Borehole seismic applications and services are an essential part of energy exploration. With a seismic receiver placed in the earth at depth and an energy source on the surface, accurate time-depth measurements are produced that can be used to calibrate well logs. By changing the location of surface sources and the number of receivers in the borehole, high-resolution data can be recorded and detailed formation properties and reservoir images can be produced. Providing high-resolution data enables 3-D images to be created which can improve vertical and provides the latest in digital data acquisition systems and services, which provide clients with quality data and proven results. VSFusion-Veritas is an industry leader in borehole seismic processing. VSFusion provides a full spectrum of borehole seismic application design, data processing and interpretation.

VSFusion’s 3-C 3-D vector migration processing can provide the most accurate structural image possible by precisely locating each reflection point in the VSP data. VSFusion also provides leading technology in processing and interpretation of 3-D VSP data.

Magnitude Baker Atlas microseismic experts, use the latest technology in seismic processing of acoustic events in microseismic monitoring and hydraulic fracture mapping. Hydraulic fracture mapping is achieved by monitoring and recording microseismic events that occur during the fracture treatment of a well. Fracture monitoring provides an independent estimate of fracture volume and direction which is crucial to the optimization of a prospects development program. Baker Atlas provides the latest technology in digital downhole equipment and wellsite operations while Magnitude provides processing and interpretation of the microseismic data.

Section Contents/Solution Highlights Matrix

Velocity Survey

Travel-time measurements are acquired with receivers placed at known depths in a well. These measurements produce accurate time-depth and seismic velocity results that can be used to calibrate well log data. Velocity survey information is presented as time-depth correlation plots and detailed velocity tables. Borehole seismic-generated velocity information is used to calibrate acoustic well log data and produce accurate synthetic seismograms. Synthetic seismograms link surface seismic time-domain information with high resolution, depth-domain well log data.

Operation

VSFusion provides calibration information and parameters derived from borehole seismic measurements to enhance surface seismic imaging, attribute processing, and reduced uncertainty in surface seismic interpretation.The Velocity Survey uses a combination of geophysical and well logging techniques to measure the one-way travel-time of a seismic pulse. This pulse is generated from an energy source, located at ground or sea level, and propagates to a geophone receiver placed at a known depth in the borehole. Using the measured total travel-time for a measured depth, the average velocity of a pulse propagating through the earth to that depth is calculated and corrected to the seismic datum. Interval velocities are calculated between check shot levels. By comparing acoustic well log integrated interval times to the measured one-way seismic travel-times from the Velocity Survey, the acoustic log values can be adjusted for errors due to borehole conditions such as borehole diameter, drilling fluid invasion and formation gas.

Highlights
  • Well log and surface seismic time-depth correlation
  • Driller’s depth and surface seismic time-depth correlation
  • Velocity analysis
  • Well log editing
  • Synthetic seismogram generation
  • Well log conversion to time scale
Benefits
  • Velocity analysis stacking and migration velocities
  • Well log and surface seismic time-depth correlation

Velocity or Checkshot Survey

Zero Offset Vertical Seismic Profile (ZVSP)

The unique geometry of a zero offset VSP allows for multiples, recorded above the depth of the deepest receiver, to be identified and removed. The end result is a zero-phase, primaries-only dataset. This dataset can be used to identify residual multiple reflections on the surface seismic data, as well as to phase and frequency match the surface seismic data to the zero-phase VSP.

Operation

With a zero offset VSP, the energy source is placed a relatively short distance from the well. The downhole receiver spacing is usually denser than that used when recording a velocity survey. In addition to measuring the elapsed time for the surface-activated energy source pulse to travel to the geophone, as is done with a velocity survey, the VSP technique is used to record and interpret the seismic response that follows the first arrival pulse. The primary output from the zero offset VSP is a single stacked trace, which represents the acoustic response of the subsurface at the well location. This single trace, consisting of primary reflections called the corridor stack trace, is compared to the surface seismic data at the well location. The VSP receiver is placed downhole rather than on the surface to provide the following benefits:

