Tip-Effect Microseismicity - Numerically Evaluating the Geomechanical Causes for Focal Mechanisms and Microseismicity Magnitude at the Tip of a Propagating Hydraulic Fracture

Agharazi, Alireza; Lee, Byungtark; Nagel, N.B.; Zhang, Feng-Shou; Sanchez, Marisela

doi:10.2118/167129-ms

Cited by 9 publications

(5 citation statements)

References 9 publications

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“…The excellent agreement between the true fracture length and the observed microseismic length implies that at least some of the microseisms are generated in the vicinity of the tip of the fracture, and the detection of those microseisms provides accurate length assessment. Such behavior is fully expected from geomechanics considerations (Warpinski et al 2004;Agharazi et al 2013). Cipolla (2015) presents a single result for a fracture hit in a horizontal pad development in the Bakken in North Dakota.…”

Section: Fracture Lengthmentioning

confidence: 63%

A Validation Assessment of Microseismic Monitoring

Warpinski

Wolhart

2016

Day 2 Wed, February 10, 2016

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Microseismic monitoring of hydraulic fracturing in unconventional reservoirs is a valuable tool for delineating the effectiveness of stimulations, completions, and overall field development. Important information, such as fracture azimuth, fracture length, height growth, staging effectiveness, and many other geometric parameters, can typically be determined from good quality data sets. In addition, there are parameters now being extracted from microseismic data sets, or correlated with microseismic data, to infer other properties of the stimulation/completion system, such as stimulated reservoir volume (SRV), discrete fracture networks (DFNs), structural effects, proppant placement, permeability, fracture opening and closure, geohazards, and others. Much of the information obtained in this way is based on solid geomechanical or seismological principles, but some of it is speculative as well.This paper reviews published data where microseismic results have been validated by experiments using some type of ground-truth or alternative measurement procedure, discusses the geomechanics and seismological mechanisms that can be reasonably considered in evaluating the likelihood of inferring given properties, and appraises the uncertainties associated with monitoring and the effect on any inferences about fracture behavior. Considerable data now exist from tiltmeters, fiber-optic sensing, tracers, pressure sensors, multi-well-pad experiments, and production interference that can be used to aid the validation assessment.Relatively limited microseismic results have actually been validated in any consistent manner. Fracture azimuth from microseismic has been verified across a wide range of reservoir types using multiple techniques. Good validation of fracture length and height were performed in sandstones for planar fractures; fracture length and height in typical horizontal completions with multiple fractures or complexity have a lesser degree of verification. Other parameters, such as complexity, discrete fracture networks, source parameters, and SRV, have little supporting evidence to provide validation, even though they might have sound physical principles underlying their application. It is clear that microseismic monitoring would benefit from more attention to validation testing. In many cases, the data might be available but have not been used for validation purposes, or such results have not been published.

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Section: Fracture Lengthmentioning

confidence: 63%

A Validation Assessment of Microseismic Monitoring

Warpinski

Wolhart

2016

Day 2 Wed, February 10, 2016

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“…The shear stimulation due to changes in stresses is mainly driven by the propagation of a new hydraulic fracture. Agharazi et al (2013) and Nagel et al (2014) showed that a high shear zone develops at the tip of a propagating hydraulic fracture that moves with the propagating fracture tip. Natural fractures that fall inside this shear zone experience an increase in shear stresses and may slip, depending upon their orientation and shear strength characteristics.…”

Section: Dominant Stimulation Mechanismmentioning

confidence: 99%

“…Behind the shear zone, a compressive zone forms on either side of the fracture with lower shear stresses (Agharazi et al 2013 andNagel et al 2014). Within this region, the buildup of pore pressure due to fluid leakoff into formation is the only mechanism that can cause shear failure (wet events) on natural fractures.…”

Section: Dominant Stimulation Mechanismmentioning

confidence: 99%

“…In a low permeability rock such as unconventional reservoirs, pore pressure can be increased in two ways; first, directly, by injecting high pressure fluid into the rock, and second, indirectly, by rapid loading of the rock (undrained loading). In the second case, the pore pressure builds up due to rock deformation (poro-elastic effect) (Jaeger et al 2007, Detournay andCheng 1993). When the rate of loading (increase of net pressure on the fracture surface) is higher than the pressure diffusion rate, the excess pore pressure generated by rock deformation cannot dissipate at the same pace as the rock (pores) deforms, resulting in the buildup of pore pressure.…”

Section: Dominant Stimulation Mechanismmentioning

confidence: 99%

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A Geomechanical Study of Refracturing Based on Microseismic Observations - Case Study of Haynesville and Eagle Ford Wells

