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Evaluation of Microseismicity Related to Hydraulic Fracking Operations of Petroleum Reservoirs and Its Possible Environmental Repercussions

Figure 1. Typical microseismic array deployed in vertical well (a) and horizontal well (b).

ABSTRACT

Petroleum reservoir operations such as oil and gas production, hydraulic fracturing, and water injection induce considerable stress changes that at some point result in rock failure and emanation of seismic energy. Such seismic energy could be large enough to be felt in the neighborhood of the oil fields, therefore many issues are recently raised regarding its environmental impact. In this research we analyze the magnitudes of microseismicity induced by stimulation of unconventional reservoirs at various basins in the United States and Canada that monitored the microseismicity induced by hydraulic fracturing operations.

Author

Abdulaziz M. Abdulaziz

Mining, Petroleum, and Metallurgical Engineering Department, Faculty of Engineering, Cairo University, Giza, Egypt.
Received 15 February 2014; revised 17 March 2014; accepted 26 April 2014

In addition, the relationship between microseismic magnitude and both depth and injection parameters is examined to delineate the possible framework that controls the system. Generally, microseismicity of typical hydraulic fracturing and injection operations is relatively similar in the majority of basins under investigation and the overall associating seismic energy is not strong enough to be the important factor to jeopardize near surface groundwater resources. Furthermore, these events are less energetic compared to the moderately active tectonic zones through the world and usually do not extend over a long period at considerably deep parts. However, the huge volume of the treatment fluids and improper casing cementing operation seem to be primary sources for contaminating near surface water resources.

Introduction

Due to the increasing demands on the traditional oil and gas resources, hydraulic fracturing became an important technology applied for enhancing production from hydrocarbon reservoirs, particularly the unconventional ones. Recently, the conjugated practices of horizontal wells with multi-stage hydraulic fracturing not only have increased the well productivity dramatically, but also lead to enormous increase in hydraulic fracturing. This increase approaches the size of massive fracture treatments carried out in the 1970 [1] . Microseismic monitoring is a new technology that typically targets the impulsive, energetic acoustic emissions to map fracture growth during hydraulic fracking stimulations. Other applications utilize these emanations to monitor the slow creeping processes within the reservoir over long period due to production operations [2].

The acoustic emissions represent the released energy during formation deformation which corresponds to small (Mw < −1.5) to medium magnitude micro-earthquakes (although high magnitudes of Mw > 0 are also reported), known as microseisms. The resulting deformation is induced by stress redistribution within the reservoir based on stress-strain interaction and may activate slippage across pre-existing structures or initiate new fractures within the stimulated reservoir volume. The analysis of the recorded microseismic data is typically useful in locating the induced fracture system [3] , monitoring the geomechanical deformation [4], mapping fracture growth [5] , and calculating the stimulated reservoir volume [6] by the stimulation operations.

To successfully monitor a hydraulic fracture treatment, the location of the monitoring array, determination of an adequate velocity structure, and management of noise are important issues to be considered [1] . Microseismic imaging usually employs a temporary string of eight to twelve triaxial geophones in a monitoring well located close to the treatment well to observe the creation the of induced fracture systems within its environ (Figure 1).

Figure 1. Typical microseismic array deployed in vertical well (a) and horizontal well (b).

Figure 1. Typical microseismic array deployed in vertical well (a) and horizontal well (b).

To capture good quality spectral responses with minimal signal interference in a microseismic record, the downhole geophones should possess sufficient sensor response, minimal tool resonances, and suitable frequency response. In addition, the acquisition system should enable sampling rate between 0.5 and 0.25 msec that corresponds to Nyquist frequency 1000 and 2000 Hz respectively. Seismic attenuation is usually encountered at high frequency signals from far events compared to the low frequency and near ones that are usually mitigated during signal processing. In most cases, microseismic monitoring utilizes downhole sensors, while near surface sensors are in some cases deployed. Being close to the source in borehole microseismic, the depth of the microseismic event is usually accurately determined, but surface arrays proved to be more efficient in determining the spatial location of hypocenter. Dynamic microseismic images are obtained as a live streaming to the fracture propagation using the time history of the microseismic activity [7]. Later, additional seismic source attributes, such as strength or magnitude, are calculated and the complete record is processed to elucidate information about the travel path, e.g. anisotropy and velocity tomograms [8] .

It is well-documented that long-term injection of fluids into deep formations induces earthquakes [9] [10]. This is particularly true in regions susceptible to tectonic activities such as waste chemicals injection in the Rocky Mountains [11], water injection in geothermal plants [10] [12] , water flooding operation for hydrocarbon production optimization [13], and water disposal in mining industry [14]. Recently a considerable attention and arguments have been raised concerning the seismicity associating hydraulic fracturing operation [11], particularly, their role in contaminating the shallow aquifer systems that could be the main water supply for domestic use. Since such operations are not risk-free, several incidents of probable water resources contamination have been reported due to hydrocarbon production stimulation by hydraulic fracking [15] [16] .

