Stimulation techniques are used to encourage production to flow from the reservoir rocks. Hydrocarbons are located in the spaces between pores of reservoir rock. Production is achieved when these pore spaces are connected and permeability, or the ability to transmit fluids, is such that the hydrocarbons flow out of the rock and into the well. In some reservoirs, the rocks have low permeability, and the hydrocarbons cannot be extracted for production. Other times, production is hampered by formation damage, when drilling into the reservoir rock lessens permeability.
Production can be achieved in these wells via a production stimulation method called well fracturing. Performed above the reservoir formation fracture pressure, well fracturing causes a highly conductive flow path between the reservoir and the wellbore. Well fracturing actually breaks or splits the reservoir rock open to encourage hydrocarbons to flow from the rocks into the well.
Performed after the well has been completed, or after the casing, tubing and perforations have been applied, well fracturing has evolved over time. Starting in the 1860s and used through the 1940s, explosive fracturing used to be the most common method of well fracturing. Explosive fracturing, also known as well shooting, detonated an explosive within the well to break the reservoir rocks.
Successful at stimulating production, yet dangerous, explosive fracturing introduced liquid nitrogen into the well via a tin cylinder referred to as a torpedo. The torpedo was lowered into the wellbore and detonated. The explosion created a huge hole that was then cleaned out and completed as an openhole, leaving the bottom of the well open into the reservoir.
Developed in the late 1940s, hydraulic fracturing, also known as a frac job, is the practice of injecting a well with large amounts of frac fluids under high pressure in order to break the rocks. Performed on both openhole and cased-well perforations, hydraulic fracturing quickly replaced explosive fracturing.
Used in a gel-like state, frac fluids consist of water and polymers, or long organic molecules that form a thick liquid. Both oil-based and foam-based frac fluids use nitrogen bubbles to achieve the fracture. Carbon dioxide can be used, as well, to minimize formation damage.
A frac job is performed in three steps. First, a large amount of frac fluids are pumped into the well. The high-pressure of the frac fluids and the continual pumping increases the pressure in the well, overcoming the strength of the reservoir rocks to break them apart. Fracing fluids are pumped into the well until the rocks are cracked to a desired length.
Then, frac fluid and propping agents are introduced into the well to extend the breaks and pack them with proppants, or small spheres composed of quartz sand grains, ceramic spheres or aluminum oxide pellets, that hold the fractures open after pumping has ceased. This is important, because then the hydrocarbons can flow through the open cracks in the reservoir rocks. Finally, the well is back flushed to remove the frac fluids.
Increasing both the rate of production and the overall production of a well, hydraulic fracturing is best employed in medium and hard formations. In fact, hydraulic fracturing is used in almost all tight gas sand reservoirs. In onshore US wells, approximately 50% of the gas wells and 30% of the oil wells are fraced.
Hydraulic fracturing can increase the production of a well by 1.5 to 30 times the initial rate of flow, as well as the overall production from 5 to 15%. Also, a well can be fraced multiple times during its production life.
For more information about reservoir stimulation, take a look at How Does Well Acidizing Work to Stimulate Production?