Low-cost drilling systems using microhole technologies can give operators reservoir access in marginal fields for half the cost of conventional rigs.

Microhole technologies are the array of technologies necessary to enable the ability to drill out of 41?2-in. casing (generally 35?8-in. and smaller holes). It is the infrastructure that will create a new class of wells: smaller; much less expensive to drill, equip and operate; and more environmentally friendly than before. Much of it is downsized versions of existing technologies, such as bits, motors, and some bottomhole assemblies (BHAs). Others are novel technologies unique to the problems of miniaturizing drilling equipment.
Drilling technologies include built-for-purpose coiled tubing (CT) drilling rigs; specialized bits and motors for faster and straighter drilling; the complete array of BHAs for steering, logging, seismic, and communications with surface; and technologies to extend reach such as tractors and friction reducing devices. Enabling technologies are those that make drilling microholes with CT less expensive or more reliable, such as tubulars with improved alloys, manufacturing and defect detection and remediation; and those technologies that expand the applications, such as low-cost, miniaturized seismic receivers for reservoir monitoring.
Advantages and limitations
Many of the advantages of a microhole well over a conventional-sized, jointed-tubing well are obviously related to size. A smaller hole, with the cross-section area a function of the square of the radius, means much less cuttings and mud volume, smaller power requirements, faster rate of penetration (less rig time) and smaller, less expensive tubulars. Smaller components mean the rig and drilling equipment can be smaller, more mobile, require fewer people, rig up faster, have a smaller footprint and require little or no site preparation. A closed mud system eliminates the pits, which when combined with the footprint and minimal preparation, is an environmentally acceptable process. Drilling with CT is faster, can be easily done underbalanced and results in a very straight hole.
Limitations are a function of the size and the weight of the CT. Depth is limited by reel size and overall weight. The hoisting capacity is limited by the injector, which carries the weight of the CT in the well. Rig dimensions are limited by highway regulations, and there is a limit on how far the tubing can be pushed horizontally, even with tractors. All these limitations continue to be pushed back with new technology.
Applications
Even with its limitations, there are a number of applications where CT drilled microholes can provide an economic and environmental advantage over current technology. Where a number of shallow- to medium-depth wells are to be drilled quickly and cheaply, this technology provides the best solution. More than 90% of the CT drilling in the world takes place in Alberta province. There are over 40 built-for-purpose CT drilling rigs, operating essentially full time, drilling more than 3,000 wells per year in Alberta. The day rates are comparable to conventional rotaries, but require half the time to drill. A 3,000-ft (915-m) well is routinely drilled in 12 hours. The initial target was the shallow (3,000 ft) conventional gas play, but the explosion in coalbed-methane drilling in Alberta has fueled recent growth from 1,000 wells annually to 3,000 drilled with CT.
The United States has similar potential. The initial US market for this technology is shallow gas in the Appalachian basin and a number of western basins. Reservoirs that generally have small to medium reserves of dry gas, long producing lives and relatively low production rates are ideal candidates. One drilling rig operating in western Kansas and eastern Colorado has drilled more than 140 wells in the shallow Niobara play for Rosewood Resources on a Department of Energy (DOE)-sponsored program. The potential for oil recovery is also substantial. The US Energy Information Agency (EIA) estimates that there are 407 billion bbl of oil not economically recoverable with current technology. Over half of those barrels are in reservoirs less than 5,000 ft (1,525 m) deep and accessible with microhole technology.
Another application, currently well established in Alaska by BP and ConocoPhillips, is the use of CT microhole re-entry into existing vertical well bores. Since 1992 over 500 re-entries have been drilled: 400 through 41?2-in. casing, 100 through 31?2-in. casing. The record horizontal length in Alaska is 3,124 ft (953 m). It is estimated that more than 270 million bbl have been recovered through this program. BP is now testing this technology in the lower 48 in a program to redevelop a mature tight gas sandstone reservoir in the Texas Panhandle. The potential for redevelopment of low pressure, mature reservoirs is enormous.
The application of microhole technology to drill low-cost exploration test wells is being investigated by Geoprober Drilling Inc., in an industry/DOE co-funded project to demonstrate the
economics of drilling a substantially smaller diameter test well in deep water, but relatively shallow sediments. Ultimately, Geoprober estimates that savings up to 59% can be achieved. The well can be logged, used for a high-resolution seismic survey and tested to determine the optimum location for the conventionally drilled development wells. While this is an offshore application, it could be applied to a large number of the 2,600 onshore wells drilled annually (EIA 2003 Annual Energy Review).
With national goals to reduce the growth of greenhouse gas emissions, considerable research is being conducted to evaluate the feasibility of the long-term storage of captured CO2 in geological formations: initially in depleted oil reservoirs, then gas reservoirs, and eventually unmineable coal and deep aquifers. Once stored, the CO2 must be monitored to ensure that it is not moving out through a fault or other geological feature. The technology best suited for detecting movement of fluids through a reservoir is vertical seismic profile (VSP) imaging, where the seismic receivers are placed in the well bore to maximize resolution of the image and changes of time (4-D seismic). Microholes are well suited to place permanent receivers for long-term monitoring at the least cost. As this storage or sequestration grows, so will the demand for inexpensive holes to monitor CO2 movement.
DOE support
The DOE and the National Energy Technology Laboratory (NETL) have a mission to promote economic development of domestic energy. Increasingly, that means enabling the development of technology to reduce the cost of accessing remaining oil and gas, finding new ways to extract formerly uneconomic reserves and transferring those technologies to the independent operators. The CT drilling in Alberta has, by and large, not migrated to the United States. There is no infrastructure to support the potential demand for lower cost wells using microhole technologies. NETL is providing co-funding to technology providers to design, build and field-test these technologies to overcome the "chicken and egg" syndrome of infrastructure and demand.
Role of PTTC
The DOE selected Petroleum Technology Transfer Council (PTTC) to institute the Microhole Technology Integration Initiative to integrate and disseminate information generated in the first and second round projects performed in the DOE Microhole Technology program and other related industry activities. In addition, PTTC will ensure the projects' timing and direction mesh to best meet the DOE goals and industry needs and to provide information to and feedback from the industry.
Defining the future
As BP has demonstrated in Alaska and elsewhere, and Advanced Drilling Technologies, Inc. has in Kansas and Colorado, the technology for CT drilling of shallow gas and half-mile long reservoir extensions in mature fields is here today. With experience, new tools being created in the DOE microhole program and demonstrated cost savings, drilling small holes with CT, either from surface to shallow gas, or re-entry in mature fields is poised to take off. This activity should complement and add to the conventional drilling fleet to develop reserves at the lowest cost. In the longer term, as infrastructure grows and costs are driven down, exploration test wells, both onshore and offshore, will enter the mix. Finally, as CO2 sequestration becomes a reality, the need for the many cheap non-production wells will be filled with the microhole technology.

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