In 2017, distributed acoustic sensing (DAS) technology was deployed in a horizontal well to conduct a vertical seismic profiling survey before and after each of 78 hydraulic fracturing stages. From two vibroseis source locations at the surface, time shifts of P- and S-waves were observed but decayed over days. Some stages also showed waves scattered off the stimulated rock volume. We have used 2D finite difference elastic wavefield modeling to understand these observations and connect them to underlying properties of the stimulated rock. We have developed an effective medium model of vertical fractures that close exponentially with time as fluid leaks off into the formation can match the distribution of P- and S-wave time shifts along the well. This has enabled estimates of the height, normal and tangential fracture compliance values, and decay time of the stimulated rock volume. Additionally, the kinematics of scattered waves observed in the data have been found to be consistent with PS conversion across the stimulated rock volume from an individual stage. With higher quality DAS data, stage-by-stage inversion for height, fracture compliance, and decay time attributes may be possible for characterizing variations in the effectiveness of hydraulic fracturing.

Optimization of well spacings and completions are key topics in research related to the development of unconventional reservoirs. In 2017, a vertical seismic profiling (VSP) survey using fiber-optic-based distributed acoustic sensing (DAS) technology was acquired. The data include a series of VSP surveys taken before and immediately following the hydraulic fracturing of each of 78 stages. Scattered seismic waves associated with hydraulic fractures are observed in the seismic waveforms. Kinematic traveltime analysis and full-wavefield modeling results indicate these scattered events are converted PS-waves. We tested three different models of fracture-induced velocity inhomogeneities that can cause scattering of seismic waves: single hydraulic fracture, low-velocity zone, and tip diffractors. We compare the results with the field observations and conclude that the low-velocity zone model has the best fit for the data. In this model, the low-velocity zone represents a stimulated rock volume (SRV). We propose a new approach that uses PS-waves converted by SRV to estimate the half-height of the SRV and the closure time of hydraulic fractures. This active seismic source approach has the potential for cost-effective real-time monitoring of hydraulic fracturing operations and can provide critical constraints on the optimization of unconventional field development.

This work experimentally investigates the impact of gas bubbles on the thermal and acoustic energy prorogation within the wellbore, at various water and air flow rates using Distributed Acoustic Sensing (DAS). This is the first study that experimentally investigates the thermal and acoustic propagation at both single and two-phase flow conditions using DAS. Our results will improve the production logging algorithms especially at multiphase flow conditions, benefit the detection of gas leakage in the vertical section of a wellbore, as well as pipeline flow monitoring. Distributed Fiber-Optic Sensing (DFOS) based production logging has drawn much attention in recent years. Comparing to conventional production logging tools, fiber-optic cables can endure much harsher borehole environments and can be deployed at a lower cost. This work presents an experimental study of using DAS to track the thermal slugging in single-phase (liquid) and two-phase (gas and liquid) flow within a vertical wellbore to estimate flow velocity and characterize multiphase flow behavior. A vertical flow loop is constructed for this research, which consists of a 7-m long transparent polyvinyl chloride (PVC) test section with a 1-inch pipe inner diameter. A single-mode optic fiber with thin plastic coating is wrapped evenly around the PVC pipe, with a fiber-to-pipe length ratio of 10.7. Tap water and compressed air are used as the testing fluids. The water and air are injected at the bottom of the test section, similar to field conditions where oil and gas mixture flows into the wellbore from the bottom. The water is directly supplied from the building water system without using an additional pump to minimize the unrelated acoustic noise. Air is supplied from an air tank charged by the building compressor with a maximum pressure of 80 psi. A peristaltic pump is used to inject a small amount of hot water (< 2% of the minimum water volume from the inlet) to generate thermal slugs at the bottom of the test section. For the case of single-phase water flow, the velocity of the thermal slugging signal is similar to the actual water velocity, as expected. For multiphase flow, the thermal signal looks almost identical with that for single-phase, although the bubble velocity is much higher than the water velocity. This observation indicates that gas, with low thermal capacity and in its bubble form, cannot carry enough heat to perturb the thermal slugs in the water severely. However, a detailed analysis of the thermal slugging velocity indicates a small increase with an increase of air flow rate. We interpret that the thermal slugging velocity is associated with the actual water velocity. The existence of the gas bubble decreases the effective water holdup (cross-sectional area occupied by the water phase) in the pipe. We find that, with a constant water volumetric rate, smaller holdup leads to higher in-situ water velocity, thus higher thermal slugging velocity. The decay of the thermal slugging signal is also analyzed. The signal decay is due to the heat exchange between the fluid and the surrounding, as well as between the warmer and cooler fluid within the pipe. We observe a faster signal decay associated with a higher bubble rate, which indicates a faster heat exchange rate with the existence of gas bubbles. Multiple physical processes may cause this correlation. First, as the gas bubbles travel through the water with a higher velocity, they generate local turbulence in the water phase and accelerate the heat exchange within the water. Another possibility is that the gas, although with much smaller thermal capacity, carries heat from the warmer section to the cooler section, therefore accelerating the thermal equilibrium process. The comparison between the single-phase water and two-phase air-water experimental results indicate that the gas bubbles generate acoustic energy as they move through the pipe. Even in the low-frequency DAS data band (<0.5 Hz), it appears that the higher background noise-level is associated with the rising bubbles. Detailed analysis of the DAS data indicates individual bubbles can be traceable if they are separated more than a gauge length.

Slug flow is one of the most common flow types encountered in surface facilities, pipelines, and wellbores. The intermittent gas phase, in the form of a Taylor bubble, followed by the liquid phase can be destructive to equipment. However, commonly used point flow sensors have significant limitations for flow analysis. Distributed acoustic sensing (DAS) can turn optical fibers into an array of distributed strain rate sensors and provide substantial insights into flow characterization. We built a 10 m vertical laboratory flow loop equipped with wrapped fiber optic cables to study the DAS response of rising Taylor bubbles. Low-passed DAS data allow for velocity tracking of Taylor bubbles of different sizes and water velocities. Moreover, we measured the velocity of the wake region following the Taylor bubble and explored the process of Taylor bubbles merging. The amplitude analysis of DAS data allows for the estimation of Taylor bubble size. We conclude that DAS is a promising tool for understanding Taylor bubble properties in a laboratory environment and monitoring destructive flow in facilities across different industries to ensure operations are safe and cost-effective.

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