High‐resolution crosswell imaging of a west Texas carbonate reservoir: Part 3—Wavefield separation of reflections

Geophysics ◽  
1995 ◽  
Vol 60 (3) ◽  
pp. 692-701 ◽  
Author(s):  
James W. Rector ◽  
Spyros K. Lazaratos ◽  
Jerry M. Harris ◽  
Mark Van Schaack
Keyword(s):  
High Resolution ◽  
Carbonate Reservoir ◽  
Data Sets ◽  
S Wave ◽  
Time Alignment ◽  
West Texas ◽  
Wavefield Separation ◽  
Vsp Data ◽  
Sampling Intervals ◽  
Multiple Domains

Using crosswell data collected at a depth of about 3000 ft (900 m) in west Texas carbonates, one of the first well‐to‐well reflection images of an oil reservoir was produced. The P and S brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution. Both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling. The brute stacks, particularly the S‐wave reflection image, also exhibited imaging artifacts. We found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high‐resolution reflection images. In this study, the most important elements for an effective wavefield separation were the time‐alignment of seismic arrivals prior to filter application and the implementation of wavefield‐separation filters in multiple domains, particularly in common offset domain. The effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals. In this study, 2.5 ft (0.76 m) vertical sampling intervals for both source and receiver were used, whereas most previous crosswell data sets were collected with much coarser sampling intervals, resulting in spatial aliasing and limiting the utility of the data for reflection processing. The wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional VSP data analysis.

High‐resolution crosswell imaging of a west Texas carbonate reservoir: Part 2—Wavefield modeling and analysis

Geophysics ◽  
1995 ◽  
Vol 60 (3) ◽  
pp. 682-691 ◽  
Author(s):  
Mark Van Schaack ◽  
Jerry M. Harris ◽  
James W. Rector ◽  
Spyros Lazaratos
Keyword(s):  
High Resolution ◽  
High Velocity ◽  
Field Data ◽  
Sampling Interval ◽  
Low Noise ◽  
Carbonate Reservoir ◽  
Modeling And Analysis ◽  
S Waves ◽  
West Texas ◽  
Wavefield Separation

We have collected low‐noise crosswell data in a high‐velocity carbonate environment with a spatial sampling interval of 2.5 ft (0.76 m). This sampling reveals a variety of coherent events not previously identified in coarsely sampled gathers. Nearly every event in our field record can be explained using simple approximations for the geology, source, and receivers without accounting for the presence of the boreholes. We have used synthetic records as a guide in a moveout‐based analysis of the field data. Our analysis shows that much of the full wavefield energy, i.e., scattered waves, in our data are converted modes arising from the direct P‐ and S‐waves. This observation suggests that for crosswell reflection imaging, the focus of acquisition and wavefield separation techniques should be on the suppression of once‐converted modes.

High resolution cross‐well imaging of a west texas carbonate reservoir: Part 1. Data acquisition and project overview

1992 ◽  
Author(s):  
J. M. Harris ◽  
Richard Nolen‐Hoeksema ◽  
J. W. Rector ◽  
M. Van Schaack ◽  
S. K. Lazaratos
Keyword(s):  
High Resolution ◽  
Data Acquisition ◽  
Carbonate Reservoir ◽  
West Texas

VSP wavefield separation: Wave-by-wave optimization approach

Geophysics ◽  
2007 ◽  
Vol 72 (4) ◽  
pp. T47-T55 ◽  
Author(s):  
Emil Blias
Keyword(s):  
Eigenvalue Problems ◽  
Real Data ◽  
Data Sets ◽  
Optimization Approach ◽  
Arrival Times ◽  
Vertical Seismic Profiling ◽  
Wavefield Separation ◽  
Vsp Data ◽  
The Waves ◽  
Sampling Event

Waves propagating across a vertical seismic profiling (VSP) array may be distinguished by their differing arrival times and linear-moveout velocities. Current methods typically assume that the waves propagate uniformly with an unvarying wavelet shape and amplitude. These assumptions break down in the presence of irregular spatial sampling, event truncations, wavelet variations, and noise. I present a new method that allows each event to independently vary in its amplitude and arrival time as it propagates across the array. The method uses an iterative global nonlinear optimization scheme that consists of several least-squares and two eigenvalue problems at each step. Events are stripped from the data one at a time. As stronger events are predicted and removed, weaker events then become visible and can be modeled in turn. As each new event is approximately modeled, the fit for all previously removed events is then revisited and updated. Iterations continue until no remaining coherent events can be distinguished. As VSP data sets are typically not large, the expense of this method is not a significant limitation. I demonstrate with a real-data example that this iterative approach can lead to a significantly better VSP wavefield separation than that which has been available when using conventional techniques.

