Inferring Hydraulic Connectivity of Induced Fractures in the Near-wellbore Region Using DAS-recorded Tube Waves Excited by Perforation Shots

We present a novel use of tube waves exited by perforation (or “perf”) shots and recorded on distributed acoustic sensing (DAS) to infer and compare the hydraulic connectivity of induced fractures near the wellbore on a stage-by-stage basis. Evaluating the fracture connectivity near the wellbore is critical since it controls the flow of the hydrocarbons from the formation to the wellbore. Currently, there are no established methods used to assess this property. However, we discuss how tube wave decay rates can be used to infer relative differences in fracture connectivity between stages and, through field observations on DAS, demonstrate the correlation between decay rates and frac effectiveness. Additionally, we consider other potential uses of this data in unconventional wells such as assessing plug integrity and constraining fracture geometry with Krauklis waves. DAS data is commonly acquired during the perf shots but primarily for fiber depth calibration purposes and has not been well studied. Our work illustrates the untapped potential of this data and how it can be easily repurposed to bring new insights about fracture characteristics in the near-wellbore region.

Theoretical analysis of acoustic emission signal propagation in fluid-filled pipes

The theoretical investigation of acoustical wave propagation in cylindrical layered media is the main interest of our research. The propagation of wire break or slip related acoustical signal in the buried water-filled Prestressed Concrete Cylinder Pipe (PCCP) is taken as a specific application. The PCCPs are widely used for potable-and waste-water distribution and transmission systems, which are generally located below the surface ground. Therefore, it is difficult to inspect or detect the damage caused by the wire-break or slip related events in the pipeline. In current practice, the acoustic emission (AE) monitoring system is used for random examination of prestressing wires by excavating or internal inspecting of the pipe walls, which is based on field data analysis. This gives only the localized knowledge of wire break or slip, which can be misleading, underestimated of the extent of corroded areas, deterioration of wire failure, due to the system resonance, acoustoelasic effect, loading effect, etc. There is no systematic theoretical analysis from the acoustic signal generation to propagation related to these effects, and hence, a common problem in AD technology is to extract the physical features of the ideal events, so as to detect the similar signals. The theoretical analysis is important to understand how the AE signal is generated by the leak, wire break or slip related events and how the path characteristics, excitation frequency, and modes of propagation physically affect the signal propagation. For this purpose, and acoustical model is developed from the Navier's equation of motion. This can simulate vibrating AE signal propagation through the fluid-filled PCCP. The interaction of this propagation with the pipe structure is modeled by using Newton's law of motion in equilibrium. The principle of virtual work is used to develop the fluid-structure interaction. In this work, the impact of the path on the spectral profiles of the vibrating AE signals in different locations throughout the pipes were investigated for low and high frequency excitation signals. At low frequency, there is only plane wave propagation, therefore the stoneley or tube mode analysis is used for this purpose. The tube wave effects on the acoustical wave propagation were observed from this analysis. At high frequencies, there also exist rayleigh or shear modes which exhibit oscillatory amplitudes in the fluid and a decaying amplitude in the pipe and the surrounding medium. The eigenfrequency and the modal analysis is used in this case. From the analyses, the phase velocity, group velocity, tube wave velocity, system resonance frequencies, cut-off frequencies were observed. The high frequency analysis has some special advantage over low frequency signal. This can provide an earlier indication of incipient faults, which is important to detect the AE event in early stage of pipe deterioration. Moreover, it was established that the frequency of propagating AE signal in the pressurizing fluid medium ranges up to 30kHz. Therefore, it is important to investigate the wave propagation of AE signal propagation through the fluid column inside the pipe within the range of sonic/ultrasonic frequency. The acoustic wave propagation in fluid-filled PCCP of various radius, stiffness and thickness of the pipe as well as different types of surrounding medium, is obtained by applying a numerical Finite Element Method (FEM). Finally, the results are compared with available analytical solutions. The proposed model is independent of sources, dimensions and medium characteristics. Therefore, it can be used for the analysis of acoustic wave propagation through any type of cylindrical shells immersed or surrounded by different types of medium. The current analysis, therefore, has fundamental importance in many applications.

