With traditional fracking, several millions of "fracked" zones have been created where oil and gas are extracted and, of course, undesirable events have occurred where seismic movements have caused both damage to buildings and destroyed groundwater.
Creating and widening cracks has been used since ancient times. From the mid-19th century, nitroglycerin and then dynamite were used to access the oil or gas.
Nuclear charges have also been used, but the results were not what was expected when the high temperatures that arose caused the rock to become glass. In Sweden, a form of cracking is used everyday when "pressurizing" water wells with high pressure to achieve a better water flow and the method is described in the governing document "Normbrunn - 16 Guidance For To Drill Well", published by Sweden's geological survey.
The concept of seismic events is perceived by many as scary and certainly there is justification for this. Major earthquakes have caused tremendous damage and many deaths.
The top layer of the earth is divided into several plates that are constantly moving in relation to each other. This layer is called the lithosphere and the plates slide around, crashing and scuffing against each other. In the boundaries between the plates there are large faults or shear zones in the earth's crust.
The friction in the zones makes the movements far from smooth. It is instead that the movements of the earth's crust are never smooth but are experienced as sudden and often the effects of them can be severe.
The seismic events occur when the natural tension in the bedrock changes. The greatest tension is the one that goes horizontally, sideways, not as one might think vertically, ie from the top down.
Seismic events or earthquakes often occur in Sweden The Swedish National Seismic Network, SNSN, records on average one to two earthquakes per day in Sweden. Over the past ten years we have had an average of 17 earthquakes per year with magnitude 2 or greater, 5 earthquakes per year with magnitude 2.5 or greater and 1 to 2 earthquakes per year with magnitude 3 or greater. The largest earthquake in the last ten years occurred on September 15, 2014 south of Lillhärdal in southernmost Jämtland and had magnitude 4.1.
The image above shows the registration of a stimulus while a major earthquake occurred in Japan. No one could say they felt the bigger earthquake.
Local earthquakes occur, for example, when blasting at construction sites and even when a fully loaded truck travels over a bump in the road or a train runs over a joint in the track.
The image above shows the elapses from passing trains at the same time as the seismic events from the stimulation were recorded. To indicate the strength of earthquakes, the Richter scale is used, which is a logarithmic scale where each step or magnitude corresponds to an increase of the wave height or shaking by 10 times. This corresponds to about 32 times more energy. An earthquake of 3 - 4 is barely noticeable, while one in 8 or more can destroy cities. Earthquakes of about 8.0 occur on average once a year, while even larger earthquakes occur less frequently.
In our methods we use pressures that are less than the rock stress and in addition we make sure not to pump in larger crack zones.
This is to follow the rule, no major cracks - no major seismic events.
More on seismic
This term is used to describe both a sudden landslide in a crack, and the resulting tremor in the ground and the emission of seismic energy caused by landslides or by volcanic or magmatic activity, or other sudden voltage changes in the earth (USGS definition). According to Merriam-Webster's definition, an earthquake is a tremor or earthquake of volcanic or tectonic origin ().
When an earthquake occurs, it releases energy in the form of seismic waves that radiate from the earthquake in all directions. The different types of energy waves shake the ground in different ways and they also travel through the earth at different speeds. The fastest wave, and therefore the first to come, is called the P-wave. The P-wave, or compression wave, alternately compresses and expands material in the same direction as it travels. The S-wave is slower than the P-wave and arrives after the P-wave and shakes the ground up and down and back and forth, perpendicular to the direction it is traveling. The surface waves follow the P and S waves.
A seismic event is a generic term for events where energy is released in the earth's crust, resulting in a series of seismic waves. Because an earthquake for the common man involves a shaking of the earth felt by humans or animals, the term seismic event or micro-seismic event is often used by geoscientists when communicating with the public about minor and micro-earthquakes. Many seismic events are too small to be felt, and can only be measured with precision instruments.
|MMI||Max ground acceleration (g)||Maximum ground speed (cm / s)||Perceived shaking||Risk of injury|
|IN||<0.0017||<0.1||Does not feel||No|
|II to III||0.0017 - 0.014||0.1 - 1.1||Very weak||No|
|IV||0.014 - 0.039||1.1 - 3.4||Weak||No|
|V||0.039 - 0.092||3.4 - 8.1||Average||Very small|
|WE||0.092 - 0.18||8.1 - 16||Strong||Small|
|VII||0.18 - 0.34||16 - 31||Very strong||Average|
|VIII||0.34-0.65||31-60||Difficult||Means for the difficult|
Earthquake size and distribution
It has long been known that small earthquakes are much more common than large earthquakes. This relationship can be expressed with a formula called the Gutenberg-Richter relationship:
log (N) = a - bM
where N is the number of events having a magnitude greater than or equal to M, and a and b are parameters that fit the data. The parameter b, called the b-value, is usually close to one, which means that for every logarithmic decrease in magnitude, there are about 10 times as many earthquakes. Most of the earthquakes generated by EGS stimulation will have a magnitude less than 2.0. It is estimated that there are over 36,000 events of this size range per day worldwide.
