Fifth EAGE Workshop on Borehole Geophysics

We observe distributed acoustic sensing records of guided waves excited by perforation shots in a low-velocity shale layer. Thanks to the high spatial and temporal resolution of distributed acoustic sensing acquisition, unaliased high frequencies of up to 700 Hz can be observed. We analyze and validate the existence of such waves by comparing the recorded data with synthetic data computed using acoustic modeling as recorded in both the horizontal and the vertical segment of the well. These guided waves are trapped within the low-velocity organic shale reservoir and hold a tremendous potential for high-resolution imaging of it.

Carbon capture and storage (CCS) is a key climate mitigation technology required to meet the Paris Agreement goal for limiting global warming. At Shell, we demonstrate that a CO2 storage site is performing as expected using a comprehensive risk-based Measurement, Monitoring and Verification (MMV) framework. Quest is the first Shell CCS project to have implemented a MMV plan for CO2 storage performance verification. We report here some of the operational data and insights from the borehole geophysics technologies deployed at Quest. The data acquired using the injection and monitoring wells have been critical in demonstrating containment and conformance within the geosphere of this CO2 storage site.

VSP data usually generates a higher resolution seismic image in the region of the borehole than surface seismic, but its imaging result often suffers migration artifacts due to the fact that there are fewer and fixed borehole receivers and therefore low and uneven CDP folds in a VSP survey. A common method to suppress the VSP migration artifacts is to reduce the migration aperture to a small angle. However, a small aperture angle can often fail to image significantly dipping structures. Here, we present an efficient and effective dip filter consistent with the local velocity model in VSP migration, which preserves true dip structures and in the meantime suppresses migration artifacts. Examples from both synthetic and field walkaway VSP data demonstrate that the dip filter consistent with the local velocity model effectively improves VSP imaging quality and thereafter interpretation of the VSP data.

Coiled tubing (CT) is a very long metal tubing on a drum that is used for intervention services as well as a conveyance method for logging tools, typically a production logging toolstring, when a well is highly deviated or horizontal. The conventional wireline method would not be able to move the toolstring down to the bottom of the well with gravity. Traditionally, there is no relationship between CT services and borehole seismic logging because CT services are mainly run in highly deviated cased hole development wells, whereas borehole seismic data is usually acquired in vertical or less deviated openhole exploration wells. However, real-time downhole CT logging services introduced fiber-optic cable inside the CT, which is used for telemetry communication originally with two multimode fiber (MMFs). Adding another MMF enabled distributed temperature sensing (DTS) while-CT-logging service. This situation created the possibility of borehole seismic acquisition-while-CT service by adding a single-mode fiber (SMF) in the fiber-optic cable. Called distributed acoustic sensing (DAS) vertical seismic profile- (VSP-) while-CT service, this is a new way of acquiring VSPs.

Analysis of surface seismic and VSP data shows that in many cases VTI, TTI, HTI anisotropy of seismic velocities are observed and can be significant. The lack of anisotropy in seismic migration leads to a distortion of reflections, errors in determining their depths and slopes, which significantly reduces the efficiency and accuracy of seismic results for the drilling projects and reserves estimates. The anisotropy parameters can be estimated accurately using various borehole methods including cross dipole sonic, walkaway VSP, walkaround VSP, 3D VSP and cross well seismic.

We compare multiple DAS datasets with different acquisition parameters, acquired in a land test well. The objective is to evaluate achieved acoustic coupling and recommend the “best-mode” recipe to acquire DAS datasets inside boreholes.

A two-stage optimization algorithm is proposed for estimating hypocenter location parameters and origin time of a passive seismic event by maintaining low computational cost while preserving accuracy. The algorithm starts with a global optimization technique on a coarsely discretized medium and ends with a gradient-based local optimization method for determining both the hypocenter and timing of each induced passive seismic event. The first stage deploys the systematic grid-search method due to its reasonably easy implementation and robustness in obtaining a global solution for the nonlinear and multimodal objective function. Using a coarsely gridded model, the grid-search method, being a global optimization technique, ensures superior results with less computational cost in contrast with local optimization methods, which require a starting solution, and may get trapped in a local minimum. The second stage deploys the Polak-Ribere conjugate gradient technique to improve the accuracy of hypocenter location and origin time estimates of each passive seismic event requiring an additional minimal computational cost since the forward modeling traveltime calculations are not needed.

Distributed acoustic sensing (DAS) technology offers full-well seismic sensor coverage for vertical seismic profiling (VSP) applications. A rapid time-lapse DAS VSP provides the opportunity to record the in-situ seismic response between individual fracturing stages. An experimental rapid time-lapse DAS VSP was acquired to detect changes in the seismic response following hydraulic stimulation. A traveltime analysis revealed that the direct P-wave arrivals were delayed relative to the baseline and the maximum time-delay was located at the center of the fracturing stage. The waveform difference between the baseline and post-frac data indicated fracture-induced seismic waves. The peak of the residual amplitude coincided with the fracturing stages. These observations were confirmed with simulated waveform data and are investigated further for applications of hydraulic stimulation monitoring and diagnostics.

By applying a vector-based 3D migration process to VSP data, we are able to generate a 3D image of the flank of a salt body, providing a significant improvement compared to traditional methods. The method allows the reconstruction of both the salt flank itself, and the sediments that tend to be truncated against the sides of the salt body. This is demonstrated by an application to a synthetic 3D dataset.

Characterizing subsurface fractures is a key to developing hydrocarbon and geothermal fields, as well as providing fundamental information on fracture system relevant to regional seismotectonics. Seismic characterization of fractures has generally been based on the effective medium theory, which considers seismically invisible small fractures. Therefore, there is a considerable scale gap between the fracture properties obtained by seismic methods and those from borehole logging. Recent studies of single-well reflection imaging using acoustic borehole logging data show the potential of filling the scale gap by providing fracture properties around the borehole up to a few tens of meter away from the borehole location. In the context of reflection imaging of individual fractures, in this study, we develop least-squares migration (LSM) coupled with linear-slip model. LSM solves the linearized waveform inversion to provide high-resolution quantitative images. Linear-slip model can describe wave reflection at a fracture accurately. We show numerical modelling examples of the proposed approach considering a vertical fracture with coupling compliances, and acoustic dipole measurements of a dipping fracture embedded in a background random velocity distribution. The results show that the proposed LSM provides higher resolution images than reverse-time migration, and more accurate images than the conventional LSM without linear-slip model.