Ben Clennell

Direct laboratory observation of patchy saturation and its effects on ultrasonic velocities

Maximizing the recovery of known hydrocarbon reserves is one of the biggest challenges facing the petroleum industry today. Optimal production strategies require accurate monitoring of production-induced changes of reservoir saturation and pressure over the life of the field. Time-lapse seismic technology is increasingly used to map these changes in space and time. However, until now, interpretation of time-lapse seismic data has been mostly qualitative. In order to allow accurate estimation of the saturation, it is necessary to know the quantitative relationship between fluid saturation and seismic characteristics (elastic moduli, velocity dispersion, and attenuation). The problem of calculating acoustic properties of rocks saturated with a mixture of two fluids has attracted considerable interest (Gist, 1994; Mavko and Nolen-Hoeksema, 1994; Knight et al., 1998. For a comprehensive review of theoretical and experimental studies of the patchy saturation problem see Toms et al., 2006).

Research of CO 2 and N 2 Adsorption Behavior in K-Illite Slit Pores by GCMC Method

Understanding the adsorption mechanisms of CO2 and N2 in illite, one of the main components of clay in shale, is important to improve the precision of the shale gas exploration and development. We investigated the adsorption mechanisms of CO2 and N2 in K-illite with varying pore sizes at the temperature of 333, 363 and 393 K over a broad range of pressures up to 30 MPa using the grand canonical Monte Carlo (GCMC) simulation method. The simulation system is proved to be reasonable and suitable through the discussion of the impact of cation dynamics and pore wall thickness. The simulation results of the excess adsorption amount, expressed per unit surface area of illite, is in general consistency with published experimental results. It is found that the sorption potential overlaps in micropores, leading to a decreasing excess adsorption amount with the increase of pore size at low pressure, and a reverse trend at high pressure. The excess adsorption amount increases with increasing pressure to a maximum and then decreases with further increase in the pressure, and the decreasing amount is found to increase with the increasing pore size. For pores with size greater larger than 2 nm, the overlap effect disappears.

Frictional and Hydraulic Behaviour of Carbonate Fault Gouge During Fault Reactivation - An Experimental Study

We presents results from an experimental program designed to shed light on the effect of stress and deformation history on the permeability and slip behaviour of faults in carbonate rocks. We investigate the mechanical and hydraulic behaviour of experimentally created fault cores and damage zones in natural travertine rock samples and also explore the role of a sealing layer on the frictional and hydraulic response of the rock. Following direct shear testing on the blocks, cylindrical plugs with diameter of 38mm were drilled across the slip surface to be tested in a conventional triaxial configuration monitoring the permeability and frictional behaviour of the samples. The results indicate that the fault cross cutting the sample is acting as seal and its permeability is negatively affected by an increase in mean effective stress; slip on the fault plane does not improve the permeability of the fault. It can be therefore concluded that leakage along an un-cemented carbonate gouge cannot be achieved by movement on the fault plane alone, at least not within the range of slip measureable with our apparatus (; other mechanisms (e.g. cementation of the gouge) need to be explored to assess the possible leaking scenarios in faulted carbonate rocks.

Sedimentary cyclicity from X-ray CT images in Campos Basin, Offshore Brazil

Small-scale changes in rock properties, such as those resulting from centimeter-scale depositional layering, are usually undetectable in standard borehole logs (Murphy et al., 1984). Even high-resolution logs with a small sampling interval (e.g., 2 inches) may still have a relatively large investigation volume. This presents a problem when we wish to capture the full variation in physical properties for purposes such as rock physics modeling.

