Hydraulic fracturing
From ShaleGasWiki
Hydraulic fracturing was first used in the late 1940s and has become a common technique to enhance the production of low-permeability formations, especially unconventional reservoirs--primarily tight sands, coal beds, and deep shales.
Hydraulic fracturing is a technically complex process. In horizontal shale gas wells, fracturing is done in multiple stages, starting at the “toe” of the well (the name given to the tip of the foot-shaped horizontal wellbore) and proceeding toward the “heel” (the end of the horizontal section of the wellbore that is closest to the vertical portion).
A wellbore that extends 5,000 feet laterally within a shale layer might be hydraulically fractured in ten to fifteen stages several hundred feet apart. Each perforation interval is isolated in sequence so that only a single section of the well is hydraulically fractured at a given time and to prevent damage to other sections of the wellbore (see Figure 1).
During a hydraulic fracturing operation, fracturing fluid is pumped at high pressure through perforations in the section of the casing. The chemical composition of the fracturing fluid, as well as the rate and pressure at which it is pumped into the shale formation, are tailored to the specific properties of each shale and, to some extent, each well. When the pressure increases to a sufficient level, a planar hydraulic fracture opens in the rock, propagating more or less perpendicularly to the path of the wellbore.
A typical hydrofracture propagates horizontally about 500-800 feet away from the well in each direction. The fracturing pressure is carefully controlled to prevent vertical propagation beyond the thickness of the layer of gas-producing shale. The pressure needed to propagate the hydraulic fracture varies and depends on depth, the pressure of the gas in the pores of the shale and other factors.
One novel and significant discovery made while working in the Barnett Shale[[1]] was the importance that using slickwater as a fracturing fluid could have in virtually all shale gas. In contrast to the highly viscous gels used as fracturing fluids in low-permeability reservoirs to suspend and transport sand as far as possible from the well being fractured, to prop open the hydrofracture during production, slickwater is water with a limited amount of sand, friction reducers and other chemical additives to improve the efficiency of hydraulic fracturing. Slickwater works well in shale gas reservoirs because its low viscosity allows the fracturing fluid to slowly leak out from the hydraulic fracture into many small, naturally occurring fractures in the shale.
The increased water pressure in these small fractures induces shear slip events, microseismic activity that can be detected only by using ultrasensitive seismometers deployed in nearby monitoring wells. See: Seismic Impact This technique is known as microseismic monitoring. The microseismic events enhance the permeability of the shale and allow natural gas to flow more easily into the wellbore.
The locations of the microseismic events generated during slickwater hydraulic fracturing provides operators with a picture of where the hydrofractures propagated and where the stimulation of permeability occurred. It thus reveals the approximate volume of the reservoir that is likely to be drained at any given well.
Figure 1. Schematic of Multi-stage Hydraulic Fracturines and Microseismic Events
While a given horizontal well may have 10-20 hydrofrac stages, only four are shown here for simplicity. The figure is not to scale.
Freshwater aquifers above shale formations are unlikely to be contaminated by direct migration of hydraulic fracturing fluids from the horizontal section of the wellbore due to the significant distance between the shale formation and freshwater aquifer. Main article: Subsurface contamination risks of hydraulic fracturing. Improper well casing or handling of flowback, however, can contaminate groundwater, surface water and land.
Significant portions of this page are excerpted and adapted from Addressing Environmental Risks from Shale Gas Development, by Mark Zoback, Saya Kitasei and Bradford Copithorne, published by the WorldWatch Institute
