Microhole Array

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Exploration Technique: Microhole Array

Exploration Technique Information
Exploration Group: Drilling Techniques
Exploration Sub Group: Exploration Drilling
Parent Exploration Technique: Exploration Drilling
Information Provided by Technique
Lithology:
Stratigraphic/Structural:
Hydrological: May be possible to assess fluid flow using small-diameter downhole tools designed for slim holes.
Thermal: May be possible to collect limited temperature data using small-diameter downhole tools designed for slim holes.
Additional Info
Cost/Time Dependency: Microholes cost less to drill than traditional wellbores, delivering greater degrees of rock contact per bore with less monetary risk.[1]
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Microhole Array:
Microholes are among the smallest boreholes that can currently be drilled for geothermal reservoir development, with diameters of <4”. Microhole arrays are drilled as a completion technique during EGS reservoir development, and are not meant to replace the vertical primary wellbores drilled using traditional techniques. A directional array is drilled from the primary wellbore in order to intersect natural or induced fractures in the reservoir, allowing fluids to access more surface area for more efficient heat mining of the geothermal resource.
Other definitions:Wikipedia Reegle


 
Introduction
High pressure abrasion nozzle used for microhole drilling.[1]

A microhole array is a series of hundreds of small-diameter (<4”) directional boreholes drilled to intersect natural or induced fracture networks in an EGS reservoir.[1] An array of holes is drilled from a primary EGS wellbore and is designed to promote hydraulic communication between the reservoir and injection (and/or production) wells. Multiple microholes allows a larger volume of rock to be accessed by working fluids via more flow paths, increases the probability of intersecting open fractures, and reduces ‘short circuiting’ of working fluids within the EGS reservoir. This results in more effective heat mining of hot rocks, lowering development risk and increasing the productive life of an EGS project.

Microholes are drilled using FLASH Abrasive Slurry Jetting (ASJ) technology using solid abrasives propelled via a supercritical nitrogen, carbon dioxide, flue gas, or steam jet stream.[1] FLASH ASJ has proven capable of drilling all rock types effectively in bench tests, including basalt. Microhole drilling is a completion technique for EGS reservoirs, and is not meant to replace the vertical primary wellbores drilled using traditional techniques.

 
Use in Geothermal Exploration
Microhole drilling techniques are currently being tested and are not yet being used in geothermal exploration and well field development. Detailed models of EGS well configurations with and without microhole arrays were constructed to estimate microbore fracture intersection probabilities, understand fluid flow, and predict temperature changes under Soultz-based reservoir conditions.[2] Modeling suggests that microhole arrays increase the number of fractures penetrated by wellbores and thereby reducing the risk of EGS developments, and can make a significant impact on the long term heat transfer efficiency of the system.

Microhole array configuration used to model fluid flow and temperature draw-down over time.[1]

 
Related Techniques
Microholes are among the smallest boreholes that can be drilled for geothermal reservoir development, with diameters of <4”. Larger-diameter boreholes, such as slim holes and core holes, are traditionally used for geothermal exploration and well field development.


 
Field Procedures
Microholes are drilled with FLASH ASJ technology using abrasive slurry consisting of solids and supercritical nitrogen, carbon dioxide, flue gas, or steam jet stream.[1] WellFlo simulations indicate that specific fluids are optimal for different depths, with carbon dioxide being most effective at <10,000 feet and nitrogen being most effective at >10,000 feet. The fluid slurry mixture is highly pressurized (up to 10,000 psig) and is ejected downhole from a specialized nozzle for cutting. Supersonic erosion of the reservoir rocks allows microholes to be drilled 4-20x faster than conventional drilling methods. A modified 1” coiled tubing rig can be used for FLASH ASJ drilling to reduce the downtime that would be needed for conventional jointed pipe drilling. Reduced bottom hole pressure is needed for high efficiency cutting, and requires the use of a downhole pump to establish an isolated zone at the kickoff point in the primary borehole.








 
Additional References

Bruce E. Galbierz, Kenneth D. Oglesby. 2005. Advanced Mud System for Microhole Coiled Tubing Drilling. Tulsa, Oklahoma: Final Report: US Department of Energy, Bandera Petroleum Exploration LLC, Impact Technologies LLC. unspecifiedp.

David Archibold Summers, Klaus Hubert Woelk, Kenneth Doyle Oglesby, Grzegorz Galecki. Method and Apparatus for Jet-Assisted Drilling or Cutting [Internet]. 03/09/2009. unspecified. David Archibold Summers. [updated 2009/03/09;cited 2009/09/10]. Available from: http://www.google.com/patents/US20090227185





 
References
  1. 1.0 1.1 1.2 1.3 1.4 1.5 Ken Oglesby. 04/2013. Microhole Arrays Drilled With Advanced Abrasive Slurry Jet Technology To Efficiently Exploit Enhanced Geothermal Systems. Washington, D.C.. U.S. Geothermal Technologies Office. Geothermal Technologies Office 2013 Peer Review Meeting; 23p.
  2. Stefan Finsterle, Yingqi Zhang, Lehua Pan, Patrick Dobson, Ken Oglesby. 07/2013. Microhole Arrays for Improved Heat Mining from Enhanced Geothermal Systems. Geothermics. 47:104-115.


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