Conditional reconstruction: An alternative strategy in digital rock physics

Digital rock physics (DRP) is a newly developed method based on imaging and digitizing of 3D pore and mineral structure of actual rock and numerically computing rock physical properties, such as permeability, elastic moduli, and formation factor. Modern high-resolution microcomputed tomography scanners are used for imaging, but these devices are not widely available, and 3D imaging is also costly and it is a time-consuming procedure. However, recent improvements of 3D reconstruction algorithms such as crosscorrelation-based simulation and, on the other side, the concept of rock physical trends have provided some new avenues in DRP. We have developed a modified work flow using higher order statistical methods. First, a high-resolution 2D image is divided into smaller subimages. Then, different stochastic subsamples are generated based on the provided 2D subimages. Eventually, various rock physical parameters are calculated. Using several subsamples allows extracting rock physical trends and better capturing the heterogeneity and variability. We implemented our work flow on two DRP benchmark data (Berea sandstone and Grosmont carbonate) and a thin-section image from the Grosmont carbonate formation. Results of realization models, pore network modeling, and autocorrelation functions for the real and reconstructed subsamples reveal the validity of the reconstructed models. Furthermore, the agreement between static and dynamic methods indicates that subsamples are representative volume elements. Average values of the subsamples’ properties follow the reference trends of the rock sample. Permeability trends pass the actual results of the benchmark samples; however, elastic moduli trends find higher values. The latter can be due to image resolution and voxel size, which are generated by imaging tools and reconstruction algorithms. According to the obtained results, this strategy can be introduced as a valid and accurate method where an alternative method for standard DRP is needed

Nano Geochemistry of Low Salinity Enhanced Oil Recovery

In this thesis a wide range of analytical techniques were used to characterise several petrophysical properties of Berea sandstone, including the mineral distribution at the pore surface and its pore structure, both playing a crucial role in determining its response to low salinity enhanced oil recovery (EOR) investigations. In addition, the role of different cations in affecting the wettability state of pure quartz and Berea sandstone was experimentally investigated in order to gain an insight on the behaviour of sandstone reservoirs during low salinity waterflooding EOR. Results from the multi-technique, multi-scale characterisation of Berea indicate that the mineralogy exposed to the pore spaces is highly heterogeneous across different length scales, going down to the often-neglected nanoscale were significant amounts of phases identified as grain coatings. In addition, analysis of the porosity and pore-connectivity also requires a multi-length approach for its full characterisation to be realised. Both aspects are crucial to understand the role of mineral surface chemistry in determining oil/water and oil/minerals interactions in both experiments and field conditions. Investigations on wettability alteration using environmental scanning electron microscopy (ESEM) and contact angle measurements on ideal quartz surfaces showed that reduced salinity leads to a more water-wet state. These measurements were complemented with atomic force microscopy adhesion measurements on quartz surfaces, the results giving further insight into the role of nano-scale roughness on quartz surfaces in wettability alteration by increasing the amount of oil retained on the surface. Finally, similar wettability alteration experiments were performed on Berea sections. The effect of brine was consistent, reproducible and reversible and again showed a low salinity effect, i.e. a change to more water-wet conditions with lower salinity. The results also demonstrate that quartz surfaces always contributes at least in part to the low salinity effect, decreasing oil wettability when salinity is low. In addition, we demonstrate that the ESEM can be an essential tool in studying the wettability alteration of rocks and minerals

Conditional reconstruction: An alternative strategy in digital rock physics

Digital rock physics (DRP) is a newly developed method based on imaging and digitizing of 3D pore and mineral structure of actual rock and numerically computing rock physical properties, such as permeability, elastic moduli, and formation factor. Modern high-resolution microcomputed tomography scanners are used for imaging, but these devices are not widely available, and 3D imaging is also costly and it is a time-consuming procedure. However, recent improvements of 3D reconstruction algorithms such as crosscorrelation-based simulation and, on the other side, the concept of rock physical trends have provided some new avenues in DRP. We have developed a modified work flow using higher order statistical methods. First, a high-resolution 2D image is divided into smaller subimages. Then, different stochastic subsamples are generated based on the provided 2D subimages. Eventually, various rock physical parameters are calculated. Using several subsamples allows extracting rock physical trends and better capturing the heterogeneity and variability. We implemented our work flow on two DRP benchmark data (Berea sandstone and Grosmont carbonate) and a thin-section image from the Grosmont carbonate formation. Results of realization models, pore network modeling, and autocorrelation functions for the real and reconstructed subsamples reveal the validity of the reconstructed models. Furthermore, the agreement between static and dynamic methods indicates that subsamples are representative volume elements. Average values of the subsamples’ properties follow the reference trends of the rock sample. Permeability trends pass the actual results of the benchmark samples; however, elastic moduli trends find higher values. The latter can be due to image resolution and voxel size, which are generated by imaging tools and reconstruction algorithms. According to the obtained results, this strategy can be introduced as a valid and accurate method where an alternative method for standard DRP is needed