Highlights
  • Improved time-depth relationship
  • True wavelet corridor stack
  • Multiple free corridor stack
  • Identify surface seismic events as primary reflection or multiple reflections
  • Identify depth at which seismic event intersects wellbore
  • Depth prediction of seismic events ahead of the drill bit
  • Improved vertical resolution compared to surface seismic data
  • Wavelet extraction and wavelet shaping
  • Phase determination and matching of surface seismic data
  • Complex structure and wellbore deviation to allow for offset imaging with improved lateral resolution Attenuation studies (Q estimation)
  • Extraction of parameters for enhanced surface seismic processing
Benefits
  • Measured averaged and interval velocities and improved time-depth relationship
  • Identify surface seismic events as primary reflections or multiple reflections

zero Offset VSP

Walkaway VSP, 3-D VSP

Benefit and Applications
  • Reflection angle to offset correlation
  • Surface-to-VSP trace amplitude calibration
  • Synthetic AVO response from well log data
  • AVO attribute analysis
  • Angle trace gather, two-way time section
  • Amplitude variation with offset/angle cross-plot
  • Gradient vs intercept cross-plot for AVO classification
  • P-wave intercept trace (P), Gradient trace (G)
  • Quick AVO synthetic from P and G and AVO synthetic stack
  • Regression coefficients or standard deviations of curve fit on AVO cross-plot
  • Combinations of intercepts and gradient traces, Poisson’s ratio, shear restricted gradient, product and math-traces

Walkaway

Synthetic AVO

Intercept AVO versus gradient crossplot

Both 2-D and 3-D VSPs improve reservoir characterization. High-resolution VSP data can be Integrated with surface seismic data to provide detailed descriptions of formation properties and identification of reservoir compartments not possible with surface seismic data alone.

Reservoir Delineation
  • Depth prediction of seismic events ahead of and offset from the drill bit
  • Time-depth correlation with surface seismic data
  • Structural imaging
  • Improves reservoir characterization by delineation of faults and pinchouts

Rig source deviated borehole seismic image (VSP-CDP transform) digitally spliced into the

AVO and Anistrophy Modeling and Analysis
  • Surface seismic AVO calibration
  • Anisotropy detection for accurate model building
  • Better understanding of lithology, porosity, pore fluids and orientation of aligned fractures
2-D VSP Imaging
  • Improved vertical and lateral resolution compared to surface seismic
  • Phase determination and matching of surface seismic
  • Extraction of parameters for enhanced surface seismic processing
  • Generation of a high-resolution velocity model at the wellbore
  • Q compensation derived from borehole VSP data improves surface seismic resolution

The offset VSP provides enhanced resolution to the surface seismic data. The offset VSP and Corridor Stack are superimposed on the surface seismic data. Data example courtesy of BP

3-D VSP Imaging
  • Simultaneous surface and borehole seismic data acquisition reduces operating costs
  • Improved vertical and lateral resolution compared to surface seismic
  • Using a shared VSP-calibrated velocity model, the 3-D VSP can be integrated with the surface seismic data
  • Volumetric estimates improved by high-resolution 3-D VSP
  • True 3-D migration in time and depth
  • 2-D out of plane migration errors solved by 3-D migration
Salt Imaging
  • Salt Flank Reflection Imaging
  • Sub-Salt Imaging Map reflections beneath salt base, which are not illuminated by surface seismic
  • 3-D Salt Proximity Refraction Methodolog

AVO gradient and intercept volumes derived from surface seismic data are used by the interpreter to provide quantitative answers regarding reservoir size, location and fluid saturation levels. While they provide good spatial coverage, the seismic amplitudes recorded on the surface have traveled twice through the sediments overlaying the reservoir. These data were affected by scattering and attenuation, and are also subject to variation due to the heterogeneity of the overburden. There are many uncertainties inherent to surface seismic AVO. These uncertainties may manifest themselves in an erroneous reservoir model. To reduce these uncertainties, a borehole seismic AVO survey should be run to calibrate the surface seismic AVO data.

Operation

Because the VSP receiver can be placed very close to the reservoir, many of the uncertainties associated with surface seismic AVO are eliminated. Having an in-situ receiver results in a one-way seismic travel path. Consequently, reflection amplitudes recorded in a VSP AVO survey are much less affected by transmission loss and scattering. In addition, with a properly designed VSP survey, wider reflection angle apertures can be acquired than with surface seismic geometry.

