Compressional velocities (Vp) are plotted against porosity (f) measured in two wells in the same depositional trend. Inset above and below are the NMR T2 spectrum and representative SEM image for rocks from each well. Based on the NMR and SEM, these rocks are drastically different. (Image courtesy of Mewbourne School of Petroleum and Geological Engineering)
Rocks, in the words of Mathew Simmons, author of Twilight in the Desert, are “the only incontrovertible sample of the reservoir.” The measurement of their physical properties therefore becomes a direct measurement of the reservoir.

However, it is not practical to sample the entire reservoir. Instead, the ideal objective is to seek a marriage of measurement and theory that allows the extension of a limited number of laboratory observations.

Petrophysics has a legacy in the seminal works of Gus Archie. Archie was a pioneer in changing otherwise qualitative information in logs, referred to as “character logs,” into quantitative information that formed the basis for economic reservoir evaluation. The relationships among formation factor, porosity, and water saturation are classic examples.

Archie recognized the need to view and correlate information at multiple scales and used careful laboratory measurements on cores to calibrate log responses and to establish empirical relationships among petrophysical properties. As a modern example, the nuclear magnetic resonance (NMR) log provides a matrix-independent measure of fluids; however, through calibration with laboratory measurements it can provide much more, such as irreducible water saturation and permeability.

Rock physics

Rock physics grew in importance with the recognition that seminal work by Biot could be used to improve the discrimination of fluids in reservoirs. This event was significant in the evolution of this discipline. Hydraulic fracturing required knowledge of Young’s modulus and Poisson’s ratio, in turn requiring the measurement of shear velocities and driving advances in sonic logging tools that then enabled more realistic elastic seismic modeling. Recognizing that shear properties could be obtained from the angular dependence of seismic amplitude provided the catalyst that allowed rock physics to stand apart from petrophysics.

Rock physics is vital to exploration and 4-D reservoir management, but it needs further research to address “resource plays.” The emphasis now is very much akin to Archie’s original approach. Understanding and establishing the petrophysical controls on elastic, anisotropic, and attenuative properties of rocks have the potential to make resource exploitation more effective.

IC3

Understanding the legacy experimental contributions to rock physics supports the current experimental programs. What has fundamentally changed is the holistic interest in rock properties for E&P purposes. Through this integration, it is possible to develop linkages across the rock physics and petrophysical domains. This philosophy is embraced in the experimental program at the Integrated Core Characterization Center (IC3) at the University of Oklahoma. In studying a geomechanical problem, the classical static elastic moduli, the failure strength, etc. are measured, but the mineralogy, permeability, porosity, seismic velocities, thin-section analysis, and scanning electron microscopy (SEM) are also included. Such a dataset is incrementally more expensive but paves the way for understanding why a suite of samples has a particular mechanical response and often leads to a surrogate means to remotely appraise behavioral changes of other wells.

Limited petrophysical measurements such as porosity, permeability, resistivity, etc., solve the immediate problem but fail to provide a mechanism to understand field-wide variations. Integration of 3-D and 4-D studies with reservoir production history-matching becomes more powerful when pressure and phase-induced production changes can be directly related to frame and fluid property changes recorded by seismic waves.

There is a growing awareness of the inextricable interdependence of rock properties and the limited predictability of geological overprints. This demands a comprehensive approach to reservoir management and physical property measurement. A direct consequence of this concept is maximizing information from limited core material. In addition to making routine porosity, permeability, density, mineralogy, and resistivity measurements, petropysicists integrate what are considered SCAL (Special Core Analysis) measurements, compressional and shear velocities, NMR, mercury injection, thin-section analysis, SEM microscopy, and mechanical testing. This creates a database that becomes a resource in addressing future problems.

Unconventional reservoirs

Unconventional reservoirs represent a switch in controls on physical properties. Packing and consolidation control reservoir properties at high porosities, while at lower porosities typical of unconventional reservoirs, the diagenetic processes dominate. Limited understanding and variability of these processes drives empirical studies.

Figure 1 illustrates some of the problems. Compressional velocities (Vp) are plotted against porosity (f) measured in two wells in the same depositional trend. Inset above and below are the NMR T2 spectrum and representative SEM image for rocks from each well. Based on the NMR and SEM, these rocks are drastically different — above is clean and quartz-dominated, while below the rock is filled with clays. Ignoring this information and interpreting the Vp-f trend, one could erroneously take a velocity in the Anderson well as a sample from Well-x with twice the porosity.

Classical measurements of porosity and permeability become more difficult and sometimes impossible, requiring new approaches. The industry made the transition from steady-state liquid permeability to routine pulse decay analysis using gas, and now methods are being evaluated to measure permeabilities at and below nanodarcy levels.

Porosities historically measurable on whole core samples are now being determined on crushed samples and by using techniques such as NMR and mercury injection. Breakthroughs come from an understanding of the effects of cracks, crack alignment, and stresses on rock properties; the roles of clays in supporting matrix or filling the pores on permeability; velocities; and resistivities.

Diagenetic signatures dominate both rock and petrophysical properties, often defining reservoir sweet spots in resource plays. More complicated experiments and sophisticated instrumentation are required to unlock these relations. In the extreme, gas shale reservoirs present the greatest challenges — traditional porosity and permeability measurement methods fail; particle size is so fine as to defy optical resolution; and the act of preparing samples for measurement, if not done correctly, can deleteriously influence the outcomes.

The frontiers of experimental petrophysics and rock physics are defined by the new unconventional reservoirs. The challenge is to define the physics that enable upscaling these observations and integrating them with classical observables, which then transforms this knowledge into tools that improve recovery and stimulation.

Developing these technologies into useful tools requires focused research on techniques, processes, and education of end users. In an effort to address the latter, labs based around modern instrumentation should be integrated into undergraduate course curricula. This will lead to a generation of new engineers who will be more receptive to the information these new technologies can provide.

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