Microstructural characterisation of organic-rich shale before and after pyrolysis

Organic-rich shales, traditionally considered as source rocks, have recently become an ambitious goal for the oil and gas industry as important unconventional reservoirs. Understanding of the initiation and development of fractures in organic-rich shales is crucially important as fractures could drastically increase the permeability of these otherwise lowpermeable rocks. Fracturing can be induced by rapid decomposition of organic matter caused by either natural heating, such as emplacement of magmatic bodies into sedimentary basins, or thermal methods used for enhanced oil recovery. In this work the authors study fracture initiation and development caused by dry pyrolysis of Kimmeridge shale, which is characterised with a high total organic carbon content of more than 20%. X-ray diffraction (XRD) analysis exhibits high carbonate (both calcite and dolomite) and low clay (illite) content. Field emission gun scanning electron microscopy (FEG-SEM) shows that kerogen is presented either as a loadbearing matrix or as a filling of the primary porosity with pores being of micron size. Cylindrical samples of the Kimmeridge shale are heated up to temperatures in the range of 330–430°C. High-resolution X-ray microtomographic (microCT) images are obtained. The microtomographic images are processed using AVIZO (Visualization Sciences Group) to identify and statistically characterise large kerogen-filled pores and pre-existing and initiated cracks. The relationship between the total area of fractures and the temperature experienced by the sample has been obtained. Total organic carbon content is determined for samples subjected to heating experiments. This approach enables a quantitative analysis of fracture initiation and development in organic-rich shales during heating.

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.

Seismic time-lapse monitoring of potential gas hydrate dissociation around boreholes - could it be feasible? A conceptual 2D study linking geomechanical and seismic FD models

Monitoring of the seafloor for gas hydrate dissociation around boreholes during hydrocarbon production is likely to involve seismic methods because of the strong sensitivity of P-wave velocity to gas in sediment pores. Here, based on geomechanical models, we apply commonly used rock physics modeling to predict the seismic response to gas hydrate dissociation with a focus on P-impedance and performed sensitivity tests. For a given initial gas hydrate saturation, the mode of gas hydrate distribution (cementation, frame-bearing, or pore-filling) has the strongest effect on P-impedance, followed by the mesoscopic distribution of gas bubbles (evenly distributed in pores or “patchy”), gas saturation, and pore pressure. Of these, the distribution of gas is likely to be most challenging to predict. Conceptual 2-D FD wave-propagation modeling shows that it could be possible to detect gas hydrate dissociation after a few days.

Wideband Electrical/dielectric Measurements From Millihertz to Gigahertz Frequencies

No single system is available to ground truth the full frequency range of geophysical and petrophysical electromagnetic measurements. We have combined a number of instruments and different measurement principles to obtain truly wide band (mHz to GHz) spectra of conductivity and permittivity of earth materials. We have designed several new cells and fixtures, and implemented a number of novel procedures for measurement and calibration. Several of our cells operate at overburden pressures (35 MPa). Our guarded parallel plate cells can be connected to more than one measurement system to cover the cross-over region adequately. The four electrode cells can be configured for time domain and electrokinetic measurements. To aid in calibration we have also implemented resonant methods giving highly accurate dielectric measurements at spot frequencies that compliment swept frequency data.

Impact of Saturation Change on Shale Properties

Over the last few years, interest in shales has sky-rocketed through their emergence as productive reservoirs in gas shale plays. Considerable interest in shale properties has also been generated through anomalous responses in shales, as opposed to in reservoirs, in highly expensive 4D seismic surveys. These issues have led to a surge in the amount of research being performed on shales and specifically, significant interest in shale properties, especially in the rock physics, petrophysics and geomechanics domains.

Computation of Elastic Properties Based on Microtomogram Images

Understanding of physical rock properties is currently of great importance both for industry and fundamental science, for it allows improving interpretation and reducing risks. Digital rock physics provides us with a promising opportunity for rock analysis and quantification. Moreover it allows running simulations on rock samples unsuitable for laboratory experiments. A detailed computational rock physics workflow including 3D rock imaging, processing and simulations of physical experiments has been created and tested on different rock samples. Here we describe this workflow and demonstrate the results of elastic simulation in comparison with experimental data obtained in physical laboratory for a sandstone sample.