Michael B. Clennell

Feasibility of a data-constrained prediction of hydrocarbon reservoir sandstone microstructures

Microstructures are critical for defining material characteristics such as permeability, mechanical, electrical and other physical properties. However, the available techniques for determining compositional microstructures through segmentation of x-ray computed tomography (CT) images are inadequate when there are finer structures than the CT spatial resolution, i.e. when there is more than one material in each voxel. This is the case for CT imaging of geomaterials characterized with submicron porosity and clay coating that control petrophysical properties of rock. This note outlines our data-constrained modelling (DCM) approach for prediction of compositional microstructures, and our investigation of the feasibility of determining sandstone microstructures using multiple CT data sets with different x-ray beam energies. In the DCM approach, each voxel is assumed to contain a mixture of multiple materials, optionally including voids. Our preliminary comparisons using model samples indicate that the DCM-predicted compositional microstructure is consistent with the known original microstructure under low noise conditions. The approach is quite generic and is applicable to predictions of microstructure of various materials.

Forced imbibition into a limestone: measuring P‐wave velocity and water saturation dependence on injection rate

Quantitative interpretation of time-lapse seismic data requires knowledge of the relationship between elastic wave velocities and fluid saturation. This relationship is not unique but depends on the spatial distribution of the fluid in the pore-space of the rock. In turn, the fluid distribution depends on the injection rate. To study this dependency, forced imbibition experiments with variable injection rates have been performed on an air-dry limestone sample. Water was injected into a cylindrical sample and was monitored by X-Ray Computed Tomography and ultrasonic timeof-flight measurements across the sample. The measurements show that the P-wave velocity decreases well before the saturation front approaches the ultrasonic raypath. This decrease is followed by an increase as the saturation front crosses the raypath. The observed patterns of the acoustic response and water saturation as functions of the injection rate are consistent with previous observations on sandstone. The results confirm that the injection rate has significant influence on fluid distribution and the corresponding acoustic response. The complexity of the acoustic response—-that is not monotonic with changes in saturation, and which at the same saturation varies between hydrostatic conditions and states of dynamic fluid flow – may have implications for the interpretation of time-lapse seismic responses.

Three-dimensional structure of experimentally produced clay smears: Implications for fault seal analysis

The geometries of clay smears produced in a series of direct shear experiments on composite blocks containing a clay-rich seal layer sandwiched between sandstone reservoir layers have been analyzed in detail. The geometries of the evolving shear zones and volume clay distributions are related back to the monitored hydraulic response, the deformation conditions, and the clay content and strength of the seal rock. The laboratory experiments were conducted under 4 to 24 MPa (580–3481 psi) fault normal effective stress, equivalent to burial depths spanning from less than approximately 0.8 to 4.2 km (0.5 to 2.6 mi) in a sedimentary basin. The sheared blocks were imaged using medical-type x-ray computed tomography (CT) imaging validated with optical photography of sawn blocks. The interpretation of CT scans was used to construct digital geomodels of clay smears and surrounding volumes from which quantitative information was obtained. The distribution patterns and thickness variations of the clay smears were found to vary considerably according to the level of stress applied during shear and to the brittleness of the seal layer. The stiffest seal layers with the lowest clay percentage formed the most segmented clay smears. Segmentation does not necessarily indicate that the fault seal was breached because wear products may maintain the seal between the individual smear segments as they form. In experiments with the seal layer formed of softer clays, a more uniform smear thickness is observed, but the average thickness of the clay smear tends to be lower than in stiffer clays. Fault drag and tapering of the seal layer are limited to a region close to the fault cutoffs. Therefore, the comparative decrease of sealing potential away from the cutoff zones differs from predictions of clay smear potential type models. Instead of showing a power-law decrease away from the cutoffs toward the midpoint of the shear zone, the clay smear thickness is either uniform, segmented, or undulating, reflecting the accumulated effects of kinematic processes other than drag. Increased normal stress improved fault sealing in the experiments mainly by increasing fault zone thickness, which led to more clay involvement in the fault zone per unit of source layer thickness. The average clay fraction of the fault zone conforms to the prediction of the shale gouge ratio (SGR) model because clay volume is essentially preserved during the deformation process. However, the hydraulic seal performance does not correlate to the clay fraction or SGR but does increase as the net clay volume in the fault zone increases. We introduce a scaled form of SGR called SSGR to account for increased clay involvement in the fault zone caused by higher stress and variable obliquity of the seal layer to the fault zone. The scaled SGR gives an improved correlation to seal performance in our samples compared to the other algorithms.

