Page 379 - Geology and Geochemistry of Oil and Gas

340 APPENDIX C

Tortuosity Square of the ratio of the effective length, L e , to the
length parallel to the overall direction of flow of the
2
pore channels, t ¼ (L e /L) .
Total depth (TD) The maximum depth reached in a well.
Tract A region or area of land that may be precisely or
indefinitely defined.
Transgression The spread of the sea over land areas; also, any
change that brings offshore, deep-water environments
to areas previously occupied by near-shore, shallow-
water environments. The term transgressive is used in
reference to sediments deposited during a
transgression. Cf: Regression. Syn: Sea invasion.
Trap [petrol] The accumulation of petroleum in a reservoir rock
under such conditions that its migration and escape is
prevented. Sealing can be effected by the abutment of
impermeable formations against the reservoir (e.g.,
stratigraphic trap, unconformity trap), by lateral
variation within the bed to reduce permeability
(e.g., pinch-out trap, shale-out trap), by the presence
of water preventing downward migration (e.g.,
structural trap), or by the presence of impermeable
fault preventing lateral migration (e.g., fault trap).
Trap [prod] A device for separating sediment from flowing oil or
water. Syn: Sand trap.
Traveling block Arrangement of pulleys on the drilling rig with an
attached hook, which moves up and down on cables
running through the crown block.
Trip Process of pulling drillstring (or tubing) out of the
wellbore and then running it back in.
Trip gas Gas that enters the wellbore when the mud pump is
shut down and pipe is being puled from the wellbore.
The gas may enter because of reduction in
bottomhole pressure when the mud pump is shut
down, because of swabbing, or because of both.
Tubing Relatively small-diameter pipe that is run into the well
to serve as a conduit for the passage of oil and gas to
the surface.
Turbidite A sediment deposited from a turbidity current. It is
characterized by graded bedding, moderate sorting,
and well-developed primary structures, especially
lamination.
Turbodrills A rotary drilling method in which fluid pumped down
the tubing turns the drill bit. The downhole motor
consists of multistage vane-type rotor and stator sec-
tion, bearing section, drive shaft, and bit-rotating sub.