P. K. Harvey

Petrophysical Properties of Crystalline Rocks

Boreholes are commonly drilled into crystalline rocks to evaluate their suitability for various applications such as waste disposal (including nuclear waste), geothermal energy, hydrology, sequestration of greenhouse gases and for fault analysis. Crystalline rocks include igneous, metamorphic and even some sedimentary rocks. The quantification and understanding of individual rock masses requires extensive modelling and an analysis of various physical and chemical parameters. This volume covers the following aspects of the petrophysical properties of crystalline rocks: fracturing and deformation, oceanic basement studies, permeability and hydrology, and laboratorybased studies. With the growing demands for sustainable and environmentally effective development of the subsurface, the petrophysics of crystalline rocks is becoming an increasingly important field.

Application of FMS images in the Ocean Drilling Program: an overview

Abstract Piecing together the evolution of the ocean basins increasingly relies on the integration of data from both recovered core and downhole measurements. This task is often complicated by the limited amount of core recovered by the Ocean Drilling Program (ODP) and the lack of understanding of the downhole data. The availability of downhole electrically based images since ODP Leg 126 in 1989 provides scientists with the visual means of examining the nature of the subsurface, and for tying disparate core to the continuous downhole data. These Formation MicroScanner (FMS) images are unfortunately based on a relatively crude resistivity measurement which provides the interpreter with only an estimate of the resistivity of the rock but, where there are variations in resistivity which correspond to variations in fabric or structure, the measurement response is often sufficient to provide a detailed visual record. Scientists participating in the ODP have explored the use of these images in tackling a wide range of problems from volcanic and sediment stratigraphy to structure and tectonic applications. The determination of core orientation and the mapping of intervals where core recovery is incomplete in particular provide the geologist with a means of carrying out field studies based on borehole and core observations which were previously unthinkable. This paper aims to provide a brief introduction to this subject, and in reviewing some of the principal results to date, illustrates the use of downhole FMS images in the ODP.

High-resolution petrophysical characterization of samples from an aeolian sandstone: the Permian Penrith sandstone of NW England

Abstract The Penrith Sandstone is an orange/red, mainly homogeneous, friable rock made up of well-rounded, highly spherical quartz grains, often showing euhedral overgrowths of quartz. Sandstone samples from Stoneraise Quarry, NW England, exhibit a remarkable degree of rounding and very high sphericity, along with frosted textures typical of aeolian deposits. Chemically, the rock is predominantly SiO2 (>95%), with no evidence of carbonate cements. Quartz predominates with a small proportion (10%) of feldspar. The grain size across heterogeneous zones varies from very fine (100 µm) to coarse sand (700 µm). There is no evidence of the presence of clay minerals. Petrophysically, based on the measurements made in this study, the Penrith Sandstone is a typical clean sandstone characterized by moderate porosity (12%) and core-plug permeability (10−14−10−12 m2), and Archie ‘m’ exponents between 1.90 and 1.91, suggesting a reasonably clean ‘Archie’ rock with no excess conductivity associated with clays or bound water. Capillary pressure curves for four samples demonstrate unimodal pore-size distributions with a single modal range that varies between 25–50 and 70–80 µm. Because of the relative simplicity of its petrophysics, the sandstone is thus potentially very useful in fundamental studies, and also in the trialling of new techniques. We use imaging techniques to investigate the degree of heterogeneity and the fabric of the Penrith Sandstone. Conventional optical images are complemented by electrical resistivity, porosity and mini-permeametry images. These two-dimensional maps of resolution of approximately 5 mm show a spatial similarity determined by the rock fabric. The detailed images show a wider degree of variation and heterogeneity than the plug-averaged values. The success of the resistivity imaging method suggests that the technique could be used in deriving correlations that could be used to interpret borehole resistivity imaging logs. However, in the present study, correlations of property values derived from the imaging do show considerable scatter: this suggests that heterogeneity even below the scale of the imaging is also important, a conclusion supported by thin-section and electronmicroscope data.

Interpretation of core and log data—integration or calibration?

Abstract Core-log interpretation requires the reconciliation of datasets from different measurements. Measurement process, resolution, scale and quality must be appreciated for each dataset. Calibration of measurements involves the use of standards to enable quantitative comparisons locally or globally; this may involve inter-dataset comparison and the process of equalization with the modification of one dataset in preference for another. Calibration should not be confused with integration which aims to maximize the information in an optimal manner and may require the selective choice of data. The clear recognition of the aims of the study at the earliest opportunity enables the best choice of strategy from measurement acquisition through to integration. The final interpretation should realize the original aims but must be compatible with all observations.

