Exploration and Production Technologies

The Application and Use of Microholes for Vertical Seismic Profiling

FEW ESD04-006

Project Goal

The project goal is to evaluate and develop “downward-looking” vertical seismic profiling (VSP) technology for microholes to be used to enhance image resolution and depth of investigation beyond current technology. The potential reduced cost and high resolution possible through this approach to subsurface imaging might allow, for the first time, geophysicists to pick borehole locations for an optimized, cost-effective VSP network for long-term monitoring, as opposed to the typical VSP using existing production holes for limited testing.

Performers
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA
Los Alamos National Laboratory (LANL), Los Alamos, NM
Rocky Mountain Oilfield Testing Center (RMOTC), Casper, WY

Results
A vertical seismic instrumentation system was developed for use in microholes that can be deployed in a low-cost manner. VSP surveys were completed at RMOTC using a 20-level hydrophone string and a 20-level geophone string. Performing the surveys demonstrated that VSP data can be collected without using expensive rigs and extensive manpower. This work will serve as baseline study in preparation of a future CO2 injection monitoring program.

Benefit
Microhole technology (providing inexpensive access to the subsurface via ultrasmall-diameter wellbores) has the potential to be the most significant technology advance for the energy industry in the last 50 years. It could be a catalyst for creating a quantum leap in imaging technology leading to a game-changing understanding of subsurface processes. A critical application is the placement of sensors in the subsurface for use with seismic techniques such as VSP, crosswell seismic, microseismic, and even high-resolution surface seismic to image and monitor previously unknown or unclarified resources.

Background
While VSP is not a new technology, the routine, low-cost application of VSP at the same scale of surface seismic has not occurred. As oil and gas resources become harder to find and produce in the United States, there is a critical need to enhance seismic resolution of the subsurface. While VSP offers such an increase in resolution, it has been held back by the use of expensive holes and large-scale deployments. Microhole technology offers a means to deploy low-cost and denser sampling than “conventional” VSP to allow much higher resolution.

Project Summary
Project researchers have sought to validate the following perceived attributes of Microhole VSP.

  • For active seismic, Microhole VSP:
    • Can penetrate to as much as 10 times hole depth.
    • Delivers resolution (due to reduced signal-to-noise ratio) that is up to three time better than conventional VSP.
    • Makes seismic surveys faster and much cheaper.
    • Allows the operator to easily customize/change surveys for changing reservoir conditions and varying reservoir conditions across any particular field
  • For passive seismic, Microhole VSP:
    • Can provide critical information on fracture generation, fluid interactions, and fluid paths from borehole seismic data.
    • Does not need sensors to be placed at reservoir level.
    • Does need sensors to be placed away from surface and insufficient azimuthal coverage to eliminate path effects and obtain data for analysis of source mechanisms critical to understanding fracture generation and analysis.

Another prime objective was to develop instrumentation that could be deployed at low cost. Modeling of the shot hole locations was performed prior to the field work to estimate shot spacing and total distance for the well. Two complete VSP multi-offset surveys (12 shot locations each)—with offset distances from 35 feet to 2,700 feet (every 250 feet)—were completed using a 20-level hydrophone string and a 20-level geophone string. In all, 40 levels were recorded for each set of sensors using 12 different shot locations. A vibroseis was used as a source (Enviroseis from IVI Inc.); this source was selected to minimize ground disturbance and for its high-frequency content (up to 300 hertz).

Current Status (July 2007)
The project’s tasks still in progress include the following:

  • Complete processing of RMOTC 2005 data.
    • Compare and contrast to 3-D surface seismic.
  • Extend application to commercial sites (in planning stage).
    • TORP Kansas site.
      – Active time-lapse monitoring of CO2 injection.
      – Passive monitoring of reservoir between time-lapse measurements.
    • Wyoming deep (>8500 ft) CO2 EOR (Perfection Oil).
    • Barnett shale hydrofracture monitoring.
  • Investigate next generation of instrumentation.
    • Fiber optic sensors.
    • Microelectromechanical systems and nano-sensors.
  • Adapt processing for improved “look-ahead” capability (in progress).
    • Improve methods of imaging vertical features in homogeneous geology.

The researchers have evaluated and developed Vertical Seismic Profiling ( VSP) technology for microholes to be used to enhance image resolution and depth penetration beyond current technology in a low-cost fashion. They have demonstrated through field testing that the use of micro-holes constitutes a viable inexpensive alternative for performing Vertical Seismic Profiles(VSP). Vertical Seismic profiling (VSP) is defined here in the broadest of terms, i.e. the use of seismic sources or sensors in boreholes, including co-location in the same hole (single well seismic) or in different wells (cross well seismic), and/or in the conventional sense of the source on the surface and the sensors in the subsurface (or vice versa). It is recognized that much improved resolution could be obtained by placing the sources and/or receivers in the subsurface. The main reason being cost of access to the subsurface, either in down time of production wells or in direct costs of carrying out the work. However, with the ever-increasing need for greater understanding of the subsurface VSP methods are receiving more and more attention. In addition, the ever increasing need to sequester CO2 in the subsurface places emphasis on accurately imaging methods to validate CO2 injection strategies. Researcher of LBNL have successfully traced CO2 mobility in Weyburn Field, Canada ,with the help of time lapse VSP and Crosswell seismic data. Valuable information of CO2 movement is imaged by VSP in a time lapse well as static application to image the efficiency of CO2 injection. These images resulted to increase oil and gas recovery significantly. The successful test results are received also from VSP microholes at the Rocky Mountain Oilfield Testing Center (RMOTC) using commercial instrumentation for microholes. Micro drilling anticipated to provide the means to do so. The reduced costs for subsurface access will open up a whole new means and industry for reservoir monitoring and characterization. Dr. Majer’s paper entitled : “Cost effective imaging of CO2 injection with bore holes”, is published by SEG in the Leading Edge of October 2006.

Project Start: March 12, 2004
Project End: March 11, 2007

Anticipated DOE Contribution: $1,105,000
Performer Contribution: $0

Contact Information
NETL - Purna Halder (purna.halder@netl.doe.gov or 918-699-2084)
Lawrence Berkeley National Laboratory - Ernest L. Majer (elmajer@lbl.gov or 510-486-6709)

The complete system used to acquire the VSP data


3-D seismic cross-section, Teapot Dome oilfield, Natrona County, WY.

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