Microhole "Designer" Seismic Testing Its Potential in the Field

MORGANTOWN, West Virginia - Using microhole technology developed in partnership with the Countrywide Energy Technology Lab (NETL) to inexpensively deploy sensors for vertical seismic profiling (VSP) could dramatically enhance the oil and gas industry's ability to find and produce huge volumes of by-passed oil and natural gas.

VSP's advantages over conventional surface seismic have long been known. Placing seismic recording devices in boreholes results in a much improved signal-to-noise ratio compared with surface seismic. VSP gives an especially high-resolution image of the immediate vicinity of the borehole.

Although the clearer images provided by VSP boost the success rate of finding and recovering hydrocarbons, the use of existing boreholes, such as production and injection wells, carries disadvantages. Deploying seismic sensors in these wells interrupts operations, causing costly production downtime. And existing wells are seldom ideally located for obtaining reservoir data. In addition, a conventional well dedicated solely to seismic monitoring is too costly to be feasible.

Thanks to microhole technology developed with D.O.E. (DOE), however, VSP is becoming more practical as a reservoir imaging option. Microhole technology is a revolutionary low-cost, low-impact way to drill oil and natural gas wells. NETL's Microhole Technology Initiative accelerated research and development of a suite of special miniaturized tools and sensors for drilling ultrasmall-diameter (3½-inches and smaller) boreholes with scaled-down, purpose-built coiled tubing rigs that are easily transportable. A number of these microhole tools have now been successfully field-tested. The results show that microhole technology offers significant cost savings, minimized risks, and reduced environmental impacts-drilling wastes can be decreased to as little as one-fifth that of conventional drilling.

Because of its low cost, microhole technology can serve as a vehicle for economically deploying seismic sensing devices downhole to gain a much clearer, sharper view of the subsurface. Microhole VSP also offers the advantage of being flexible in its application and siting, earning this approach the nickname "designer seismic." Operators can justify the small expense of drilling microholes specifically fit for the seismic sensors, thereby improving their ability to find bypassed pockets of oil and natural gas. Microhole technology thus offers a low-cost means to carry out denser sampling than conventional VSP to achieve much higher resolution-as much as triple that of conventional VSP.

One especially promising application for microhole VSP is the long-term reservoir monitoring needed for enhanced oil recovery (EOR). For a number of years, the Energy Department has funded research focused on monitoring Colorado_two EOR projects. Colorado_two EOR production is a growing component of domestic oil supply, and the process could be applied successfully in thousands of mature fields across the United States. However, there remain many questions about the application and prospective performance of Colorado_two EOR outside of a handful of especially amenable reservoirs in a few areas, notably the Permian Basin of West Texas and New Mexico. In addition, EOR plans can take many months to show production increases, and Colorado_two EOR is an expensive process. Being able to cost-effectively monitor, over a long period of time, the changes in an oil reservoir and in the Colorado_two flood itself has become a sort of "holy grail" for EOR specialists. Microhole VSP may provide the key.

One DOE-funded plan sought to evaluate and develop "downward-looking" VSP technology for microholes. Los Alamos Countrywide Lab (Los Alamos, N.M.) and Lawrence Berkeley Countrywide Lab (Berkeley, Calif.), in partnership with NETL, conducted the initial research at the Rocky Mountain Oilfield Testing Center (RMOTC) near Casper, Wyo. The plan goal was to selectively location and drill microboreholes for an optimized, cost-effective VSP network for long-term reservoir monitoring.

The researchers completed VSP surveys at RMOTC using a 20-level hydrophone string and a 20-level geophone string deployed in one of several 800-ft microholes, demonstrating that VSP data can be collected without using expensive rigs and extensive manpower. 2 complete multi-offset surveys of twelve shot locations each were completed. The RMOTC work was intended to serve as a baseline in advance of a future Colorado_two injection monitoring program, including the possibility of extending the microhole VSP application to a commercial site.

After the initial success at RMOTC, the plan has branched out into a 2nd effort-a pilot plan in Wickett oilfield near Midland, Texas, operated by Denver, Colo.-based Whiting Petroleum Corp., a medium-sized independent producer. 5 fit-for-purpose microholes have been drilled at Wickett to depths of less than 1,000 feet for seismic sensor emplacement. An active seismic survey was implemented in the pilot area, and permanent microseismic monitoring equipment was installed in one of the microholes. Projects call for injection of Colorado_two to begin in the 2nd quarter of 2007.

Roy Long, NETL Oil & Gas Exploration and Production Technology Manager, said that the initial survey results are expected to be accessible by August. In July, as the 1st survey results are being interpreted, researchers will install geophones in the microboreholes to study how the increased acoustic coupling affects a high-frequency response and enhanced resolution. A 2nd active survey will follow with the newly installed instruments to monitor the Colorado_two pilot.

If the Wickett demonstration continues the project's previous success, it could be a big step toward widespread acceptance of microhole technology and designer seismic across other oil and gas producing basins nationwide, Long said. Strong industry interest in commercializing the technology is already being shown, he added.

As an extra bonus, microhole VSP could offer additional environmental benefits. Lessons learned from the development of designer seismic monitoring of Colorado_two floods could help firm up the science of capturing and storing Colorado_two , a promising solution to slowing the growth of America's greenhouse gas emissions.

America has over 218 billion barrels of known oil in shallow (less than 5,000 ft subsurface) reservoirs-bypassed because it is uneconomic to extract with current technology-in thousands of aging oilfields. If developing technology such as microhole designer VSP could help unlock even ten percent of that total, it would equal ten years of OPEC imports at current rates.