This disclosure relates generally to methods and systems for analyzing images of rock samples to determine petrophysical properties.
In hydrocarbon production, obtaining accurate subsurface estimates of petrophysical properties of the rock formations is important for the assessment of hydrocarbon volumes contained in the rock formations and for formulating a strategy for extracting the hydrocarbons from the rock formation. Traditionally, samples of the rock formation, such as from core samples or drilling cuttings, are subjected to physical laboratory tests to measure petrophysical properties such as permeability, porosity, formation factor, elastic moduli, and the like. As known in the art, some of these measurements require long time periods, extending over several months in some cases, depending on the nature of the rock itself. The equipment used to make these measurements can also be quite costly.
Often, petrophysical rock properties are measured in the laboratory at ambient environmental conditions, with the rock sample at room temperature and surface atmospheric pressure. However, the sub-surface environment of the rock in the formation can differ significantly from that of ambient laboratory conditions. For example, the weight of overburden sedimentation on formation rock, which increases with increasing burial depth, causes compaction of the formation rock, which is reflected in reduced porosity and permeability as compared with surface ambient conditions.
Subsurface rock formations are also subjected to changes in in situ stress/strain conditions as a result of hydrocarbon development and production. For instance, the stress conditions at a point in a rock formation adjacent to a drilled borehole will differ from the original in situ stress conditions at that same point prior to drilling. In addition, the injection and extraction of pore fluids, as occurs in field production, sets up changes in pore fluid pressure from that prior to production, which also causes changes in in situ stress conditions. Different stress or strain conditions from these and other causes can significantly alter the petrophysical properties of rock relative to the same rock under ambient conditions. Of course, it is the subsurface petrophysical properties of the rock under its in situ stress conditions that are of most interest for purposes of appraisal, development, and production of the field.
To compensate for the effect of changes in in situ stress, conventional laboratory measurements of porosity, permeability, electrical conductivity, and other petrophysical properties can be physically measured in the laboratory under a variety of stress and strain conditions. It has been observed, however, that the equipment and technician time required to artificially apply these physical conditions in the laboratory can be prohibitively expensive, as compared with tests performed under room ambient conditions, and can also require significantly more time to carry out, especially for complicated rock types. Moreover, the range of laboratory-applied stress and strain conditions for the measurement of a particular petrophysical property is often quite limited, and may not accurately represent the in situ subsurface conditions.
Even if equipment for measuring rock properties under confining stresses and pressures is available, the estimation of petrophysical properties of a given rock sample under several different stress/strain conditions is often not possible, because the microstructure of the rock sample may be permanently deformed by one or more of the loading and unloading stress/strain cycles. This deformation may occur, for instance, when measuring petrophysical properties of a given rock sample initially under hydrostatic stress conditions (i.e., where the sample is subjected to uniform confining pressure) and then measuring the petrophysical properties of the same rock under uniaxial stress conditions (i.e., where stress is applied in only a single direction, with no applied stress in all other directions). In that case, subsequent iterations of the measurement experiment on the same sample can result in a different petrophysical property value or other change in physical behavior that is not representative of the true stress/strain response of the rock. The measured petrophysical properties in the second and subsequent stress experiments may thus differ significantly from the true in situ values sought for those stress experiments.
Because of the cost and time required to directly measure petrophysical properties, the technique of “direct numerical simulation” has been developed for efficiently estimating physical properties, such as porosity, absolute permeability, relative permeability, formation factor, elastic moduli, and the like of rock samples, including samples from difficult rock types such as tight gas sands or carbonates. According to this approach, a three-dimensional tomographic image of the rock sample is obtained, for example by way of a computer tomographic (CT) scan. Voxels in the three-dimensional image volume are “segmented” (e.g., by “thresholding” their brightness values or by another approach) to distinguish rock matrix from void space. Numerical simulation of fluid flow or other physical behavior such as elasticity or electrical conductivity is then performed, from which porosity, permeability (absolute and/or relative), elastic properties, electrical properties, and the like can be derived. A variety of numerical methods may be applied to solve or approximate the physical equations simulating the appropriate behavior. These methods include the Lattice-Boltzmann, finite element, finite difference, finite volume numerical methods and the like.
However, conventional direct numerical simulation is generally limited to rock samples under ambient stress/strain conditions, in that images obtained by X-ray tomographic images or other imaging techniques (e.g., FIBSEM) are generally acquired under ambient conditions. This is because the mechanical equipment required to induce stress/strain conditions are not routinely attached to imaging equipment, or cannot feasibly be so attached, due to the nature of either or both of the imaging and mechanical devices. In those cases in which imaging and mechanical testing have been combined, such as by using special sample holders that are transparent to X-ray tomography, such combined experimental apparatus is highly specialized and extremely expensive, and may involve health and safety risks.