Hydraulic-Fracture Monitoring

Microseismic events are detected in an observation well that is located away from the treatment well. The Geoch GeoWaves and MSR tool systems are used for data acquisition in the observation well. These micoseismic events are used to map fractures that may occur away from the treatment well. They can provide an independent estimate of fracture volume and verify fracture direction. Verification of fracture volume and direction is crucial to optimization of a prospect development program.

Magnitude can detect microseismic events from nearby observation wells or in some circumstances, from the treat- ment well itself. Monitoring from the treatment well allows the measurement of trapped-mode acoustic emissions, which directly measure azimuths of the open fractures. Treatment well detectors can also see shear-slip events away from the borehole. Ideally, a hydro-frac monitoring network would include detectors in both treatment and observation wells. Magnitude’s expertise in seismic processing of acoustic events for quantitative results.

Shear-slip event locations map fracture volume and distribution

Benefits
  • Independent estimate of fracture volume and orientation Location and orientation of pre-existing sub-seismic faults activated during injection
  • Validation of hydro-frac program parameters Improved well spacing and field development planning
  • Reduced fracturing costs through efficiency improvements Optimization of development program for minimum completion costs and maximized production
Applications
  • Independent verification of fracture program success
  • Dynamic 4-D fracture growth characterization
  • Estimate of rock mechanic parameters from fracture statistics

Treatment well receivers record events during pressure fall-off reduced noise conditions. Adapted from Parotidis et al. (GRL, 2004)

Vertical Seismic Profile(VSP)

Down hole Seismic Surveys

The present-day level of development of seismic surveys is based on obtaining reliable information about structure and characteristics of the wave field. With this purpose, the company Asmary has conducted a big scope of research and development operations along with practical field acquisition operations for creation of equipment and instrumentation, methodological, algorithmic and software tools for best-quality performance of down hole seismic measurements. The Company has created the new multilevel multi-component tool for down hole seismic measurements. Operations are conducted in both onshore and offshore wells.

The main specific features of this tool are:

  • availability of special devices in receive modules for determining spatial orientation of geophones in the course of each recording cycle;
  • the possibility to increase the number of acquisition module in the tool, interchangeability of modules;
  • low level of intrinsic instrumental noise allowing measurement and recording weak seismic signals and utilize non-explosive low-energy seismic sources;
  • availability in receive modules of controllable calibrators making it possible to monitor metrological characteristics of electronic measuring channels and parameters of geophones;
  • small dimensions and weight of receive modules and availability in the modules of a powerful electromechanical clamping device minimizing the adverse effects of resonance of system “tool borehole wall” as well as of noise waves (mud wave, cable waves) on recored seismic vibrations;
  • in order to reduce the time of clamping/freeing the modules, a possibility to simultaneously actuate up to 10 modules is provided;
  • versatility of the hardware-software system providing for the opportunity of utilizing it various modifications of down hole seismic surveys, the possibility to test the entire complex, quality control of acquired data.
Equipment Specifications:

The available equipment capability makes it possible to arrange simultaneous operation of up to three crews working in different regions. For working in remote and had-to-reach areas, necessary equipment and personnel may be delivered on a helicopter. Depending on problems to solve and surface conditions, seismic signals may be excited by explosive or by down hole air guns. Down hole equipment makes it possible to conduct surveys in cased well and open hole with deviation up to 63. Surveys may be conducted in boreholes with diameter from 54 mm (thru-tubing operation) to 420 mm, a depths up to 7000 m, pressure up to 140 MPa and temperature up to 200C. The software used makes it possible to design optimal observation systems and perform quick-look data processing.

Seismic Energy Source
Pressure 150 bar
Volume 1.5-2.0-3.0 liters

Down hole tool (VSP10*48)

Length 1456 mm
Mass 11 kg
Diameter 48 mm
Maximum pressure 70 MPa
Maximum temperature 200C
Down hole geophone SMC-1850-14
Number of levels up to 10
Clamping force 180 mm 125 kg

220 mm 110 kg

320 mm 80 kg

Main problems solved with the use of offset VSP

Main problems solved with the use of offset VSP
  1. Constructing a velocity model of medium
  2. Predicting below-the-bit geological section
  3. Stratification of seismic reflections and tie-up to CDP data

The VSP equipment comprises software-controlled digital instruments for conducting down

hole seismic surveys (zero-offset VSP, offset VSP) acquisition of seismic vibrations in the near- wellbore space, converting them into electric digital signal and transmitting them to a computer.