Mark E. Willis

Spatial orientation and distribution of reservoir fractures from scattered seismic energy

Wepresentthedetailsofanewmethodfordeterminingthereflection and scattering characteristics of seismic energy from subsurface fractured formations. The method is based upon observations we have made from 3D finite-difference modeling of the reflected and scattered seismic energy over discrete systems of vertical fractures. Regularly spaced, discrete vertical fracture corridors impart a coda signature, which is a ringing tail of scatteredenergy,toanyseismicwaveswhicharetransmittedthrough or reflected off of them. This signature varies in amplitude and coherence as a function of several parameters including: 1 the difference in angle between the orientation of the fractures and the acquisition direction, 2 the fracture spacing, 3 the wavelength of the illuminating seismic energy, and 4 the compliance, or stiffness, of the fractures. This coda energy is most coherent when the acquisition direction is parallel to the strike of thefractures.Ithasthelargestamplitudewhentheseismicwavelengths are tuned to the fracture spacing, and when the fractures have low stiffness. Our method uses surface seismic reflection tracestoderiveatransferfunctionthatquantifiesthechangeinan apparent source wavelet before and after propagating through a fracturedinterval.Thetransferfunctionforanintervalwithnoor low amounts of scattering will be more spikelike and temporally compact. The transfer function for an interval with high scattering will ring and be less temporally compact. When a 3D survey is acquired with a full range of azimuths, the variation in the derived transfer functions allows us to identify subsurface areas with high fracturing and to determine the strike of those fractures.Wecalibratedthemethodwithmodeldataandthenapplied ittotheEmiliofieldwithafracturedreservoir.Themethodyielded results which agree with known field measurements and previously published fracture orientations derived from PS anisotropy.

Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A

On Leg 78A we drilled Sites 541 and 542 into the seaward edge of the Barbados Ridge complex, and Site 543 into the adjacent oceanic crust. The calcareous ooze, marls, and muds at Sites 541 and 542 are lithologically and paleontologically similar to the upper strata at Site 543 and are apparently offscraped from the down-going plate. A repetition of Miocene over Pliocene sediments at Site 541 documents major thrust or reverse faulting during offscraping. The hemipelagic to pelagic deposits offscraped in the Leg 78A area include no terrigenous sand beds, but they contain numerous Neogene ash layers derived from the Lesser Antilles Arc. Hence, this sequence is quite unlike the siliciclastic-dominated terranes on land that are inferred to be accretionary complexes. The structural fabric of the offscraped deposits at Sites 541 and 542 is disharmonic, probably along a decollement, with an underlying acoustically layered sequence, suggesting selective underthrusting of the latter. The acoustically layered sequence correlates seismically with pelagic strata cored at Site 543 on the incoming oceanic plate. Cores recovered from the possible decollement surface at both Sites 541 and 542 show scaly foliation and stratal disruption. Approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits. In the incoming section, a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata. In the Leg 78A area, the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the Barbados Ridge complex.

Automatic P and S velocity determination from full waveform digital acoustic logs

Automatic methods of determining P and S velocities from full waveform acoustic logs are studied and compared. The suggested P‐wave method is an event detector which is based on threshold detection in a window near the previous picks and fine adjustment by a semblance correlation. The moveouts found by the correlation process are used to find common source P velocities as well as effective borehole compensated (BHC) P velocities. Compensated velocities are derived from waveforms from complementing tool positions which compare favorably to velocities from standard BHC sonic logs. We call the most reliable method for S waves the P‐correlated S‐method. It consists of correlating the P waveform with the rest of the record to find the S arrival. The S waveform is then correlated with the next record to determine the S velocity. This method is compared with published techniques and proves more stable and reliable. The method does not necessitate a distinct S‐wave arrival and can utilize the existence of the ref...

A Novel Application of Time Reversed Acoustics: Salt Dome Flank Imaging Using Walkaway VSP surveys

In this paper we present initial results of applying Time-Reversed Acoustics (TRA) technology to saltdome flank, seismic imaging. We created a set of synthetic traces representing a multilevel, walkaway VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded salt dome. We first applied the concepts of TRA to the synthetic traces to create a set of redatummed traces without having to perform velocity analysis, moveout corrections, or complicated processing. Each redatummed trace approximates the output of a zero-offset, downhole source and receiver pair. To produce the final salt-dome flank image, we then applied conventional, poststack, depth migration to the zero-offset section. Our results show a very good image of the salt when compared to an image derived using data from a downhole, zero-offset source and receiver pairs. The simplicity of our TRA implementation provides a virtually automated method to estimate a zero-offset, seismic section as if it had been collected from the reference frame of the borehole containing the VSP survey.