Fault transmissibility in clastic-argillaceous sequences controlled by clay smear evolution

The continuity of clay smears evolving in sealed direct shear experiments of initially intact sandstone-mudrock sequences was quantified to large displacements up to more than ten times the thickness of the sealing layer. The sample blocks consisted of a preconsolidated clay-rich seal layer, which was embedded and synthetically cemented in quartz sand. The mineralogy and mechanical properties of the clay layer and the reservoir sandstones were varied systematically to mimic a range of natural clastic rock sequences. The fluid-flow response across the fault zone was monitored continuously during deformation using a new type of direct shear cell. The displacement at which seals break down is closely linked to the amount of phyllosilicates in the seal layer. Contrary to expectations, softer seal layers do not seal better than stiff seal layers for a given clay content. In the testing range of normal effective stresses between 4 to 24 MPa (580–3481 psi) covering maximum burial depth conditions of approximately 800 m (2625 ft) to approximately 4 km (2 mi) (assuming normal fault tectonics), a systematic trend is also observed, indicating better smear continuity by increasing the effective normal stress. Predominantly brittle processes such as slicing and wear, and not ductile drag or plastic flow, appear to be responsible for the generation of clay smears. The test results offer the prospect of incorporating critical shale smear factors (i.e., normalized displacement at which seal breakdown occurs) into probabilistic fault seal algorithms that consider important properties that can be measured or estimated, namely, clay content and fault-normal effective stress.

Effect of supercritical CO2 on carbonates: Savonnières sample case study

ABSTRACT CO2 geosequestration is an efficient way to reduce greenhouse gas emissions into the atmosphere. Carbonate rock formations are one of the possible targets for CO2 sequestration due to their relative abundance and ability to serve as a natural trapping reservoir. The injected supercritical CO2 can change properties of the reservoir rocks such as porosity, permeability, tortuosity, and specific surface area due to dissolution and precipitation processes. This, in turn, affects the reservoir characteristics, i.e., their elastic properties, storage capacity, stability, etc. The tremendous progresses made recently in both microcomputed X‐ray tomography and high‐performance computing make numerical simulation of physical processes on actual rock microstructures feasible. However, carbonate rocks with their extremely complex microstructure and the presence of microporosity that is below the resolution of microcomputed X‐ray tomography scanners require novel, quite specific image processing and numerical simulation approaches. In the current work, we studied the effects of supercritical CO2 injection on microstructure and elastic properties of a Savonnières limestone. We used microtomographic images of two Savonnières samples, i.e., one in its natural state and one after injection and residence of supercritical CO2. A statistical analysis of the microtomographic images showed that the injection of supercritical CO2 led to an increase in porosity and changes of the microstructure, i.e., increase of the average volume of individual pores and decrease in the total number of pores. The CO2 injection/residence also led to an increase in the mean radii of pore throats, an increase in the length of pore network segments, and made the orientation distribution of mesopores more isotropic. Numerical simulations showed that elastic moduli for the sample subjected to supercritical CO2 injection/residence are lower than those for the intact sample.

Fault reactivation in travertine and its impact on hydraulic transmissibility: laboratory experiments and mesoscale structures

Direct shear experiments were undertaken to investigate the effect of faulting and reactivation on the hydromechanical characteristics of faults in continental carbonate samples. The tested rock is a travertine of continental, microbial origin with a calcite content of 99 wt%, with a strongly laminated texture. Analyses of the intact and sheared samples performed using medical X-ray computed tomography (CT) revealed that the porosity is mainly composed of subplanar pores and vugs. Permeability is high along the laminations, controlled by interconnected pores and fractures. The travertine is a lithological analogue for Aptian pre-salt oil reservoir rocks found in South Atlantic offshore basins. Three samples, with dimensions of 240 × 110 × 150 mm, were sheared to a maximum displacement of 120 mm under different levels of effective vertical stress (6–45 MPa), resulting in the formation of cataclastic fault gouge surrounded by a dense fracture network. A new experimental method was used to reactivate the artificially formed fault by performing cyclic vertical loading at different shear displacements on a previously sheared sample, while keeping a constant pore-pressure differential throughout the test. Pore-fluid responses across the fault zone were monitored continuously during both deformation (dynamic transmissibility) and hold periods (static transmissibility). Results show that the transmissibility reduces in all the samples for all values of the applied effective vertical stress and during shear reversal. The static transmissibility also decreases over time, which may be a result of creep deformation or the blocking of pore channels with gouge material. Our results indicate that once the gouge material is developed in the core of a carbonate fault zone, the dynamic transmissibility across that fault is permanently decreased, with little dependence on subsequent kinematics of reactivation, or changes in stress, so long as the gouge zone is not breached by a new structure.