Drinking water could be contaminated during hydraulic fracking operations through hidden pathways connecting the producing horizon with shallow aquifer, casing-cement failure, and flowback water used during the treatment. Reference [17] evaluated the potential impacts associating gas-well drilling and fracturing in Marcellus Shale on shallow groundwater systems in northeast Pennsylvania and upstate New York using groundwater analysis. The dissolved-methane concentrations and carbon and hydrogen isotope ratio documented the existence of thermogenic methane in numerous wells with high concentrations (a potential for explosion hazard) close to active natural-gas wells [18] . However, no signs of brine mixture from deeper formations or traces frack treatment fluids were detected. Such observations could be supportive for gas contamination from a near surface origin such as wellbore cement failure, but the deeper origin cannot be entirely excluded due to the extremely low density of methane that enables swift gas to flow across probable hidden pathways, compared to brine and frack treatment fluids. In this paper a detailed discussion on microseismicity associating hydraulic fracking is introduced to assess the potential of hydraulic fracturing to induce minute earthquakes that may jeopardize the surface and near surface environment using microseismic records of different fields, mostly from North America. The results of this research may help recognize the potential/role of hydraulic fracturing of tight petroleum reservoirs in contaminating the potable groundwater aquifers.

Methods

Faults are physically static if the in-situ stresses are creating enough frictional forces along fault planes. Fluid injection results in shear stresses within the rock by increasing the pore pressure and therefore weakening the rock fabrics. When the shear stress increases enough to overcome the in-situ stresses, the rock initiates a fracture followed by a slip or directly starts slip on a pre-existing fault plain, resulting in an earthquake.

Reference [19] , approximated the maximum static friction (Fmax static) by:

approximated the maximum static friction (Fmax static)

where:

µstatic is the static friction coefficient; Fnormal is the normal force.

Since microseismic records represent a graphical demonstration to stress decay, fracture geometry and growth behavior can be identified using standard earthquake seismology principles [13] [20] . To evaluate the seismicity associating hydraulic fracturing operation, it is essential to calculate the magnitude for each recorded event and compare the calculated values with injection parameters. Seismic moment can be calculated using source parameters with several techniques, the simplest of them was introduced by reference [21] . He utilized Fourier transform of S-wave displacement to estimate the event moment and radius. In this research, moment and other source parameters data are calculated using Brune’s method.

Reference [21] calculated the Moment (Mо) and the source radius (rо) by

calculated the Moment (Mо) and the source radius (rо) by

where: ρ is the density, Vs is the shear velocity, R is the distance from the receivers to the event, Ωo is the low-frequency amplitude of the displacement spectrum, and Fc is a radiation pattern factor. Since the hydraulic fracturing occurs within a small intervals of the producing horizon, the source-receiver distance {R} and radiation pattern factor {Fc} are most likely the influential parameters in Equation (2) to determine the Mo value.

To determine Ωo value for each event, the amplitude spectrum is plotted versus frequency of the microseism after correcting for attenuation. Then the corner frequency is graphically identified by the intersection of the power-law decay at high frequency with the line that approximately represents the low frequency amplitude (Figure 2). The intersection of those two lines indicates the likely corner frequency, which is approximately 350 Hz in the example shown in Figure 2, and the low-frequency amplitude of a little less than 2E−10 m-sec.

Figure 2. Graphical determination of the corner frequency using amplitude spectrum and frequency after correction for attenuation.

Figure 2. Graphical determination of the corner frequency using amplitude spectrum and frequency after correction for attenuation.

Reference [21] approximated the source radius (ro) as

approximated the source radius (ro)

where: Kc is a constant, and fc is the corner frequency. The value of the constant Kc has been used to equal (~2.2) by Reference [21] that provides, a conservative value of the size of the slippage plane.

The seismic energy (E) released during the slippage along the fault plane is approximated by Reference [22] as a function of seismic moment by:

The seismic energy (E) released during the slippage along the fault plane is approximated by

Analogous to Richter scale of earthquakes, the moment magnitude (Mw) is a more convenient value to represent seismic moment and/or seismic energy that is obtained by:

the moment magnitude (Mw) is a more convenient value to represent seismic moment and/or seismic energy that is obtained by

where Mo and E in this equation are expressed in dyne-cm. The slip displacement and area can be determined as a function of the seismic moment by:

The slip displacement and area can be determined as a function of the seismic moment by

where: μ is the shear modulus of the rock (typically 2.2 × 106 psi for shale), d is the slip distance, and A is the slippage area.

In the present work the calculated moment magnitude from different unconventional reservoirs is subjected to statistical analysis and plotted against various parameters related to the reservoir (e.g. depth) and treatment parameter (e.g. injection rate and volume). This helps understanding the capability of hydraulic fracturing operation to disturb the surface and subsurface environment at the vicinity of stimulated wells.

Results and Discussion

Microseismicity and Treatment Parameters

Thousands of fracturing stimulations have been monitored using a microseismic technique, in which broad variation in the magnitudes of the recorded seismicity is documented. Based on the estimated microseismic magnitudes calculated by Brune’s method [21] described earlier (Equations (2) to (7)), a plot of these values versus depth was constructed for Barnett Shale reservoir (Figure 3, data from Reference [1] ). Although the data points are relatively scattered in this plot, there is a trend of increasing the number of high-magnitude events, shown as dense clusters, with increasing the depth (Figure 3).

Evaluation of Microseismicity Related to Hydraulic Fracking Operations of Petroleum Reservoirs

In this research we analyze the magnitudes of microseismicity induced by stimulation of unconventional reservoirs at various basins in the United States and Canada that monitored the microseismicity induced by hydraulic fracturing operations.
Emanuel Martin
Emanuel Martin is a Petroleum Engineer graduate from the Faculty of Engineering and a musician educate in the Arts Faculty at National University of Cuyo. In an independent way he’s researching about shale gas & tight oil and building this website to spread the scientist knowledge of the shale industry.
http://www.allaboutshale.com

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