ALBERT

Description: Multiple potential shale-gas plays have been identified in Australia. The total estimates of recoverable dry and wet shale gas exceed 157 tcf. Technological advances and more informed estimations of total organic carbon (TOC) abundance and distribution might further increase the assessments of recoverable shale gas. TOC content is known only at the depths where laboratory measurements on recovered core samples are performed. However, reliable estimation of potential resources can be based only on information about vertical and lateral distribution in organic matter throughout the prospective gas-shale reservoir. This information commonly is obtained from conventional wireline logs such as gamma ray, density, transit time, and resistivity. Methods routinely used for these estimations were developed for organic-rich shales from Northern American basins, where organic matter of marine Type I and Type II typically is observed. In prospective Australian basins, organic matter ranges from marine Type I to terrestrial Type III. This fact, along with different depositional history of Australian reservoirs, might lead to quite different logging-tool responses. Thus, the relations between the amount of TOC and estimation from logs might be quite different.

Description: Prediction of sonic velocities in shales from well logs is important for seismic to log ties if the sonic log is absent for a shaly section, for pore pressure anomaly detection, and for data quality control. An anisotropic differential effective medium (DEM) was used to simulate elastic properties of shales from elastic properties and volume fractions of silt and wet clay (a hypothetical composite material that includes all clay minerals and water). Anisotropic elastic coefficients of the wet clay were assumed as a first-order approximation to be linearly dependent on wet clay porosity (WCP). Here, by WCP we mean a ratio of a pore volume occupied by water to a total volume of the wet clay. Effects of silt inclusions on elastic coefficients of shales were taken into account by using the anisotropic differential effective medium model. Silt inclusions were modeled as spherical quartz particles. Simulated elastic coefficients of shales were used to calculate compressional and shear velocities, and these were in a good agreement with the sonic velocities observed on a test data set from an offshore Mid-Norway well penetrating a 500-m vertical section of shale. To further study the elastic properties of wet clays, elastic coefficients calculated from compressional and sonic velocities measured in shales were inverted for vertical profiles of wet clay elastic coefficients. Analysis of these coefficients found that in the well considered, the increase in elastic coefficients of shales was controlled by the increase of silt fraction with depth. Elastic coefficients of wet clay found no increase with depth. The inverted elastic moduli of wet clay found much stronger correlation with WCP than do the moduli of shale. This confirmed the hypothesis that silt fraction is one of the key parameters for the modeling of elastic properties of shale.

Description: Knowledge about the pressure dependency of elastic and electrical properties is important for a variety of geophysical applications. We present a technique to invert for the stiff and compliant porosity from velocity measurements made as a function of differential pressure on saturated sandstones. A dual porosity concept is used for dry rock compressibility and a squirt model is employed for the pressure and frequency dependent elastic properties of the rocks when saturated. The total porosity obtained from inversion shows satisfactory agreement with experimental results. The electrical cementation factor was determined using the inverted porosity in combination with measured electrical conductivity. It was found that cementation factor increased exponentially with increasing differential pressure during isostatic loading. Elastic compressibility, electrical cementation factor and electrical conductivity of the saturated rocks correlate linearly with compliant porosity, and electrical cementation factor and electrical conductivity exhibit linear correlations with elastic compressibility of the saturated rocks under loading. The results show that the dual porosity concept is sufficient to explain the pressure dependency of elastic, electrical and joint elastic-electrical properties of saturated porous sandstones.

Description: Knowledge about the pressure dependency of elastic and electrical properties is important for a variety of geophysical applications. We present a technique to invert for the stiff and compliant porosity from velocity measurements made as a function of differential pressure on saturated sandstones. A dual porosity concept is used for dry rock compressibility and a squirt model is employed for the pressure and frequency dependent elastic properties of the rocks when saturated. The total porosity obtained from inversion shows satisfactory agreement with experimental results. The electrical cementation factor was determined using the inverted porosity in combination with measured electrical conductivity. It was found that cementation factor increased exponentially with increasing differential pressure during isostatic loading. Elastic compressibility, electrical cementation factor and electrical conductivity of the saturated rocks correlate linearly with compliant porosity, and electrical cementation factor and electrical conductivity exhibit linear correlations with elastic compressibility of the saturated rocks under loading. The results show that the dual porosity concept is sufficient to explain the pressure dependency of elastic, electrical and joint elastic-electrical properties of saturated porous sandstones.

Description: Seismic waves propagating in porous rocks saturated with two immiscible fluids can be strongly attenuated. Predicting saturation effects on seismic responses requires a sound understanding of attenuation and velocity dependencies on the fluid distribution. Decoding these effects involves interpreting laboratory experiments, analyzing well-log data, and performing numerical simulations. Despite striking differences among scales, flow regimes, and frequency bands, some aspects of wave attenuation can be explained with a single mechanism — wave-induced pressure diffusion. Different facets of wave-induced pressure diffusion can be revealed across scales.

Description: 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.

Description: 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.

Description: 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 x 110 x 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.