Page 161 - Geothermal Energy Renewable Energy and The Environment

Drilling 147

it, either by drilling through it or reducing its diameter once it has been deployed. Several designs
have been developed to accomplish these ends.
The most commonly used packer is an inflatable packer that is constructed as a bag from elasto-
meric materials. This type of device is lowered into the hole and inflated using high pressure fluid
(air or liquid) so that it seals tightly against the pipe or well wall. Once emplaced, it acts as a bar-
rier, preventing communication up and down the well. Emplacement of multiple inflatable packers
allows individual regions to be isolated.
Inflatable packers have been utilized extensively in the oil and gas industry to allow so-called
hydrofracturing or stimulation of a particular zone in a well. Stimulation is the process whereby the
permeability of a selected region is increased. The most common way this is accomplished is by iso-
lating the zone of interest and then injecting into it fluid at a very high pressure. If the rock possesses
natural fractures that are sealed, these will often reopen during the stimulation process, providing
increased fracture permeability. In some instances, intact rock will fail if its fracture strength is
exceeded by the high pressure fluid, forming new fractures. In either case, the stimulation process
requires that the injected fluid be contained within the selected region during the stimulation pro-
cess. Inflatable packers are currently the primary routine means for accomplishing this since they
allow containment of the fluid and can then be removed once the process is complete.
Packers were used successfully in Iceland to stimulate a low temperature zone that supplied warm
(<100°C) water for a geothermal district heating system. In this instance, pressures of about 15 MPa
were utilized, and injection rates of 15–100 kg/s were realized. The result was that fluid flow from the
district heating system increased from 300 l/s–1500 l/s (Axelsson and Thórallsson 2009).
Unfortunately, however, elastomeric materials generally are unsuitable for applications above about
225°C. As geothermal wells go to deeper levels, extract hotter fluids, and pursue stimulation programs,
as is the case for enhanced geothermal systems (EGS; Chapter 14) new forms of packers will need to
be developed. Efforts have been made to employ expandable metal packers and drillable packers, with
varying degrees of success (Tester et al. 2006). This remains an important research topic.

losT circulaTion
It has been emphasized that drilling requires the constant circulation of drilling mud to cool the
bit, lubricate the rotating pipe, and remove the cuttings. For that reason, drillers constantly monitor
the rate at which fluid is pumped down the hole and the rate at which it returns. A significant drop
in the rate of return of drilling mud signals drilling mud is being diverted into highly permeable
wall rock. This happens when a zone of highly fractured rock is encountered, which is possible at
almost any depth and drilling circumstance. Loss of circulation can be a catastrophic event, in the
sense that the drilling would have to cease and the hole abandoned if the zone of high permeability
cannot be sealed off.
Strategies for sealing such zones vary. In some cases, increasing the rate at which mud is pumped
into the hole will ultimately fill the fractures and drilling can continue. In some instances, the frac-
ture network may be so extensive and pervasive that increased pumping will not fill the fractures
sufficiently. In that case, other material may be added to the mud and pumped down the hole. Such
things as cubed alfalfa, granular coal, diesel oil, and sawdust, or any other of a host of materials
that have the possibility of clogging the permeability have been tried. In such cases, there is no
guarantee of success, but the investment that has been made in siting and initiating drilling justifies
exhaustive efforts to recover circulation before the well is abandoned.

direcTional drillinG
Until the 1970s, nearly all industrial drilling produced wells that were vertical. But, the advantages
of being able to deviate a well at some angle away from the vertical was apparent. One impor-
tant advantage was that a well that was drilled vertically initially, but then was deviated toward a