Sliding is the relative displacement of previously adjacent points on opposite sides of a crack, measured on the crack surface. Shear slip can occur seismically or aseismically (without creating seismic waves).
Seismometer and seismogram
A seismometer is an instrument used to record seismic waves generated by earthquakes on a seismogram.
Below are the number of earthquakes in the world by magnitude.
|Class||Magnitude||Average per year||Average per day|
|Big||8 and higher||1|
|Very small||2-2,9||1,300,000 (estimated)||3 600|
|micro||1-1,9||13,000,000 (estimated)||36 000|
Many seismometers are installed in networks or arrays spread over the area of interest for locating seismic events in the region. To determine the location of seismic events, seismologists identify the arrival times of P and S waves on the seismograms of all instruments that have recorded the seismic waves. These arrival times are often called P ‐ picks and S ‐ picks. Theoretically, 3 P-picks and 3 S-picks can be used to triangulate the location of a seismic event. In practice, 5 P-picks and 2 S-picks provide acceptable site accuracy on a microseismic matrix such as the one described below, and 7 P-picks and 3 S-picks provide good site accuracy (Gillian Foulger, personal communication).
Hypocenter and Epicenter
Hypocenter is the point in the earth where the earthquake begins. The epicenter is the point directly above the hypocenter of the earth's surface.
The magnitude of an earthquake is determined from the logarithm by the amplitude of the waves recorded on a seismogram at a given period. The original magnitude scale was the Richter scale, commonly referred to as ML.
Torque and Torque magnitude
Momwent is a physical quantity proportional to the slip on the fault times the area of the fault surface that slips. It is related to the total energy released in the seismic event, and is denoted Mo. The moment can be estimated from seismograms. The moment is then converted to a number similar to other earthquake magnitudes with a standard formula. The result is called the moment magnitude (MW). Moment magnitude gives an estimate of the earthquake magnitude that is valid across the entire scale of magnitude, a characteristic that was lacking in previous magnitude scales such as the Richter scale. Therefore, seismologists now prefer moment magnitude as a scale and it is common to use only magnitude and M to refer to moment magnitude.
Compartive Energy Release
The formula for relating torque magnitude (Mw) to torque (Mo) in duvet centimeters (dyn-cm):
MW = log10 (Mo) / 1.5 - 10.7
One dune cm is equal to 1 × 10-7 Newton meters.
In practical terms, this shows that for every increase in torque magnitude, the total seismic energy increases 31.6 times (in the formula more precisely 10 1,5). This means that an event with M 3.5 releases the same amount of energy as about thirty-two events with M 2.5 and an earthquake with magnitude 6 releases about 32 times more energy than a magnitude 5 (and a magnitude 7 consequently approx. 1000 times more energy than one with magnitude 5).
The Richter scale and the currently used moment-magnitude scale are a measure of the magnitude, strength of earthquakes. The scale is logarithmic and a unit increase in magnitude means a 10x increase in strength. For example, an earthquake of magnitude 6 is 10 times stronger than an earthquake of magnitude 5. Furthermore, an earthquake of magnitude 7 is 100 times stronger than an earthquake of magnitude 5.
The subjective scale, which measures the observed impact of the earthquake on buildings and on nature, is usually the "Modified Mercalli Intensity Scale" (MMI). It is a 12-point scale where the different intensities are described in terms such as "hanging objects sway" (MMI II), "plaster and weak chimneys crack" (MMI VI) and "most brick buildings collapse" (MMI X). The MMI scale has 12 intensity levels where level 1 is lowest and is characterized by minor shakes that are perceived by seismic instruments. The highest level 12 is described as total destruction.
Ground speed and acceleration
Ground velocity is a measure of how fast a point on the ground shakes as a result of seismic waves passing through an earthquake. During an earthquake, tremors in the ground also cause acceleration, the transition from one speed to another. Ground velocity and acceleration decrease with distance from the epicenter of the earthquake. Maximum ground speed (PGV) and maximum ground acceleration (PGA) is the maximum speed and acceleration recorded by a particular station during an earthquake. Both PGV and PGA can be used to quantify the risk of damage from an earthquake. Engineers typically use PGV, or particle velocity, while seismologists use PGA more often. Ground velocity and acceleration are both measured on special seismometers called Strong Motion Sensors (SMS). PGA is usually quantified with respect to gravity (g).