Theoretical analysis of acoustic emission signal propagation in fluid-filled pipes

The theoretical investigation of acoustical wave propagation in cylindrical layered media is the main interest of our research. The propagation of wire break or slip related acoustical signal in the buried water-filled Prestressed Concrete Cylinder Pipe (PCCP) is taken as a specific application. The PCCPs are widely used for potable-and waste-water distribution and transmission systems, which are generally located below the surface ground. Therefore, it is difficult to inspect or detect the damage caused by the wire-break or slip related events in the pipeline. In current practice, the acoustic emission (AE) monitoring system is used for random examination of prestressing wires by excavating or internal inspecting of the pipe walls, which is based on field data analysis. This gives only the localized knowledge of wire break or slip, which can be misleading, underestimated of the extent of corroded areas, deterioration of wire failure, due to the system resonance, acoustoelasic effect, loading effect, etc. There is no systematic theoretical analysis from the acoustic signal generation to propagation related to these effects, and hence, a common problem in AD technology is to extract the physical features of the ideal events, so as to detect the similar signals. The theoretical analysis is important to understand how the AE signal is generated by the leak, wire break or slip related events and how the path characteristics, excitation frequency, and modes of propagation physically affect the signal propagation. For this purpose, and acoustical model is developed from the Navier's equation of motion. This can simulate vibrating AE signal propagation through the fluid-filled PCCP. The interaction of this propagation with the pipe structure is modeled by using Newton's law of motion in equilibrium. The principle of virtual work is used to develop the fluid-structure interaction. In this work, the impact of the path on the spectral profiles of the vibrating AE signals in different locations throughout the pipes were investigated for low and high frequency excitation signals. At low frequency, there is only plane wave propagation, therefore the stoneley or tube mode analysis is used for this purpose. The tube wave effects on the acoustical wave propagation were observed from this analysis. At high frequencies, there also exist rayleigh or shear modes which exhibit oscillatory amplitudes in the fluid and a decaying amplitude in the pipe and the surrounding medium. The eigenfrequency and the modal analysis is used in this case. From the analyses, the phase velocity, group velocity, tube wave velocity, system resonance frequencies, cut-off frequencies were observed. The high frequency analysis has some special advantage over low frequency signal. This can provide an earlier indication of incipient faults, which is important to detect the AE event in early stage of pipe deterioration. Moreover, it was established that the frequency of propagating AE signal in the pressurizing fluid medium ranges up to 30kHz. Therefore, it is important to investigate the wave propagation of AE signal propagation through the fluid column inside the pipe within the range of sonic/ultrasonic frequency. The acoustic wave propagation in fluid-filled PCCP of various radius, stiffness and thickness of the pipe as well as different types of surrounding medium, is obtained by applying a numerical Finite Element Method (FEM). Finally, the results are compared with available analytical solutions. The proposed model is independent of sources, dimensions and medium characteristics. Therefore, it can be used for the analysis of acoustic wave propagation through any type of cylindrical shells immersed or surrounded by different types of medium. The current analysis, therefore, has fundamental importance in many applications.

Tube-wave monitoring as a method to detect shear modulus changes around boreholes: A case study

Monitoring the shear modulus of formations around boreholes is of interest for various applications, ranging from near-surface investigation to reservoir monitoring. Downhole logging tools and borehole seismic are common techniques applied to measure and characterize formation properties. These methods rely on transmitted and reflected waves to retrieve the rock properties. Wave modes travelling along the interface between the well and the formation, such as tube waves, are often considered as noise. However, tube waves are less attenuated than body waves, and contain information about the shear modulus of the formation surrounding the well. Hence, a potential use of this interface wave is of interest. As tube-wave properties depend on several parameters, e.g. well geometry, highly accurate measurements should be performed for use in inferring rock properties. We study the feasibility of tube-wave measurements as a monitoring method. Different experiments are conducted using a hydrophone array in two boreholes, with depths of 30 m and 95 m. The experiments are used to investigate how accurate the tube-wave velocity can be measured, and which parameters have most impact on the measurements. Our results suggest that it is hard to estimate the absolute shear modulus of the geological formation using tube-wave velocities only. However, it seems feasible to use them to monitor changes of the shear modulus, depending on the borehole set up and geological formation. The tube-wave monitoring can be used as a first step method to determine the depth along the well where changes occur before more accurate measurements are performed in a second step.

Bayesian full-waveform inversion of tube waves to estimate fracture aperture and compliance

The hydraulic and mechanical characterization of fractures is crucial for a wide range of pertinent applications, such as geothermal energy production, hydrocarbon exploration, CO2 sequestration, and nuclear waste disposal. Direct hydraulic and mechanical testing of individual fractures along boreholes does, however, tend to be slow and cumbersome. To alleviate this problem, we propose to estimate the effective hydraulic aperture and the mechanical compliance of isolated fractures intersecting a borehole through a Bayesian Markov chain Monte Carlo (MCMC) inversion of full-waveform tube-wave data recorded in a vertical seismic profiling (VSP) setting. The solution of the corresponding forward problem is based on a recently developed semi-analytical solution. This inversion approach has been tested for and verified on a wide range of synthetic scenarios. Here, we present the results of its application to observed hydrophone VSP data acquired along a borehole in the underground Grimsel Test Site in the central Swiss Alps. While the results are consistent with the corresponding evidence from televiewer data and exemplarily illustrate the advantages of using a computationally expensive stochastic, instead of a deterministic inversion approach, they also reveal the inherent limitation of the underlying semi-analytical forward solver.