Fracture mapping with electrical core images

Abstract Naturally fractured reservoirs often contain a range of different fracture types and networks; fractures that are relatively permeable and relatively impermeable, unconnected and connected to the part of the fracture network that carries fluid flow, and naturally occurring or drilling induced. Consequently, in terms of their fluid connectivity, fractures may be open or closed, while individual fractures may be isolated or well connected. We have adapted our approach to imaging sedimentary fabric in the laboratory, where we related electrical core images to properties such as porosity, permeability, grain size and cementation, to enable electrical imaging of fractures in core. Our approach uses similar principles to those employed in down-hole electrical imaging. The results demonstrate an ability to image conductive fractures in fully saturated low-porosity water-bearing core: these fractures being electrically connected from the flat measurement surface through to the outer surface of the core. Published results for numerical modelling of down-hole electrical imaging tools show the electrical response is related to fracture depth and fracture aperture. Our experimental results on fractured core in the laboratory support these numerical observations, increased current flowing into the fracture as the aperture increases. The finite size of the electrode, however, means that this technique cannot distinguish between a single fracture and smaller groups of fractures intersecting the electrode.

Measurement scale and formation heterogeneity: effects on the integration of resistivity data

Abstract Core and downhole logging resistivity data gathered during Leg 133 of the Ocean Drilling Program are used to illustrate the wide range of scales of resistivity data available for reservoir characterization. The differences in scale and sampling interval between quantitative log resistivity data and conventional core plug data is shown to be central to reconciling these two datasets. Resistivity images of fine scale sedimentary structures taken on half-round cores are presented (at the same resolution as the downhole borehole wall imaging tools) and these fine structures are shown to be ‘lost’ if investigated using conventional core plugs and downhole resistivity logging tools. The limitations of conventional measurements on core plugs are presented and contrasted with the benefits of logging all of the core in the laboratory at a resolution comparable to the borehole wall imaging tools. An example of integrating different scales of resistivity data using a modelling approach is presented and is shown to be applicable to both core and log data. Visualizing and comparing the scale content of different resistivity datasets has been achieved in an intuitive way using a spectral method which illustrates the ‘data gap’ in quantitative resistivities which exists between core and log data.

A non-contacting resistivity imaging method for characterizing whole round core while in its liner

Abstract Recent laboratory experimentation has shown that non-contacting whole-core resistivity imaging, with azimuthal discrimination, is feasible. It has shown the need for very sensitive coil pairs in order to provide resistivity measurements at the desired resolution. Independent high-resolution ‘galvanic’ resistivity estimations show the ‘non-contacting’ measurements to be directly proportional to the resistivity of core samples. The response of the technique to a variety of synthetic ‘structures’ is presented. A whole-core image of a dipping layer is used to demonstrate the three dimensional response of the technique and to show that the resolution of the measurements is of the order of 10 mm. Experiments are described which show that the technique is capable of investigating to different depths within the whole round core. The results agree with theoretical predictions and indicate that the technique has the potential to assess invasion near the surface of the core. The technique is intrinsically safe and has the potential to be packaged in a form that would be suitable for whole-core imaging at the well site, or laboratory, without taking core from their liners. Thus it is possible to acquire information crucial for core selection, in addition to acquiring resistivity data at a resolution not too far removed from that of the downhole imaging tools.

Petrophysical estimation from downhole Mineralogy logs

Abstract A number of physical and chemical properties of rocks, many of which are important in petrophysics are simple, usually linear functions, of a rock’s mineralogy. Examples include matrix density, porosity, magnetic susceptibility and cation exchange capacity. Mineralogy logs can be obtained directly from geochemical logs through an inversion process and, in turn, estimates of petrophysical properties such as those noted above can be obtained. The accuracy of such estimates is directly dependent on the quality of both the inversion and the geochemical measurements. Examples from producing oil fields and from deep-sea environments are used to show that accurate estimates of derived parameters can be obtained provided that appropriate inversion procedures are adopted, and that these estimates are generally better than those obtained by conventional log analysis. Logs of some parameters, such as cation exchange capacity, cannot be obtained except through the mineral inversion.