Page 349 - Geology and Geochemistry of Oil and Gas

310 APPENDIX C

Effective pressure Grain-to-grain stress, which is equal to the total
overburden pressure minus the pore (fluid) pressure.
Elasticity A property a material may have of recovering its
original shape and size ‘‘immediately’’ after a
deforming force has been removed. Elasticity arises
from the forces between atoms or molecules and
shows that the forces are repulsive when the atoms are
closer than some natural spacing, and attractive when
they are farther away.
Electric log Measures the electric characteristics of a formation;
the tool transmits signals to the surface. Downhole
tools include either normal or lateral device. In both
devices (sondes) a constant electric current is passed
between electrodes A and B; the resultant potential
difference is measured between electrodes M and N.
In the normal device, electrodes A and M are on the
sonde, and electrodes B and N are, theoretically,
located an infinite distance away. In the lateral device,
electrodes A, M, and N are on the sonde, and
electrode B is located an infinite distance away.
Electric submersible lift Downhole electric motor and pump, which are used
for high volumes of fluid production.
Elevators Clamps for lifting rods, tubing, and casing.
Enhanced recovery Techniques that supplement the natural primary
recovery mechanism to increase the flow of the
reservoir fluids to the wellbore.
Entropy, information A special index in the theory of information which is
theory employed as a criterion of a priori (before the
communications were received) indefiniteness of the
system. The entropy of a set of events,
n
H ¼ P p log p
i i
i ¼1
where p i is a probability of each event. The entropy
properties: (1) it becomes zero, if one of the states of
the system is fully known and the others are
impossible; (2) at a given number of states, it becomes
maximum, if all the states are equally probable, but
increases with the increasing number of states; (3) it is
additive, i.e., it is possible to add up entropy of
different systems.
Entropy, thermodynamics A macroscopic, thermodynamic quantity, ultimately
or statistical mechanics reflecting the degree of microscopic randomness or
disorder of a system. The entropy in a closed system
increases to a maximum at equilibrium as the