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.
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.
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).