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.
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA
Los Alamos National Laboratory (LANL), Los Alamos, NM
Rocky Mountain Oilfield Testing Center (RMOTC), Casper, WY
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.
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.
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 researchers have sought to validate the following perceived attributes of Microhole VSP.
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).
The project’s tasks still in progress include the following:
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.