Joel Sarout

Geomechanical and ultrasonic characterization of a Norwegian Sea shale

Anisotropy of velocity in shaly overburden is known to cause significant problems for geophysical interpretation, including depth conversion and fluid identification. In addition, mechanical and dynamic elastic shale behavior is not well understood because few tests have been performed on well-preserved samples. Multiple stage triaxial tests were performed upon horizontal core plugs of a shale from the Norwegian Sea with a view to evaluating rock strength and the evolution of ultrasonic response during rock deformation. In addition, standard rock physical properties were characterized as well as composition. The shale microfabric is seen to be strongly laminated, with alternating thick clay-rich laminae and thin silt-rich laminae. Occasional microfractures are also noted parallel to these laminations. The shale has low friction coefficient and cohesive strength, and shows anisotropy of these parameters when the maximum principal stress is oriented parallel to and at 45 � to the microfabric. The orientation of the maximum principal stress parallel to the intrinsic fabric and microcracks was seen to significantly impact on velocity normal to the fabric as stress parallel to the fabric increased. S-wave anisotropy was significantly affected by the increasing stress anisotropy. Stress orientation with respect to fabric orientation was therefore found to be an important control on the degree of anisotropy of dynamic elastic properties in this shale.

Mechanical instability induced by water weakening in laboratory fluid injection tests

To assess water-weakening effects in reservoir rocks, previous experimental studies have focused on changes in the failure envelopes derived from mechanical tests conducted on rocks fully saturated either with water or with inert fluids. So far, little attention has been paid to the mechanical behavior during fluid injection under conditions similar to enhanced oil recovery operations. We studied the effect of fluid injection on the mechanical behavior of the weakly consolidated Sherwood sandstone in laboratory experiments. Our specimens were instrumented with 16 ultrasonic P wave transducers for both passive and active acoustic monitoring during loading and fluid injection to record the acoustic signature of fluid migration in the pore space and the development of damage. Calibration triaxial tests were conducted on three samples saturated with air, water, or oil. In a second series of experiments, water and inert oil were injected into samples critically loaded up to 80% or 70% of the dry or oil-saturated compressive strength, respectively, to assess the impact of fluid migration on mechanical strength and elastic properties. The fluids were injected with a low back pressure to minimize effective stress variations during injection. Our observations show that creep takes place with a much higher strain rate for water injection compared to oil injection. The most remarkable difference is that water injection in both dry and oil-saturated samples triggers mechanical instability (macroscopic failure) within half an hour whereas oil injection does not after several hours. The analysis of X-ray computed tomography images of postmortem samples revealed that the mechanical instability was probably linked to loss of cohesion in the water-invaded region.

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.

Ultrasonic monitoring of spontaneous imbibition experiments: Precursory moisture diffusion effects ahead of water front

Fluid substitution processes have been investigated in the laboratory on 14 carbonate and siliciclastic reservoir rock analogues through spontaneous imbibition experiments on vertical cylindrical specimens with simultaneous ultrasonic monitoring and imaging. The motivation of our study was to identify the seismic attributes of fluid substitution in reservoir rocks, and to link them to physical processes. It is shown that (i) the P-wave velocity either decreases or increases when the capillary front reaches the Fresnel clearance zone, (ii) the P-wave amplitude is systematically impacted earlier than the velocity is, (iii) this precursory amplitude decrease occurs when the imbibition front is located outside of the Fresnel zone, (iv) the relative variation of the P-wave amplitude is always much larger than that of the P-wave velocity. These results suggest that moisture diffuses into the pore space ahead of the water front. This postulate is further supported by a quantitative analysis of the time evolution of the observed P-wave amplitudes. In a sense, P-wave amplitude acts as a precursor of the arrival of the capillary front. This phenomenon is used to estimate the effective diffusivity of moisture in the tested rocks. The effective moisture diffusivity estimated from the ultrasonic data is strongly correlated with permeability: a power-law with exponent 0.96 predicts permeability from ultrasonic monitoring within a factor 3 without noticeable bias. When the effective diffusivity is high, moisture diffusion affects ultrasonic P-wave attributes even before the imbibition starts and impacts the P-wave reflectivity as evidenced by the variations recorded in the waveform coda.

