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Current oil and gas drilling technologies adaptable to EGS


There are a number of approaches that can be taken to reduce the costs of casing and cementing deep EGS wells: expandable tubular casings, low-clearance well casing designs, casing while drilling, multilaterals, and improved rates-of-penetration are developments that will dramatically improve the economics of deep EGS wells.


The first three concepts, which relate to casing design, are widely used in the oil and gas industry and can easily be adapted for EGS needs. The use of multilaterals to reduce the cost of access to the reservoir has also become common practice for hydrothermal and oil/gas operations. Adaptation, analysis, and testing of new technologies are required to reduce deep EGS well costs.


Expandable tubulars casing

The expandable tubing casing process utilizes a product, patented by Shell Development (Lohbeck, 1993), which allows in situ plastic deformation of the tubular casing. The interval is drilled using a bit just small enough to pass through the deepest casing string.


There is an under-reamer behind the lead bit. The under-reamer is used to widen the bottom of the well and allow cementing of the casing, after running and expanding. The result is that the inner surfaces of adjacent casings are flush (i.e., the inner diameter is constant with depth). This allows two possible approaches to be taken: (i) the resulting casing may be used as the production string; and (ii) a liner may be run and cemented in the well after progress through the production interval is completed. Technology improvements are needed if this approach is to be taken in deep, large-diameter EGS wells.


Low-clearance casing design. An alternative approach to using expandable tubulars is to accept reduced clearances. A well design using smaller casing and less clearance between casing strings may be appropriate (Barker, 1997). This may also require the use of an under-reamer to establish clearance between the casing and the borehole for cementing. Although closer tolerances may cause problems with cementing operations, this can usually be remedied by the use of under-reamers before cementing.


Drilling-with-casing is an emerging technology that has the potential to reduce cost. This approach may permit longer casing intervals, meaning fewer strings – and, therefore, reduced costs (Gill et al., 1995). Research is needed to improve our understanding of cementing practices that apply to the drilling-with-casing technique. As with expandable tubulars, the development of reliable under-reamers is key to the advancement of this technology.