Applications of digital core analysis and hydraulic flow units in petrophysical characterization

Conventional petrophysical characterizations are often based on direct laboratory measurements. Although they provide accurate results, such measurements are time-consuming and limited by instrument and environment. What’s more, in the geo- resource energy industry, availability and cuttings of core plugs are difficult. Because of these reasons, virtual digital core technology is of increasing interest due to its capability of characterizing rock samples without physical cores and experiments. Virtual digital core technology, also known as digital rock physics, is used to discover, understand and model relationships between material, fluid composition, rock microstructure and macro equivalent physical properties. Based on actual geological conditions, modern mathematical methods and imaging technology, the digital model of the core or porous media is created to carry out physical field numerical simulation. In this review, the methods for constructing digital porous media are introduced first, then the characterization of thin rock cross section and the capillary pressure curve using scanning electron microscope image under mixed wetting are presented. Finally, we summarize the hydraulic flow unit methods in petrophysical classification.Cited as: Chen, X., Zhou, Y. Applications of digital core analysis and hydraulic flow units in petrophysical characterization. Advances in Geo-Energy Research, 2017, 1(1): 18-30, doi: 10.26804/ager.2017.01.0

Estimating the permeability of reservoir sandstones using image analysis of pore structure

In this thesis, a method is developed for predicting the permeabilities of a core using only a small number of SEM images, without resorting to computationally intensive procedures. The pore structure is idealised as consisting of a cubic network of pore tubes having an arbitrary distribution of cross-sectional areas and shapes. The areas and perimeters of the individual pores are estimated from image analysis of scanning electron micrographs of thin sections, with appropriate stereological corrections introduced to infer the true cross sections of the pores. Effective medium theory is used to find the effective single-tube conductance, based on the measured distribution of individual conductances, thereby allowing a prediction of the permeability. The methodology has been applied to several reservoir sandstones from the North Sea, and also an outcrop sample from Cumbria, UK, yielding predictions that fall within a factor of two of the laboratory measurements in most cases. The procedure, although based on Kirkpatrick's intrinsically isotropic effectivemedium approximation, is not only capable of yielding reasonably accurate estimates of the permeabilities, but also gives a qualitatively correct indication of the anisotropy ratio. It also found that the use of an Bernasconi's anisotropic effective-medium approximation does not lead to a systematic improvement in the results, perhaps because the samples used in this study were insufficiently anisotropic for the approaches to yield different results. The validity of the effective medium approximation was also tested against exact pore network calculations. For the rocks examined in this study, with pore conductance distributions having log-variances less than 3, the effective medium approximation was found to be accurate to within a few percent.Open Acces

PORE-LEVEL FLUID MIGRATION IN RESERVOIR SANDSTONES

The void space properties of a set of gas reservoir sandstone samples have been measured. The properties include porosity, absolute gas permeability, electrical resistivity formation factor and tortuosity. The mineralogy of each sandstone was determined by scanning electron microscopy and energy dispersive x-ray analysis. Mercury intrusion and extrusion data have been measured for most of the sandstone samples. A new procedure for measuring the degree and range of void size correlations within resin-filled sandstones has been developed. Image analysis of backscattered electron micrographs of these samples supplies void size and positional information. A "semi-variogram" study of void size and coordinate data ascertains the degree and range of void size correlation. Measurable correlation has been found in two sandstone samples, but was absent from four others. Diffusion coefficients of methane, iso-butane and n-butane through dry sandstones have been measured using an adaptation of a non-steady state method, using a redesigned apparatus. A repeatability and error analysis of diffusion coefficient measurement has also been performed. A correlation between diffusion coefficients, absolute permeability, porosity and formation factor was detected for sandstones containing little clay. The diffusion coefficients measured for clay affected sandstones did not correlate with any petrophysical properties of these samples. A computer model capable of simulating porous media has been previously developed. It consists of a 10x10x10 network of cubic pores and cylindrical throats, and simulates die mercury intrusion curve. The void size distribution is modified until both simulated and experimental curves closely match. New void size distribution input and curve fit algorithms have been developed to increase the speed and accuracy of die simulations and a new modelling procedure allows the modelling of samples with void size correlation. The model is capable of simulating porosity, permeability, tortuosity and mercury extrusion. Each of the reservoir sandstones has been modelled and their characteristic properties simulated. Successful simulations were obtained for all relatively clay-free reservoir sandstones. Clay affected sandstone simulations were less successful due to the high complexity of these samples. A study into formation damage witiiin reservoir sandstones was also undertaken. The effect of colloidal particulate void space penetration is measured and simulated.British Gas, Michael Road Research Station, Londo

