Distributed Acoustic Sensing (DAS) is a Fiber Optic (FO) cable based technology which is gaining importance for VSP surveys, especially for time-lapse monitoring of reservoirs. DAS offers advantages over geophones but it also poses unique challenges: receiver depth uncertainty and low signal-to-noise ratio. Here, we present an analysis of a VSP acquisition where the FO cable was installed in a treatment well for monitoring a multi-stage hydraulic fracture treatment in the same well. We describe methods for depth calibration and discuss various sources of noise and a processing flow to enhance the signalto-noise ratio. Large spike noise, most likely from the DAS acquisition system, appears to be a major source of noise in the raw DAS data, but can be removed with careful processing. Other sources of noise include temperature fluctuations in the well and optical noise. The field trial showed that excellent quality VSP data can be recorded in a hydro-frac treatment well.

Monitoring hydraulic fracturing is important for optimizing well-completion and well-spacing. Monitoring efforts, so far, are limited to observing micro-seismic activity, shear-wave shadowing and velocity changes from a neighboring monitoring well. The advent of Distributed Acoustic Sensing (DAS) has allowed us to monitor changes from the treatment well itself. We describe a novel active-source seismic experiment with DAS in a treatment well and discuss time-lapse changes due to hydraulic fracturing. We observe amplification and attenuation of direct P-waves above and below the plug for each stage, respectively. These strong time-lapse changes appear to be long-lived, at least over a period of 10 days. The time-lapse phase changes are small and hard to interpret. We believe that the amplification in the stimulated zone is related to formation changes and the attenuation is probably related to fiber coupling changes. Though the current geometry is not ideal, DAS is promising for hydraulic fracture monitoring.

SUMMARYSurface topography and the weathered zone (i.e., heterogeneity near the earth's surface) have great effects on elastic wave propagation. Both surface waves and body waves are contaminated by scattering and conversion by the irregular surface topographic features. In this paper, we present a 2D numerical solver for the elastic wave equation that combines a 4 th -order ADER scheme (Arbitrary high-order accuracy using DERivatives) with the characteristic variable method at the free surface boundary. The method is based on the velocity-stress formulation. We demonstrate the method by calculating synthetic seismograms for simple features.

The nonlinear elastic response of rocks is known to be caused by internal microstructure, particularly cracks and fluids. In order to quantify this nonlinearity, this paper presents a method for characterizing the interaction of two nonresonant traveling waves: a low-amplitude P-wave probe and a high-amplitude lower frequency S-wave pump with their particle motions aligned. We measure changes in the arrival time of the P-wave probe as it passes through the perturbation created by a traveling S-wave pump in a sample of room-dry Berea sandstone (15 × 15 × 3 cm). The velocity measurements are made at times too short for the shear wave to reflect back from the bottom of the sample and interfere with the measurement. The S-wave pump induces strains of 0.3—2.2 × 10−6, and we observe changes in the P-wave probe arrival time of up to 100 ns, corresponding to a change in elastic properties of 0.2%. By changing the relative time delay between the probe and pump signals, we record the measured changes in travel time of the P-wave probe to recover the nonlinear parameters β~ −102 and δ ~−109 at room-temperature. This work significantly broadens the applicability of dynamic acousto-elastic testing by utilizing both S and P traveling waves.

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