High‐resolution crosswell imaging of a west Texas carbonate reservoir: Part 5—Core analysis

Geophysics ◽  
1995 ◽  
Vol 60 (3) ◽  
pp. 712-726 ◽  
Author(s):  
Richard C. Nolen‐Hoeksema ◽  
Zhijing Wang ◽  
Jerry M. Harris ◽  
Robert T. Langan
Keyword(s):  
Wave Velocity ◽  
Field Experiments ◽  
Velocity Ratio ◽  
Carbonate Reservoir ◽  
P Wave ◽  
Core Analysis ◽  
S Wave ◽  
Traveltime Tomography ◽  
West Texas ◽  
P Wave Velocity

We conducted a core analysis program to provide supporting data to a series of crosswell field experiments being carried out in McElroy Field by Stanford University’s Seismic Tomography Project. The objective of these experiments is to demonstrate the use of crosswell seismic profiling for reservoir characterization and for monitoring [Formula: see text] flooding. For these west Texas carbonates, we estimate that [Formula: see text] saturation causes P‐wave velocity to change by −1.9% (pooled average, range = −6.3 to +0.1%), S‐wave velocity by +0.6% (range = 0 to 2.7%), and the P‐to‐S velocity ratio by −2.4% (range = −6.4 to −0.3%). When we compare these results to the precisions we can expect from traveltime tomography (about ±1% for P‐ and S‐wave velocity and about ±2% for the P‐to‐S velocity ratio), we conclude that time‐lapse traveltime tomography is sensitive enough to resolve changes in the P‐wave velocity, S‐wave velocity, and P‐to‐S velocity ratio that result from [Formula: see text] saturation. We concentrated here on the potential for [Formula: see text] saturation to affect seismic velocities. The potential for [Formula: see text] saturation to affect other seismic properties, not discussed here, may prove to be more significant (e.g., P‐wave and S‐wave impedance).

High‐resolution crosswell imaging of a west Texas carbonate reservoir: Part 1—Project summary and interpretation

Geophysics ◽  
1995 ◽  
Vol 60 (3) ◽  
pp. 667-681 ◽  
Author(s):  
Jerry M. Harris ◽  
Richard C. Nolen‐Hoeksema ◽  
Robert T. Langan ◽  
Mark Van Schaack ◽  
Spyros K. Lazaratos ◽  
...  
Keyword(s):  
High Resolution ◽  
High Frequency ◽  
Large Scale ◽  
Carbonate Reservoir ◽  
P Wave ◽  
Small Scale ◽  
Observation Well ◽  
Traveltime Tomography ◽  
Seismic Profiling ◽  
West Texas

A carbon dioxide flood pilot is being conducted in a section of Chevron’s McElroy field in Crane County, west Texas. Prior to [Formula: see text] injection, two high‐frequency crosswell seismic profiles were recorded to investigate the use of seismic profiling for high‐resolution reservoir delineation and [Formula: see text] monitoring. These preinjection profiles provide the baseline for time‐lapse monitoring. Profile #1 was recorded between an injector well and an offset observation well at a nominal well‐to‐well distance of 184 ft (56 m). Profile #2 was recorded between a producing well and the observation well at a nominal distance of 600 ft (183 m). The combination of traveltime tomography and stacked CDP reflection amplitudes demonstrates how high‐frequency crosswell seismic data can be used to image both large and small scale heterogeneity between wells: Transmission traveltime tomography is used to image the large scale velocity variations; CDP reflection imaging is then used to image smaller scale impedance heterogeneities. The resolution capability of crosswell data is clearly illustrated by an image of the Grayburg‐San Andres angular unconformity, seen in both the P‐wave and S‐wave velocity tomograms and the reflection images. In addition to the imaging study, cores from an observation well were analyzed to support interpretation of the crosswell images and assess the feasibility of monitoring changes in [Formula: see text] saturation. The results of this integrated study demonstrate (1) the use of crosswell seismic profiling to produce a high‐resolution reservoir delineation and (2) the possibility for successful monitoring of [Formula: see text] in carbonate reservoirs. The crosswell data were acquired with a piezoelectric source and a multilevel hydrophone array. Both profiles, nearly 80 000 seismic traces, were recorded in approximately 80 hours using a new acquisition technique of shooting on‐the‐fly. This paper presents the overall project summary and interpretation of the results from the near‐offset profile.

Multidomain analysis and wavefield separation of cross‐well seismic data

Geophysics ◽  
1994 ◽  
Vol 59 (1) ◽  
pp. 27-35 ◽  
Author(s):  
James W. Rector ◽  
Spyros K. Lazaratos ◽  
Jerry M. Harris ◽  
Mark Van Schaack
Keyword(s):  
Seismic Data ◽  
Common Source ◽  
Data Set ◽  
Traveltime Tomography ◽  
Wavefield Separation ◽  
Vsp Data ◽  
Well Data ◽  
Multiple Domains

While cross‐well traveltime tomography can be used to image the subsurface between well pairs, the use of cross‐well reflections is necessary to image at or below the base of wells, where the reservoir unit is often located. One approach to imaging cross‐well reflections is to treat each cross‐well gather as an offset VSP and perform wavefield separation of direct and reflected arrivals prior to stacking or migration. Wavefield separation of direct and reflected arrivals in VSP is accomplished by separating the total wavefield into up and downgoing components. Since reflectors can exist both above and below the borehole wavefield, separation of cross‐well data into up‐ and downgoing components does not achieve separation of direct and reflected arrivals. In our technique, we use moveout filters applied in the domain of common vertical source/receiver offset to extract reflected arrivals from the complex total wavefield of a cross‐well seismic data set. The multiple domains available for filtering and analysis make cross‐well data more akin to multifold surface seismic data, which can also be filtered in multiple domains, rather than typical VSP data, where there is only one domain (common source) in which to filter. Wavefield separation of cross‐well data is shown to be particularly effective against multiples when moveout filters are applied in common‐offset space.

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