is trapped not in a reservoir protected by cap rock, but inside uncapped rock formations. In these “unconventional” cases, hydraulic fracturing is necessary to free the resource for production. Unconventional natural gas resources are common in places like the East Coast’s Marcellus Shale gas deposits. The Marcellus Shale covers parts of New York, Pennsylvania, Ohio, Maryland, Virginia and West Virginia. In California, by contrast, hydraulic fracturing is principally a means of ensuring that individual, conventional wells attain maximum production, often a preferable alternative to drilling additional wells to produce the same resources.
There are other differences between the typical use of hydraulic fracturing in California and elsewhere. For instance, in other states the extraction of unconventional natural gas resources requires lengthy fracturing periods along lengthy stretches of horizontally-drilled production wells. Millions of gallons of water are injected under constant pressure, a process that may take days or weeks in order to effectively open the reservoir rock. In California, much less water is used and the period of pressurizing the reservoir rock is much shorter. In other states, the extent of fracturing in unconventional rock stretches for hundreds of yards along the horizontal well and the fractures stretch farther away from the well. In California, fracturing projects tend to use far less fluid to fracture within a narrow vertical band along a well, generally starting at a point several thousand feet underground, with the fractures extending only tens to hundreds of feet away from the well.
