TABLE 5.1 Probability of Damage Increases with Number of Wells
|Total Number of Wells (N)||[PM]N wells||Expected Number of Wells Causing Moderate+ Damage|
ability of at least one observation of moderate+ damage given that N wells are in operation can be calculated2 as
This probability increases with the number of wells N (for PM = 1%), as shown in Table 5.1.
This example illustrates that, as an industry begins operation with a few wells, there might be no apparent problem with induced seismicity. As the industry expands to 100, 1,000, or more wells, there can be a significant likelihood that induced seismicity will cause damage to structures somewhere, as a result of the large number of earthquakes and ground motions that are induced, even though the probability of any one well producing such ground motions is small.
Tectonic earthquakes cause some level of earthquake risk for buildings, primarily in areas like California with relatively frequent events. Seismic building codes provide some level of protection but are not a guarantee against earthquake damage. In other regions, building codes provide lower levels of seismic protection, and earthquakes (whether tectonic or induced) may cause damage, depending on the level of ground motion associated with them.
Several steps can be taken to quantify hazard and risk. As described in the previous section, the quantification of hazard and risk requires probability assessments, which may be
2 This is a special case of the Bernoulli distribution with N independent trials and probability PM of occurrence of the phenomenon of interest (moderate+ damage). The probability of at least one observation of this damage is 1 minus the probability of no observations of this damage, given N independent trials. Any dependence among ground motions for a given technology can be examined as part of the hazard assessment step identified in the section Quantifying Hazard and Risk (this chapter), in particular step 3 in Table 5.2.