Valeriya Shulakova

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).

3D diffraction imaging of linear features and its application to seismic monitoring

Many subsurface features, such as faults, fractures, cracks, or fluid content terminations are defined by geological discontinuities. The seismic response from such features is encoded in diffractions. We develop an algorithm for imaging such discontinuities by detecting edge diffractions. The algorithm exploits phase-reversal phenomena of edge diffractions and uses them as a criterion to separate these diffractions from specular reflections and diffractions produced by a leaner object. The performance of the method is demonstrated on both synthetic and real 3D seismic data. The output image focuses the diffracted energy back to its origin and shows high semblance values at the edge of the object. The method is applied on conventionally stacked data producing an image that contains only diffraction events called the D-volume. We also reveal the potential of diffractions to image and track the changes of a CO2 plume using time-lapse analysis and detect any possible CO2 seepage from its primary containment.

Burying receivers for improved time‐lapse seismic repeatability: CO2CRC Otway field experiment

4D seismic is widely used to remotely monitor fluid movement in subsurface reservoirs. This technique is especially effective offshore where high survey repeatability can be achieved. It comes as no surprise that the first 4D seismic that successfully monitored the CO2 sequestration process was recorded offshore in the Sleipner field, North Sea. In the case of land projects, poor repeatability of the land seismic data due to low S/N ratio often obscures the time-lapse seismic signal. Hence for a successful on shore monitoring program improving seismic repeatability is essential. Stage 2 of the CO2CRC Otway project involves an injection of a small amount(around 15,000 tonnes) of CO2/CH4 gas mixture into a saline aquifer at a depth of approximately 1.5 km. Previous studies at this site showed that seismic repeatability is relatively low due to variations in weather conditions, near surface geology and farming activities. In order to improve time-lapse seismic monitoring capabilities, a permanent receiver array can be utilised to improve signal to noise ratio and hence repeatability. A small-scale trial of such an array was conducted at the Otway site in June 2012. A set of 25 geophones was installed in 3mdeep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1–12m depth. In order to assess the gain in the signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms. With such an amplified background noise level, we found that the noise level for buried geophones is on average 20 dB lower compared to the surface geophones. The levels of repeatability for borehole geophones estimated around direct wave, reflected wave and ground roll are twice as high as for the surface geophones. Both borehole and surface geophones produce the best repeatability in the 30–90 Hz.

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.

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.

Subsurface Imaging Using Buried DAS and Geophone Arrays - Preliminary Results from CO2CRC Otway Project

A permanent geophone array along with a fibre optic distributed acoustic sensing (DAS) array were deployed at the CO2CRC Otway Project site in order to conduct seismic monitoring of a CO2 plume during a small-scale injection test. This study aims to assess the ability for a permanent geophone array to overcome issues related to different acquisition (receiver) designs, high ambient noise level and seasonal variations in the near surface, as well as to test the DAS system for performing cost-effective time lapse seismic measurements. The acquisition of 3D seismic data is performed for this purpose using ~3000 vibroseis source points. We show the preliminary results of seismic reflection imaging conducted using DAS data. We observe the differences in performance between a standard commercially available tactical fibre optic cable and a custom helically wound cable. The results of this study and the workflows established will be used for processing a complete 3D seismic dataset acquired with a DAS array before being compared to a conventional geophone array.

Improving Time-lapse Seismic Repeatability CO2CRC Otway Site Permanent Geophone Array Field Trials

The next stage of CO2CRC Otway project involves injection of a small amount (around 15,000 tonnes) of CO2/CH4 gas mixture into saline aquifer (Paaratte formation) at a depth of ~1.5 km. The seismic time-lapse signal will depend largely on the formation properties and the injection scenario, but is likely to be relatively weak. In order to improve time-lapse seismic monitoring capabilities by decreasing the noise level, a buried receiver arrays can be used. A small-scale trial of such array was conducted at Otway site in June 2012. A set of 25 geophones was installed in 3 m deep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1 to 12 m depth. In order to assess the gain in signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms increasing the noise level. We found that noise level for buried geophones is on average 20 dB lower compared to the surface ones. Furthermore, the combination of active and passive experiments has allowed us to perform a detailed classification of various noise sources.

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.

Time-lapse seismic data inversion for CO2 sequestration CO2CRC Otway Project

The second stage of the CO2CRC Otway Project aims to explore the capabilities of various technologies - including time-lapse seismic - for the monitoring of small amounts of supercritical CO2/CH4 mixture. To this end, 15 kt of CO2/CH4 mixture was injected at the depth of 1.5 km into the saline sandstone reservoir in 2015-2016. A part of the monitoring program completed to date consists of a baseline survey followed by three monitor surveys after the injection of 5 kt, 10 kt and 15kt. Time-lapse seismic paired with simultaneous 4D inversion is a proven tool in reservoir monitoring because of its effectiveness in tracking the movement of CO2 and providing quantitative estimation of the changes in reservoir acoustic impedances. Here we demonstrate our workflow and the results of post stack model-based 4D seismic data inversion. The workflow was first tested and calibrated on the 4D synthetic time-lapse data. The calibrated workflow was then applied to the field seismic data.

Feasibility of Cross-well Seismic as CO2 Monitoring Tool

The next stage of CO2CRC Otway project involves exploration of the ability of various CO2 geosequestration techniques, including cross-hole seismic, to detect and monitor presence of CO2. Despite the limited spatial coverage of a cross-well survey, the acquired data could be used to improve reliability of the whole monitoring and verification program. Prior to any field experiment we evaluate the feasibility of cross-well seismic using computer modelling. We utilize finite-difference time-domain (FDTD) method for pre- and post- injection stages. Here we present the results of our study and validate the detectability of CO2/CH4 gas mixture on time-lapse cross-well seismic data on the direct as well as the reflected wave fields. We demonstrate that the presence of 15,000 t of a gas plume can lead to changes in transit times of up to 1.4 ms (in cross-well setting). The computed seismic tomography detects the difference in velocities up to 80 m/s. The difference caused by gas is also detectable in the migrated time section of reflected waves.