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 low-permeable 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 load-bearing 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 (micro-CT) 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

Evaluation of opening of fractures in the Logovskoye carbonate reservoir, Perm Krai, Russia

The development of naturally fractured carbonate reservoirs is extremely challenging. Such reservoirs have a dual pore structure consisting of low-permeable matrix with large pore volume and high-permeable fractures constituting main paths for fluid flow. Productivity of wells drilled in such formations tends to decrease rapidly due to the drop in the reservoir pressure and closure of fractures. Therefore, it is crucial to monitor opening of fractures for the effective development of carbonate reservoirs. Three methods for monitoring of opening of fractures including tracer indicators method, Warren and Root method and Victorin’s empirical relation, are applied in the Logovskoye oil reservoir, a carbonate Tournaisian-Famennian formation in Upper Kama Region, Perm Krai, Russia. The three methods provide reliable estimation of the opening of fractures, which match the reported laboratory data obtained on thin sections of core samples. The limitations of each method are also discussed. The tracer indicator method is time-consuming, the Warren and Root method includes hydrodynamic studies and requires shutdown of wells influencing the oil production, and the application of Victorin’s relation requires estimation of initial opening and current compressibility of fractures, which can be done using analysis of cores or tracer indicators studies. The appropriate method for monitoring of opening of fractures should be chosen according to available resources, time, and economic targets of the development project

Ultrasonic velocity measurements on thin rock samples: Experiment and numerical modeling

The ultrasonic pulse transmission (UPT) method has been the gold standard for laboratory measurements of rock elastic properties for decades, and it is used by oil and gas industry and service companies routinely. In spite of the wide acceptance and use of the UPT method, experimentalists are still looking for ways to further extend the limits of its applicability and to improve its state-of-the-art practices. One of the problems that limits wider application of the method is the length of the standard samples used (approximately 40-100 mm). This is a crucial limitation either in the case of a damaged core when preparation of a standard size sample is impossible or in the case when an ultrasonic experiment is combined with saturation or desiccation processes that might be extremely time-consuming on the long samples. On the other hand, thinner samples are not typically used due to the implication of inhomogeneity of stress fields inside and whereas few results of the measurements on thin disc samples have been reported in the literature, detailed justifications of the procedures have not been done yet. To fill this gap, we compare ultrasonic velocities measured at confining stresses up to 50 MPa done on standard and thin samples with lengths of 60 and 15 mm, respectively. First, we evaluate a new developed experimental setup for ultrasonic measurements on thin discs and develop a detailed experimental procedure. Then, we use finite-element modeling to numerically simulate stress fields in both types of samples. Finally, we compare the ultrasonic velocities measured on the thin discs and on standard samples and determine how to obtain reliable elastic properties on thin samples

Ultrasonic measurements on thin samples: Experiment and numerical modeling

Ultrasonic velocity measurement method is a common practice for measuring elastic properties of rocks in laboratories. Standard cylindrical plugs of 40-100 mm length and 20-38 mm in diameter are usually used for such measurements. However, the maximal sizes of samples are sometimes restricted, especially when ultrasonic measurements are combined with desaturation/rehydration or dielectric analysis experiments. Such experiments are performed on relatively thin discs (~15 mm in length). However, the reliability of the results obtained on thin discs is unclear, as no direct comparison with results obtained on standard samples has been reported yet. Here we present results of laboratory ultrasonic measurements for a suite of thin and standard samples conducted under high confining pressure. Compressional and shear waves velocities obtained on thin and standard samples match each other within the experimental errors. We also present results of numerical simulations to support the outcome of the experimental work and to improve the understanding of wave propagation in the samples during laboratory ultrasonic measurements. The finite element method is used to simulate wave propagation along the experimental set-up caused by transmitted ultrasonic pulse. The results of the numerical modeling prove that transducers working in share mode also produce a compressional wave that propagates along the sample and can be recorded by a receiver. Simulated travel times of elastic waves are in a good agreement with experimentally obtained results

Fracturing of organic-rich shale during heating

Organic-rich shales, traditionally considered as source rocks, have recently become an ambitious goal for oil and gas industry as important unconventional reservoirs. Understanding of initiation and development of fractures in organic-rich shales is crucially important as they drastically increase permeability of these low permeable shales. 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 study we integrate laboratory experiment and numerical modeling to study fracture development in organic-rich shale. At the first step, we heat a cylindrical sample up to the temperature of 330 degrees Celsius. At the second step, we obtain high resolution microtomographic images of the sample. Large kerogen-filled pores and cracks initiated by the heating can be identified from these images. We repeat these steps for several temperatures in the range 330-430 degrees. The microtomographic images are processed using AVIZO (Visualization Sciences Group) to estimate the dependency between the total area of fractures and the temperature experienced by the sample. Total organic carbon content is tested in the samples experienced the same temperatures. This approach enables a quantitative analysis of fracture initiation and development in organic-rich shales during heating