Comment on “Physical constraints on c13 and δ for transversely isotropic hydrocarbon source rocks” by F. Yan, D.‐H. Han and Q. Yao, Geophysical Prospecting 57, 393–411

Comment on “Physical constraints on c13 and δ for transversely isotropic hydrocarbon source rocks” by F. Yan, D.-H. Han and Q. Yao, Geophysical Prospecting 57, 393–411

Experimental Evidence of Calcite Dissolution and Induced Precipitation during supercritical CO2 Residence

Prior to injecting CO2 in water-saturated carbonate reservoirs, one needs to investigate the effect of the residence of supercritical CO2 (SCCO2) on the rock integrity and overall physical properties. In this study, a Savonnieres limestone is characterised in terms of its physical properties, pore chemistry and textural features prior and after 2 or 4 hours SCCO2 residence under in situ stress/temperature conditions. More precisely, elastic waves (Vp and Vs) at ultrasonic frequencies, electrical resistivity (Rt), helium porosity-permeability and pore chemistry are measured before and after SCCO2 aging. In addition, X-ray CT monitoring is carried out during the different steps. While water chemistry highlights an enhanced calcite dissolution related to the duration of SCCO2 residence, a change in the physical properties is observed between the two residence steps. It is shown from the physical properties that (i) the rock building minerals were dissolved after 2 hours; and (ii) the rock overall integrity increases after 4 hours, highlighting a possible re-precipitation phenomenon.

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.

Pressure-dependent elastic and transport properties of porous and permeable rocks: Microstructural control

Although several studies aimed at linking electrical and hydraulic transport properties in rocks, the existing models remain at most incomplete. Based on this observation, in addition to the transport properties, this contribution investigates the pressure dependence of P wave velocities and porosity for three porous rocks. Apart from hydraulic conductivity, all physical properties show an important dependence to the confining pressure. In particular, electrical resistivity reaches an asymptote at low confining pressures. Using the measured P wave velocities and effective medium theories, the microcrack density (ρ) and its evolution with confining pressure are estimated. For the three rocks, the microcrack density at which electrical resistivity reaches a plateau is of ρ ~0.13. This value corresponds to the threshold for crack percolation in media containing microcracks exclusively. This suggests that in porous and microcracked rocks, electrical resistivity is controlled by two independent hydraulic pathways of tubes and cracks acting in parallel. In contrast, this percolation threshold is not observed in the permeability data. A simple conceptual model is finally introduced that explains the fundamental differences between transport properties.

Estimation of Anisotropy Parameters Using the P-wave Velocities On a Cylindrical Shale Sample

Summary In this paper we present a new approach to the estimation of the Thomsen anisotropy parameters and symmetry axis coordinates from the P-wave traveltime measurements on cylindrical shale samples. Using the tomography-style array of transducers, we measure the ultrasonic P-wave ray velocities to estimate the Thomsen anisotropy parameters for a transversely isotropic shale sample. This approach can be used for core samples cut in any direction with regard to the bedding plane, since we make no assumption about the symmetry axis directions and will estimate it simultaneously with the anisotropy parameters. We use the very fast simulated re-annealing to search for the best possible estimate of the model parameters. The methodology was applied to a synthetic model and an anisotropic shale sample.

Linking Thermal and Elastic Properties in Sandstones Reservoir Rocks

Summary Although not measurable at the field scale, thermal properties of reservoir rocks at depth are important for many applications. Motivated by the fact that field-scale elastic properties may be obtained from measurements at the surface, a new physics-based theoretical model has recently been developed to link thermal and elastic properties through their common dependences in rocks. The model aims to ultimately predict thermal properties from elastic ones. But verifying this model with existing datasets obtained from different rock samples measured under different experimental conditions proves to be challenging. New joint measurements of thermal and elastic properties acquired on reference sandstone samples at identical experimental conditions allow an improved assessment.