3.2 POROSITY AND PERMEABILITY
63
3.2.1.2.3 Direct method of permeability measurement
Permeability may be directly measured at outcrop or from a hand specimen or core
in the laboratory. Permeability at outcrop may be measured using a probe- or minipermeameter. This is a portable handheld piece of kit. The technique involves placing
a nozzle against the rock face and puffing a fluid, commonly nitrogen, into the rock and
measuring the flow rate (Hurst and Goggin, 1995).
Permeability measurement in a laboratory involves pumping gas through a carefully
dried and prepared rock sample. This may be either a whole core or a plug cut from a
core or hand specimen.The apparatus records the length and cross-sectional area of the
sample, the pressure drop across it, and the rate of flow for the test period (refer back
to Fig. 3.12). The permeability is then calculated by applying the Darcy equation (Section 3.2.1.1). The viscosity of gas at the temperature of the test can be found in printed
tables of physical data. Permeability values from minipermeameter readings are comparable to those from laboratory measurements of whole cores, but are lower than those
measured from core plugs (Hurst and Rosvoll, 1991).
3.2.1.2.4 Indirect methods of permeability measurement
Permeability can be calculated by recording the amount of fluid which a known length
of borehole can produce over a given period of time. This is applicable in hydrology
where, unless the well is artesian, pump tests are carried out. These record the amount
of water which can be extracted in a given period, and the length of time required for
the water table to return to normal if it was depressed during the test. Both the productivity under testing and the recharge time are measures of the permeability of the
aquifer.
Oil and gas reservoirs typically occur under pressure. The permeability of the reservoir can be measured from drill-stem tests and from lengthier production tests. These
record the amount of fluid produced in a given period, the pressure drop during this
time, and the buildup in pressure over a second time interval when the reservoir is not
producing. By turning Darcy's law inside out to apply to the shape of the borehole, it is
possible to measure the mean permeability of the reservoir formation over the interval
that was tested.
Permeability, unlike porosity, cannot yet be directly measured by using geophysical
sondes in boreholes. There are now, however, various geophysical techniques that can
measure directional variations in permeability once an actual measurement has been
made using the techniques outlined earlier.
3.2.1.3 Capillarity
Fluid movement through sediments is also contingent on capillarity. Consider a trough
of liquid within which are arranged vertical tubes of varying diameters (Fig. 3.16). The
level of liquid within wide tubes is the same as that without. In narrow tubes, however,
liquid is drawn up above the common level. The height is proportional to the tube diameter. This gravity-defying effect, contrary to hydrostatic principles, is due to surface
63
3.2.1.2.3 Direct method of permeability measurement
Permeability may be directly measured at outcrop or from a hand specimen or core
in the laboratory. Permeability at outcrop may be measured using a probe- or minipermeameter. This is a portable handheld piece of kit. The technique involves placing
a nozzle against the rock face and puffing a fluid, commonly nitrogen, into the rock and
measuring the flow rate (Hurst and Goggin, 1995).
Permeability measurement in a laboratory involves pumping gas through a carefully
dried and prepared rock sample. This may be either a whole core or a plug cut from a
core or hand specimen.The apparatus records the length and cross-sectional area of the
sample, the pressure drop across it, and the rate of flow for the test period (refer back
to Fig. 3.12). The permeability is then calculated by applying the Darcy equation (Section 3.2.1.1). The viscosity of gas at the temperature of the test can be found in printed
tables of physical data. Permeability values from minipermeameter readings are comparable to those from laboratory measurements of whole cores, but are lower than those
measured from core plugs (Hurst and Rosvoll, 1991).
3.2.1.2.4 Indirect methods of permeability measurement
Permeability can be calculated by recording the amount of fluid which a known length
of borehole can produce over a given period of time. This is applicable in hydrology
where, unless the well is artesian, pump tests are carried out. These record the amount
of water which can be extracted in a given period, and the length of time required for
the water table to return to normal if it was depressed during the test. Both the productivity under testing and the recharge time are measures of the permeability of the
aquifer.
Oil and gas reservoirs typically occur under pressure. The permeability of the reservoir can be measured from drill-stem tests and from lengthier production tests. These
record the amount of fluid produced in a given period, the pressure drop during this
time, and the buildup in pressure over a second time interval when the reservoir is not
producing. By turning Darcy's law inside out to apply to the shape of the borehole, it is
possible to measure the mean permeability of the reservoir formation over the interval
that was tested.
Permeability, unlike porosity, cannot yet be directly measured by using geophysical
sondes in boreholes. There are now, however, various geophysical techniques that can
measure directional variations in permeability once an actual measurement has been
made using the techniques outlined earlier.
3.2.1.3 Capillarity
Fluid movement through sediments is also contingent on capillarity. Consider a trough
of liquid within which are arranged vertical tubes of varying diameters (Fig. 3.16). The
level of liquid within wide tubes is the same as that without. In narrow tubes, however,
liquid is drawn up above the common level. The height is proportional to the tube diameter. This gravity-defying effect, contrary to hydrostatic principles, is due to surface