Agharazi

Kashikar

2016

Proceedings of the 4th Unconventional Resources Technology Conference

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Microseismic monitoring of refracturing of depleted horizontal wells frequently shows a concentration of microseismic activity at the heel area when no mechanical isolation is used. This observation suggests that a considerable length of the well toward the toe does not benefit from refracturing and remains unstimulated. Different completion techniques, ranging from injecting diverters to using mechanical intervention methods, are usually used to avoid the localized stimulation and to enhance the treatment effectiveness. However, often overlooked is the effect of the reservoir rock's mechanical characteristics and how they contribute to the treatment results.In this study we investigated the potential contributing factors to the observed microseismic response: i) fluid pressure drop along the lateral, ii) diverter ineffectiveness, and iii) stimulation of pre-existing fractures versus developing fresh fractures from new perforations. Estimation of pressure losses along the well for the common casing diameters and fracturing fluids indicates that a high pressure gradient develops along the well during refracturing. It results in significantly higher injection pressures at the heel than at the toe, leading to higher discharge rates at the heel. If the added diverters fail to seal off the perforations at the heel area, this condition persists throughout the treatment and causes the localized stimulation of the rock, as is usually observed by microseismic monitoring.The numerical simulation of refracturing indicates that under a non-uniform treatment pressure profile and in the absence of effective diverters, the initiation and propagation of new hydraulic fractures is unlikely. The dominant stimulation mechanism is the shear failure of natural fractures, driven by the increase of fluid pressure by injection of fluid through the old perforations. This result is consistent with the observed long delay in microseismic response to refracturing and the increasing event counts as pumping continues.Based on these findings, we developed an alternative refracturing method that aims at increasing the reservoir effective complexity and enhancing the conductivity of the pre-existing hydraulic fractures uniformly along the well. The proposed method consists of a prolonged low-pressure and low-rate pad stage to pressurize the reservoir, followed by a high-pressure injection stage to stimulate the pressurized natural fractures and to place proppant in the new fractures. Critical to the success of this method is to avoid a high pressure contrast along the well. This can be achieved by proper selection of injection pressure and fluid viscosity with respect to the reservoir stresses and pressure, and the well characteristics. Numerical simulations indicate that the proposed method can considerably enhance the efficiency of refracturing, at no additional cost compared to the common refracturing methods.

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“…The worldwide boom in coalbed methane, shale gas and other unconventional developments is now well-known, and the boom has had a significant impact on the balance of hydrocarbon production [1] . There's general agreement that coal is composed of micro and nanopores, with the majority being nanopores.…”

Section: Introductionmentioning

confidence: 99%

Mechanism of Liquid-Phase Adsorption and Desorption in Coalbed Methane Systems - A New Insight into an Old Problem

Li³

et al. 2015

All Days

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Coalbed methane (CBM) recovery is a process of desorption rather than adsorption. Traditional experimental studies of methane adsorption/desorption on moist coal samples illustrate that, there is little hysteresis between adsorption and desorption processes, and these two behavior could be described by Langmuir equation. However, it is widely believed that coal porosity is normally occupied by water, and few researches focus on the gas sorption mechanism in condition of pore systems saturated with water. In this condition, there is an equilibrium between methane molecular dissolution into pore water and adsorption on (or desorption from) pore surface. And it is a typical liquid-solid internal interaction rather than gas-solid internal interaction.This paper presents experiments with methane adsorption/desorption on coal samples saturated with water and illustrates the significant difference between liquid-solid adsorption and gas-solid adsorption. In this work, we carry out three sets of experiments with coal samples immersed in water, and the equilibrium pressure is Pϭ5.445 MPa, 5.220 MPa and 2.965 MPa respectively. Subsequently, we conduct desorption experiments based on the former adsorption experiments to simulate the production process in aqueous environment with decreasing pressure. During desorption process, we are surprised to find that desorption amount changes slightly with different pressure drops, and desorption ratio for four sets of experiments is only 3.36%, 4.98% and 4.21% respectively. This dramatic results indicate that the desorption amount from saturated aqueous solution is not sensitive to pressure, which differs considerably from gas-phase desorption case.Furthermore, we analyze the Gibbs free energy change during adsorption/desortion processes in these two different systems: gas-solid and liquid-solid interface systems. It should be noted that, in an ideal gas-solid interface system (such as adsorption/desortion on flat surface case without the effect of pore structure), the Gibbs free energy change in adsorption are approximately equal to that in desortion processes, thus, the phase transition between absorbed layer and bulk is quasi-reversible, and no hysteresis exist in these two processes. However, in the liquid-solid interface system, due to the additional energy consumed by nucleation, it requires more energy for unit molar of methane during desorption process than adsorption process, which indictes that adsoption/desorption of liquid-solid interface are unreversible. Based on this theory, we established a hysteretic model to describe the hysteresis phenomenon during adsoption/desorption from aqueous environment, and this model can be perfectly verified by our experiment results.Our result demonstrates that the process of the methane desorption from micropores under water saturated condition is hysteretic seriously. This meaningful finding indicates that when almost no free gas exists in matrix pores, i.e., only absorbed gas and dissolved gas exist, and adsorbed gas almost cannot deso...

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Tip-Effect Microseismicity - Numerically Evaluating the Geomechanical Causes for Focal Mechanisms and Microseismicity Magnitude at the Tip of a Propagating Hydraulic Fracture

Cited by 9 publications

References 9 publications

A Validation Assessment of Microseismic Monitoring

A Validation Assessment of Microseismic Monitoring

A Geomechanical Study of Refracturing Based on Microseismic Observations - Case Study of Haynesville and Eagle Ford Wells

Mechanism of Liquid-Phase Adsorption and Desorption in Coalbed Methane Systems - A New Insight into an Old Problem

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