A beam implementation is presented for efficient full‐volume 3-D prestack Kirchhoff depth migration of seismic data. Unlike conventional Kirchhoff migration in which the input seismic traces in time are migrated one trace at a time into the 3-D image volume for the earth’s subsurface, the beam migration processes a group of input traces (a supergather) together. The requirement for a supergather is that the source and receiver coordinates of the traces fall into two small surface patches. The patches are small enough that a single set of time maps pertaining to the centers of the patches can be used to migrate all the traces within the supergather by Taylor expansion or interpolation. The migration of a supergather consists of two major steps: stacking the traces into a τ-P beam volume, and mapping the beams into the image volume. Since the beam volume is much smaller than the image volume, the beam migration cost is roughly proportional to the number of input supergathers. The computational speedup of beam migration over conventional Kirchhoff migration is roughly proportional to [Formula: see text], the average number of traces per supergather, resulting a theoretical speedup up to two orders of magnitudes. The beam migration was successfully implemented and has been in production use for several years. A factor of 5–25 speedup has been achieved in our in‐house depth migrations. The implementation made 3-D prestack full‐volume depth imaging feasible in a parallel distributed environment.
Hess Corporation performed microseismic monitoring of hydraulic fracturing in two infill horizontal wells in the Middle Bakken Play of North Dakota during 2011. Six vertical observation wells were drilled to monitor microseismic events. Each observation well was fitted with a pressure gauge in the Middle Bakken interval, capped by a bridge plug, and instrumented with 40 geophones. The experiment was specifically designed to observe the interaction of the fracs with a pre-existing Middle Bakken horizontal well, which had been on production for about two and a half years. The producing well was shut in for the duration of the experiment and instrumented with a downhole pressure gauge.Experimental results show the hydraulic fracturing did not propagate as anticipated. Instead, the microseismic observations provided a very interesting result early during the stimulation of the first infill well. While fracturing the fourth stage of the first infill well, microseismic events appeared along the length of the nearby production well at distances up to 9,000 ft away from the open frac port. During this stage, and just prior to the appearance of the distant events, a pressure connection was established to the original well as recorded by the pressure gauge in that well. Our interpretation is that these microseismic events were distributed throughout the depleted zone surrounding the original production well. We provide evidence for the development of a cloud of depletion points through the use of pressure measurements, microseismic event speed plots, and Mohr's circle analysis.This experiment provided a unique opportunity to "see" the shape of the depleted zone and to understand its influence in developing Bakken resources. By analyzing the outline of the depleted zone outline, we can now propose a series of reasonable infill well locations at a minimum distance from the original producer. We are leveraging microseismic as a tool to optimize the spacing of development wells. Our objective is to infill drill at the minimum distance required to maximize fracture contact in-zone near the infill wells and yet avoid significant overlap into previously depleted zones. Thus, a new application for microseismic monitoring is envisioned, in which one would pressure-up a producing well in order to measure its actual area of depletion prior to planning subsequent development wells and executing field-wide strategies for hydraulic fracturing.
In May, 2010 Hess acquired its first microseismic survey in the Beaver Lodge area, North Dakota, over a 2-day time period. In conjunction with this project Hess also acquired a walk-around, offset, and zero-offset VSP to enable estimation of azimuthal anisotropy and generation of a 3D velocity model for proper microseismic event placement. Three different companies were contracted to process the data resulting in widely varying microseismic locations. Rather than accepting externally processed microseismic events that show completely different fracture geometries, Hess is developing an internal methodology to review event picking, 3D velocities, and survey geometries that will lead to dependable results. This presentation will discuss general processing methodology differences and acquisition problems that may have contributed to the inconsistencies. Integrating surface and pumping data with the microseismic reveals that incorporating a 3D anisotropic velocity model produces more reliable results.
While low-wavenumber components of an Earth model can be constructed using reflection tomography, full-waveform inversion (FWI) is capable of deriving a model with high accuracy that covers the full range of the spatial spectrum, from low to high wavenumbers, supported by the seismic data. This data-domain algorithm minimizes the mismatch in both amplitude and phase between the recorded data set and a numerically simulated one. The inversion is commonly solved using a local gradient-based iterative scheme, like the steepest decent or conjugate-gradient method. The model update is proportional to the gradient of the objective function and is computed by crosscorrelating a forward extrapolated source wavefield with a backward propagated data residual, which is computationally equivalent to reverse time migration of the data residual.
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