Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 26

cause a reduction of the slip resistance of a fault located in the region of pore pressure in-
crease. In assessing the potential for induced seismicity, two basic questions arise: (1) What
is the magnitude of the pore pressure change? and (2) What is the extent of the volume
of rock where the pore pressure is modified in any significant manner? The magnitude
of the induced pore pressure increase and the extent of the region of pore pressure change
depend on the rate of fluid injection and total volume injected, as well as on two hydraulic
properties of the rock, its intrinsic permeability ( k ) and its storage coefficient ( S ), and on
the fluid viscosity (μ).
The permeability ( k ) is a quantitative measure of the ease of fluid flow through a rock; it
depends strongly on the porosity of the rock (the volume percentage of voids in the rock
volume) but also on the connectivity between pores. The storage coefficient ( S ) is a mea-
sure of the relative volume of fluid that needs to be injected in a porous rock in order to
increase the pore pressure by a certain amount; the storage coefficient depends on the rock
porosity in addition to the fluid and rock compressibility. The permeability ( k ) can vary by
many orders of magnitude among rocks; for example, the permeability of a basement rock
such as granite could be up to a billion times smaller than the permeability of oil reservoir
sandstone (Figure 2.1).
However, the storage coefficient increases only by about one order of magnitude
between a tight basement rock and high-porosity sandstone. The ratio k :μ S is the hydraulic
diffusivity coefficient ( c ), which provides a measure of how fast a perturbation in the pore

Middle-
East
Reservoirs

Hydraulic Fracturing Required to Produce

Beach
Sand

Shales Granite Clay Concrete Brick Building Stone

Tight Gas

0.000001 0.00001 0.0001 0.001 0.01 0.1 1 mD 10 100 1,000

Permeability Range of Producing Formaions and Where Fracturing Is Required

FIGURE 2.1 Comparison of permeability in oil and gas reservoirs utilizing permeability values for typical
rock types and common building materials. The higher the connectivity between the pore spaces, the higher
the permeability; for oil and gas reservoirs, higher permeability generally indicates greater ease with which
the hydrocarbons will flow out of the reservoir and into a production well. Permeability is most commonly
measured using a unit called a millidarcy (mD), and permeabilities can range between 1,000 mD (high
permeability, comparable to beach sand) to very low permeability (0.000001 mD, which would describe
the least permeable rocks such as shales). Other common materials (such as granite or brick) are noted
on the upper part of the scale in this figure to give a sense of the range of permeabilities on the millidarcy
scale. Although hydraulic fracturing has been used for decades to stimulate some conventional reservoirs,
hydraulic fracturing is required to produce from low-permeability reservoirs such as tight sands and shales
(left-hand side of the diagram). SOURCE: Adapted from King (2012).