Reservoir engineering and petrophysical applications of conventional coreanalysis data necessitate representation of insitu porosity and permeabilitycharacteristics. As a result, the impact of reservoir confining stress onporosity and permeability and the effect of gas slippage (KlinkenbergEffect1) on air permeability must be evaluated. Failure to considerthese effects may result in erroneous and optimistic porosity log calibrationand optimistic productivity and injectivity estimates.
These effects are classically evaluated by selecting a subset of core samplesfrom the entire sample suite to represent compressibility and pore geometrycharacteristics. The entire sample suite is generally not studied as themeasurements are time consuming, labour intensive and therefore costprohibitive.
The sensitivity of porosity and permeability to reservoir confining pressure isinvestigated by simulating effective overburden pressure (EOP) in a hydrostaticload cell and by measuring porosity and permeability as a function ofincreasing EOP. These data are typically corrected to uniaxial strainconditions to more closely represent reservoir confinement and correlationsbetween the ambient condition and simulated reservoir condition data areobtained. Corrections obtained in this fashion are then applied to the entiredata base.
Klinkenberg effects are classically evaluated by steady state Kairmeasurements at several mean gas (air) pressures on each subset sample. Thesemeasurements are EOP sensitive. A plot: of Kair vs. reciprocal meangas pressure (Pm−1) is constructed permeability value atinfinite Pm−1 (oil reservoirs) or Kair at aspecific reservoir pressure (gas reservoir) is obtained (Figure 1).
The permeability intercept at infinite Pm−1 is referredto as the Klinkenberg corrected permeability (K 8). The relationship between K8 at effective overburden pressure and Kair at minimum confiningpressure and minimum mean gas pressure is established. Klinkenberg correctionsare then applied to the entire data base. An example is found in Figure2.
An automated core measurement system has been developed and overcomes thehistorical impracticalities and restrictions placed on evaluating overburdenand Klinkenberg effects on every core sample in a conventional core analysis.Core sample handling, instrument calibration, confinement pressure application, measurements and calculations are performed automatically and are orchestratedand monitored by computer. Measurements included pore volume by isothermalhelium gas expansion and K 8 by the unsteady state method.
Data generated on the system have been gathered, from selected sandstone andcarbonate reservoirs in the Western Canadian Basin, reported and discussed.These data illustrate the sensitivity of porosity and permeability to effectiveoverburden pressure and the effect of gas slippage. The significance of havingthese data available on entire sample suites is illustrated.
The system is fundamentally comprised of a carousel-type sample feed, anautomated sample load/unload device, a hydrostatic core holder and confinementpressure source assembly and computer hardware/software components (Figure 3). The Klinkenberg permeability is determined by un unsteady-state method and porevolume is measured by Boyle's Law (Eq. l) using helium as the source gas.