Redatuming through a salt canopy and target-oriented salt-flank imaging

We describe a new shortcut strategy for imaging the sediments and salt edge around a salt flank through an overburden salt canopy. We tested its performance and capabilities on 2D synthetic acoustic seismic data from a Gulf of Mexico style model. We first redatumed surface shots, using seismic interferometry, from a walkaway vertical seismic profile survey as if the source and receiver pairs had been located in the borehole at the positions of the receivers. This process creates effective downhole shot gathers by completely moving surface shots through the salt canopy, without any knowledge of overburden velocity structure. After redatuming, we can apply multiple passes of prestack migration from the reference datum of the bore-hole. In our example, first-pass migration, using only a simple vertical velocity gradient model, reveals the outline of the salt edge. A second pass of reverse-time, prestack depth migration using full two-way wave equation was performed with an updated velocity model that consisted of the velocity gradient and salt dome. The second-pass migration brings out dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank, forming prismatic reflections. In this target-oriented strategy, the computationally fast redatuming process eliminates the need for the traditional complex process of velocity estimation, model building, and iterative depth migration to remove effects of the salt canopy and surrounding overburden. This might allow this strategy to be used in the field in near real time.

Fracture Properties From Seismic Scattering

The seismic trace is a complex aggregate of reflected and scattered signals from subsurface formation interfaces and heterogeneities. Although many varieties of random noise may also be present in the trace, we know from reacquiring the same seismic survey that seismic data are highly repeatable, indicating that significant information about the subsurface is contained in the trace but not yet used by our standard analysis methods. Seismic scattering is a type of signal contained in the data that is generally not utilized.

Theory and Laboratory Experiments of Elastic Wave Scattering by Dry Planar Fractures

Remote sensing of fractures with elastic waves is important in fields ranging from seismology to nondestructive testing. In many geophysical applications, fractures control the flow of fluids such as water, hydrocarbons or magma. While previous analytic descriptions of scattering mostly deal with very large or very small fractures (compared to the dominant wavelength), we present an analytic solution for the scattering of elastic waves from a fracture of arbitrary size. Based on the linear slip model for a dry fracture, we derive the scattering amplitude in the frequency domain under the Born approximation for all combinations of incident and scattered wave modes. Our analytic results match laser-based ultrasonic laboratory measurements of a single fracture in clear plastic, allowing us to quantify the compliance of a fracture. Copyright

Fracture quality from integrating time-lapse VSP and microseismic data

Tight gas reservoirs are problematic to produce, often requiring multiple stages of hydraulic fracturing in order to create connected pathways through which hydrocarbons may flow. In this paper, we propose a new methodology to characterize the quality of hydraulic fractures. Using synthetic VSP and microseismic data, we test the concept that the rock volume containing open, gas-filled fractures will scatter seismic energy more profusely than a volume containing closed, nonproductive fractures. By measuring the amount of scattered energy in a time-lapse 3D VSP study taken before and after the hydraulic fracturing episode, we hope to compare the productive flow quality of different regions of the hydraulically fractured rock. The microseismic recordings allow us to locate areas which have been hydraulically fractured and create imaging operators to extract the scattered signals from the time-lapse VSP data.

Locating Microseismic Events With Time Reversed Acoustics: A Synthetic Case Study

We investigated the applicability of using the Time Reversed Acoustics (TRA) technique, and thus the whole waveform of the recorded signal, to locate microseismic events in a reservoir monitoring system (Lu, 2007). Reservoir monitoring has attracted a lot of attention over the last twenty years. One of the challenges in reservoir monitoring is locating the microseismic events that result from fluid injection and fracturing processes. Conventionally, a seismic event is located using the arrival times of P and S phases. The picking of arrival times could potentially be very time-consuming and difficult in certain situations, especially when the passive seismic events are continuously recorded by the monitoring systems. The retro-focusing feature of TRA can be applied to such a location problem and might provide an opportunity to locate events using the whole waveforms without picking different arrival phases. The basic concept involved in TRA is the fundamental symmetry of time reversal invariance. In the TRA approach, we first record the full seismograms at an array of stations. The traces are then time-reversed and numerically sent back into the medium at those station locations assuming an a priori velocity model. The wavefield of the back-propagation is tracked and, at the appropriate time, energy concentrates at the focal locations of each event. The TRA technique is particularly amenable at reservoir scale, in that a detailed subsurface velocity structure is usually available. We investigate TRA’s retrofocusing properties by experimenting with different receiver coverage on a 3D elastic reservoir model using full elastic finite difference simulations. The advancements in modern reservoir networks can provide a wide receiver coverage that can potentially make TRA practical. In this numerical study, we try to address issues involved in the implementation of the TRA technique in reservoir monitoring projects, such as what kind of monitoring network is suitable for good focusing, how sparse can the stations in a network be, and what is the focal resolution in different setups, etc.

Imaging dipping sediments at a salt dome flank - VSP seismic interferometry and reverse-time migration

Summary We present results of applying seismic interferometry to image dipping sediments abutting a salt dome. We create a set of synthetic traces representing a multi-level, walk away Vertical Seismic Profile (VSP) for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded overhanging salt dome. The sediment reflectors in the model dip up towards the salt dome flank. To process these data, we create a set of redatummed traces using seismic interferometry. This is done without having to perform any velocity analysis or moveout corrections. Each of these redatummed traces mimics the output of a downhole source and down-hole receiver pair. The linear v(z) gradient enables the redattumed data set to illuminate and capture reflections from both the salt-dome flank and the upward turning sediments. We then apply pre-stack depth migration to these traces to produce the final image of the beds and the salt dome flank. The final migrated results demonstrate that the reflected turning ray energy from both the salt flank and sediments are adequate to create structurally correct images using the combination of seismic interferometry and prestack depth migration.