Seismic monitoring of a small CO2 injection using a multi-well DAS array: Operations and initial results of Stage 3 of the CO2CRC Otway project

Active time-lapse seismic is widely employed for monitoring CO2 geosequestration due to its ability to track the distribution of fluids in space and time. However, standard 4D seismic monitoring suffers from several challenges, including high cost, disruption to other land uses, and, consequently, relatively large intervals between monitor surveys. Some of these challenges can be mitigated using permanently installed sources and receivers. Such an approach was tested at the CO2CRC Otway site by continuous offset VSP monitoring of 15,000 t of supercritical CO2 injected into an aquifer 1,500 m deep with nine permanent seismic sources (surface orbital vibrators or SOVs) and five downhole fibre-optic receivers. This continuous monitoring is complemented by multi-well 4D VSP using a mobile vibroseis source and the same DAS receivers, which included one baseline and two monitor surveys after injection of 4,000 and 12,000 t of CO2. The continuous DAS-SOV monitoring detected an abrupt increase of travel times below the injection interval on the second day of injection (after injection of 300 t of CO2) and tracked the growth of the areal CO2 plume by mapping changes of reflection amplitudes. The plume is also detected by time-lapse changes of reflection amplitudes in multi-well 4D VSPs. The plume images obtained from continuous offset VSP and 4D VSP are broadly consistent with each other but with some differences due to differences in illumination, lateral variations of velocities and seismic anisotropy. These differences also serve as a measure of uncertainty of 4D VSP images

An automated system for continuous monitoring of CO2 geosequestration using multi-well offset VSP with permanent seismic sources and receivers: Stage 3 of the CO2CRC Otway Project

A permanent automated continuous seismic CO2 geosequestration monitoring system for was installed at CO2CRC Otway Project site (Victoria, Australia) in early 2020. The system is composed of five deviated ∼1600 m deep wells equipped with distributed acoustic sensing (DAS) acting as seismic receivers and nine seismic orbital vibrators (SOV) as seismic sources. DAS recording is performed continuously by three iDASv3 units. Each SOV operates for 2.5 h at a time, and hence all SOVs operating sequentially (during daytime only) produce in a single vintage every two days. Each vintage consists of 45 offset VSP transects covering predicted CO2 plume migration paths over ∼0.7 km2 area. An automated data processing implemented on-site reduces data size from ∼1.3 TB/day to ∼500 MB/day with the results transmitted to the office daily. The repeatability analysis based on pre-injection data (acquired from May to October 2020 before the injection start in December 2020) shows that variability of SOV performance is the main source of non-repeatability while borehole measurements are stable. An SOV waveform could reach NRMS value from 20 to 100 % within a few days. However, deconvolution of the seismograms with the waveform of the direct wave reduces the repeatability to within 10–15 % NRMS

Evidence of Nonlinear Seismic Effects in the Earth from Downhole Distributed Acoustic Sensors

Seismic velocities and elastic moduli of rocks are known to vary significantly with applied stress, which indicates that these materials exhibit nonlinear elasticity. Monochromatic waves in nonlinear elastic media are known to generate higher harmonics and combinational frequencies. Such effects have the potential to be used for broadening the frequency band of seismic sources, characterization of the subsurface, and safety monitoring of civil engineering infrastructure. However, knowledge on nonlinear seismic effects is still scarce, which impedes the development of their practical applications. To explore the potential of nonlinear seismology, we performed three experiments: two in the field and one in the laboratory. The first field experiment used two vibroseis sources generating signals with two different monochromatic frequencies. The second field experiment used a surface orbital vibrator with two eccentric motors working at different frequencies. In both experiments, the generated wavefield was recorded in a borehole using a fiber-optic distributed acoustic sensing cable. Both experiments showed combinational frequencies, harmonics, and other intermodulation products of the fundamental frequencies both on the surface and at depth. Laboratory experiments replicated the setup of the field test with vibroseis sources and showed similar nonlinear combinations of fundamental frequencies. Amplitudes of the nonlinear signals observed in the laboratory showed variation with the saturating fluid. These results confirm that nonlinear components of the wavefield propagate as body waves, are likely to generate within rock formations, and can be potentially used for reservoir fluid characterization