A
7
mary Report
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must efficiently exploit the EG
de highly deviated directional
s, etc. Cost reducing well desig
ing casing all the way to the su
s (including motors, turbines
logging-while-drilling devices) are required to increa flexibility in well design. Alternatives to traditional casing systems can improve economics. Casing-while-drilling, expandable tubulars, and low-clearance casing designs are needed. Robust hard-rock underreamers are required to take advantage of lean casing designs.
High-temperature packers and other zonal isolation tools are needed to address well
timulation and interventions i
ical submersible pumps capab
ate flow. Additional high-tem
ill be required for EGS applica
temperatures and pressures m
om advances in drill bit perfor
ns, and Wellfield Construction
th aspects most critical to EGS tivate facilitated discussion on
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ncompass all rervoir acc
including well (and wellfield) design, drilling, well integrity, interventions, stimulation, logging and monitoring, and production. The two general topic areas covered were: • System-level issues associated with drilling EGS wells: Differences and
onal hydrothermal and EGS w
; possible EGS well locations;
struction of EGS wells: Curren
as and geothermal; high-tempe
dvanced drilling tools and syst
for drilling; stimulation, moni gn issues.
Workshop participants were enco
ome latitude to encourage dialo
workshop were not transcribed, a
similarities between geothermal drilling and oil & gas; where oil & gas is today;
General Issues of EGS Well Construction
This workshop was designed to identify technology gaps for constructing wells in EGS environments, but specific needs will be determined by the environment in which the wells are constructed; the technology to be ud is likely to be site-specific, and it is not possible to define the economics of drilling without knowing the location. The question “What is the site?” was asked in all of the technology needs workshops. It may be
preferable to lect dimentary formations for early sites in order to be able to model the site better and avoid creation of “stranded” wells that cannot be connected to the
rervoir. Also, it may be possible to avoid drilling issues at first by lecting sites where existing wells have high heat flow, enabling inexpensive rervoir development
experiments that would otherwi cost hundreds of millions of dollars for drilling. In
encouraged to provide written comments to the workshop organizers following the day’s discussion. The key discussions from the workshop are covered here.
mal systems are often
ion.) While oil & gas wells ar
ciated with geothermal wells ar
vailable than in the oil & gas
to drill to given target depth苹果8屏幕多少寸
circulation during the drilling process, steer and survey the well using motors (albeit with older technologies), and complete wells open-hole or with slotted liners. Drilling technology limitations hav
e not prevented geothermal development, however the costs of drilling geothermal wells remains high and drilling costs continue to be an issue. The geothermal industry drills wells at temperatures in excess of 250°C.
l & gas and geothermal well co
in drill bits (e.g., bearing asm
rock drilling in geothermal en
gging tools, underreamers (hol
ms, and other technologies em
e to the geothermal market be
ion with high temperatures an
he geothermal industry is a nich
develop the tools. However, ad
梦到老人去世developed in the future due to th
the oil & gas industry.
Geothermal and oil & gas industry costs are linked to some degree becau the geothermal industry competes with the oil & gas industry for drilling rigs. As the price of oil increas, so does the price of drilling rigs and supporting rvices, and therefore the cost of constructing geothermal wells goes up as well. However, historic cost data show that the cost of geothermal well construction has decread relative to the cost of oil and gas drilling over the last 30 years due to improved technologies and methods. As described in the MIT report, cost models bad on hydrothermal experience indicate that the cost of constructing EGS wells may be lower than that for oil & gas wells at depths below about 5000 m for a number of reasons ( e.g., well control issues are anticipated to be simpler for EGS and longer casing intervals are possible for geothermal wells.) The cost impact of low rates of penetration (ROP) in deep drilling is often underappreciated, and the concept that deep EGS wells would be less expensive than oil & gas wells was viewed with skepticism by workshop participants.
tions
will require a three- to four-fo
ccommodating this productivity
tion by a similar factor is un
cannot be done with current technology.) The alternative is to construct wells and wellfields that exploit the resource more efficiently. Well design alternatives should be considered that balance cost (including options that reduce cost, such as not cementing casing to surface) against productivity to provide more favorable overall economics (i.e., higher-cost well options may result in lower overall electricity costs.) The goal is to maximize the efficiency of the well.
2035
ng and cementing and other ta
geothermal wells. In traditiona
nto open-hole ctions by nest
surface in geothermal wells.)
uce costs. Alternatives for doin
g the u of low-clearance cas
ng strings is reduced, making
and expandable tubulars or casin
trings of casing). At the extreme
be ud to produce a well drilled
well construction practices, man
well construction. The options are not currently employed in geothermal well construction for a number of reasons: lean casing systems are expensive; the available material diameters are too small; expanding tubulars generally employ elastomers for annulus aling (which are problematic at high temperatures); and quality underreamers needed for the advanced casing programs are not available for hard rock environments. (Underreamers are bits that can drill hole diameters larger than the diameter of a hole they can fit through, either by expanding or rotating eccentrically.)
The geothermal well construction practice of cementing all casing strings to the surface derves ev
aluation. This practice constrains the thermal expansion of the casing when flowing high-temperature fluids (it is also employed in steam injection wells ud for enhanced oil recovery). Casing and wellhead design alternatives that would allow the casing to float could reduce the cost of cementing the casing and also mitigate the difficulties associated with circulating cement in deep wells.
阿尔寨石窟
(vibrations, whirl, etc.) at depth
bil’s well publicized “Fast Drill
颚之巨人future EGS well design and
could be the topic of a study to
report reportedly included th
fracturing, but stimulation was not a major cost element in the analys. The fracturing cost may not be modeled correctly; the modeled costs are bad on industry input, rather than objective data.
ng is employed in geothermal w
the state of the art in oil & gas
ced beyond basic “point and sh
motors and bent subs are ud t
hed with wet-connect wireline
WD devices have been problem
ng light aerated muds have resu
emperatures within operationa
angle and formation character
engineer’s options.
园心
Development of wells to access greater volumes of stimulated rock should be a focus of efforts to ad
vance EGS. Active steering of the well trajectory to optimally orient the hole relative to the producing fractures can increa the effectiveness of the well. Development of a ries of multilaterals off a single wellbore to produce from a larger portion of the rervoir may increa productivity. U of lower-cost drilling systems to effectively increa the diameter of the primary wellbore can be considered; for example, small microholes using coiled tubing may provide a low cost method of creating “capillaries” for monitoring or production/injection wells. While small-diameter coil tubing drilling is currently limited to soft formations and shallow depths, advanced technology may provide a less expensive solution to rervoir access. Depending on site conditions, drilling many inexpensive wells might improve EGS economics. While more complicated well completion scenarios will certainly increa development cost, it is possible the economics of the system can be improved using more expensive completion methods.