An experimental study of porosity collapse and deformation band formation in high-porosity sandstones

Compaction bands are millimetre to several centimetre thick sub-seismic bands of localised deformation in sandstones, which form approximately perpendicular to the maximum principal stress. They are associated with intense grain crushing and pore collapse, which generally results in an intra-band reduction in porosity and permeability. Consequently, these structures can exert a significant control on fluid flow, with potential implications for industrial processes such as fluid extraction during petroleum or groundwater production, or fluid injection during geothermal or CO2 sequestration projects. However, due to the heterogeneous nature of sandstones, determining the role that specific microstructural properties have on band formation is extremely challenging and consequently, much is still unknown regarding compaction band formation. The aim of this study is to attempt to better understand the microstructural properties and external factors which promote and govern the formation of compaction bands in sandstones. A new methodology has been developed which enables the production of high-porosity sandstone samples for laboratory testing which have reproducible petrophysical properties that can be systematically controlled. The technique uses the chemical reaction between sodium silicate and hydrochloric acid to precipitate cementing amorphous quartz between initially incohesive sand grains. This enables the production of sandstone samples for laboratory testing which have reproducible petrophysical properties that can be systematically controlled. Microstructural and mechanical analysis of the synthetic sandstones shows them to have realistic and reproducible uniaxial compressive, tensile and hydrostatic yield strengths, and to also exhibit yield curves with comparable geometries to natural sandstones of similar porosity and grain size. They also display elastic moduli within the expected range for natural sandstones. The effect of porosity and grain size on compaction localisation is investigated using synthetic sandstones produced using the new methodology developed. Twelve sandstones are produced with 3 different starting porosities (27, 32, 37%) and 4 different mean grain sizes (314, 411, 747 and 987 µm). The samples are each shortened by 5% axial strain at an effective stress equivalent to 85% of their grain crushing pressure (P*). Discrete compaction bands (≤3 grain diameters in width) oriented normal to the axial loading direction are only observed in the sample with the lowest starting porosity (27%) and smallest grain size (314 µm), while diffuse bands (>3 grain diameters in width) are observed for the same porosity at a larger grain size of 411 µm. No compaction bands develop for any grain size in either the 32% or 37% starting porosity samples. Porosity analysis indicates grain size reduction does not necessarily ii correspond to porosity reduction indicating that compaction by grain rearrangement is as effective as localisation through comminution for these high-porosity synthetic sandstones. The role of cement in compaction band formation is examined using three sandstones, Bentheim, Castlegate and a synthetic sandstone that each possess similar porosities (~26-29%) and grain sizes (~230-300 µm), but which are cemented differently, with syntaxial quartz overgrowths, clay, and amorphous quartz cement respectively. Each sample forms discrete compaction bands when taken to 5% axial strain at a starting effective stress equivalent to 85% of its hydrostatic yield (P*) value. The compaction bands are only located at the sample ends in Bentheim Sandstone, whereas, in Castlegate Sandstone they are distributed throughout the whole sample and in the synthetic sample, the bands are only located within the sample centre. The results suggest that cement type plays a significant role in the mechanics of deformation within each of the samples, which in turn, determines where the compaction bands nucleate and develop. Since all the compaction bands identified are discrete, cement is not the primary control regarding the preference for the formation of diffuse or discrete compaction bands. The nature of strain localisation with increasing effective confining pressure is examined in Castlegate Sandstone. At low effective pressures, deformation localises into sets of dilational conjugate shear bands orientated ~30° to the maximum compressive stress

Experimental study of localised deformation in porous sandstones

This PhD thesis presents a laboratory study aiming at a better understanding of the stress-strain response of the Vosges sandstone (porous rock) tested at a range of confining pressures (i.e., 20-190 MPa) and different axial strain levels. Localised deformation was captured at different scales by a combination of full-field experimental methods, including Ultrasonic Tomography (2D), Acoustic Emissions (3D), X-ray Tomography (3D), and 3D volumetric Digital Image Correlation, plus thin section and Scanning Electron Microscope observations (2D). These experimental methods were performed before, during and after a number of triaxial compression tests. The combined use of the experimental techniques, which have different sensitivity and resolution, described the processes of shear band and shear-enhanced compaction band generation, which formed at low to intermediate and relatively high confining pressures, respectively. Pure compaction bands were not identified. The deformation bands were characterised as zones of localised shear and/or volumetric strain and were captured by the experimental methods as features of low ultrasonic velocities, places of inter- and intra-granular cracking and structures of higher density material. The two main grain-scale mechanisms: grain breakage (damage) and porosity reduction (compaction) were identified in both shear band and shear-enhanced compaction band formation, which presented differences in the proportions of the mechanism and their order of occurrence in time