Characterization of Darai Limestone Composition and Porosity Using Data-Constrained Modeling and Comparison with Xenon K-Edge Subtraction Imaging.

Data-constrained modeling is a method that enables three-dimensional distribution of mineral phases and porosity in a sample to be modeled based on micro-computed tomography scans acquired at different X-ray energies. Here we describe an alternative method for measuring porosity, synchrotron K-edge subtraction using xenon gas as a contrast agent. Results from both methods applied to the same Darai limestone sample are compared. Reasonable agreement between the two methods and with other porosity measurements is obtained. The possibility of a combination of data-constrained modeling and K-edge subtraction methods for more accurate sample characterization is discussed.

Impact of Rock Heterogeneity on Interactions of Microbial-Enhanced Oil Recovery Processes

Residual oil saturation reduction and microbial plugging are two crucial factors in microbial-enhanced oil recovery (MEOR) processes. In our previous study, the residual saturation was defined as a nonlinear function of the trapping number, and an explicit relation between the residual oil saturation and the trapping number was incorporated into a fully coupled biological (B) and hydrological (H) finite element model. In this study, the BH model is extended to consider the impact of rock heterogeneity on microbial-enhanced oil recovery phenomena. Numerical simulations of core flooding experiments are performed to demonstrate the influences of different parameters controlling the onset of oil mobilization. X-ray CT core scans are used to construct numerical porosity-permeability distributions for input to the simulations. Results show clear fine-scale fingering processing, and that trapping phenomena have significant effects on residual oil saturation and oil recovery in heterogeneous porous media. Water contents and bacterial distributions for heterogeneous porous media are compared with those for homogenous porous media. The evolution of the trapping number distribution is directly simulated and visualized. It is shown that the oil recovery efficiency of EOR/MEOR will be lower in heterogeneous media than in homogeneous media, largely due to the difficulty in supplying surfactant to unswept low-permeability zones. However, MEOR also provides efficient plugging along high-permeability zones which acts to increase sweep efficiency in heterogeneous media. Thus, MEOR may potentially be more suited for highly heterogeneous media than conventional EOR.

The Role of Specific Surface Area and Cation Exchange Capacity in Determining Shale Rock Properties

Surface area, surface charge and the exchangeability of cations have long been known as factors in determining the physical and mechanical properties of shales. Such properties are related to the fine grain size of the various clay minerals and also the particle shapes, edge-face microstructural arrangements and are impacted by the likes of salinity and depositional environment. In general, the siltier a given clay or shale, the lower their cation exchange capacity (CEC) and specific surface area (SSA). CEC and SSA have direct impacts on mechanical and flow properties. Wellbore instability can be caused by the development of osmotic pressures resulting from differences between pore fluid composition and drilling fluid composition and the resultant time dependent effects based on permeability. Such instability can be mitigated by the use of oil-based muds or for example high KCl water-based muds. Electrical and dielectric properties are also dependent on mineral surface charge and related directly to the CEC of a given shale, especially properties such as resistivity (or conductivity) and permittivity (dielectric constant). This paper will detail SSA and CEC values of shales from multiple basins worldwide and also from individual boreholes to demonstrate their role for shale rock properties.

The Ultrasonic Response of North Sea Shale to Undrained Loading

A kaolinite-rich shale core from the North Sea was characterized in terms of mineralogical composition and physical properties. Subsampled plugs were then subjected to undrained multi-stage triaxial tests close to failure in order to determine the shear failure envelope. During undrained loading, ultrasonic waveforms were recorded parallel to, perpendicular to and at off-axis angles to bedding, such that the evolution of the full elastic tensor could be monitored with increasing stress anisotropy. Results indicate the importance of fabric elements and their orientation with respect to the prevailing stress field.