Assessment of Fluid Transport Mechanisms in Shale Gas Reservoirs

The complex interplay between the physical and flow properties of shales was investigated. A methodology was developed to estimate free and bound porosity fractions from NMR-T2 experiments on shales, while a second order flow model was proposed to interpret gas permeability data. Slippage effects appeared to be influenced by characteristic pore lengths, while poroelastic behaviour was linked to compositional data. Potential associations emerged between FFI/BVI, pore sizes, fluid dynamic phenomena, and shale composition

Investigation of Changing Pore Topology and Porosity during Matrix Acidizing using Different Chelating Agents

Core flooding acidizing experiments on sandstone/carbonate formation are usually performed in the laboratory to observe different physical phenomena and to design acidizing stimulation jobs for the field. During the tests, some key parameters are analyzed such as pore volume required for breakthrough as well as pressure. Hydrochloric acid (HCl) is commonly used in the carbonate matrix acidizing while Mud acid (HF: HCl) is usually applied during the sandstone acidizing to remove damage around the well bore. However, many problems are associated with the application of these acids, such as fast reaction, corrosion and incompatibility of HCl with some minerals (illite). To overcome these problems, chelating agents (HEDTA, EDTA and GLDA) were used in this research. Colton tight sandstone and Guelph Dolomite core samples were used in this study. The experiments usually are defined in terms of porosity, permeability, dissolution and pore topology. Effluent samples were analyzed to determine dissolved iron, sodium, potassium, calcium and other positive ions using Inductively Coupled Plasma (ICP). Meanwhile Nuclear Magnetic Resonance (NMR) was employed to determine porosity and pore structure of the core sample. Core flood experiments on Berea sandstone cores and dolomite samples with dimensions of 1.5 in × 3 in were conducted at a flow rate of 1 cc/min under 150oF temperature. NMR and porosity analysis concluded that applied chemicals are effective in creating fresh pore spaces. ICP analysis concluded that HEDTA showed good ability to chelate calcium, sodium, magnesium, potassium and iron. It can be established from the analysis that HEDTA can increase more amount of permeability as compared to other chelates

Considerations for the acquisition and inversion of NMR T2 data in shales

© 2018 Elsevier B.V. Low-field Nuclear Magnetic Resonance (NMR) is a non-invasive method widely used in the petroleum industry for the evaluation of reservoirs. Pore structure and fluid properties can be evaluated from transverse relaxation (T2) distributions, obtained by inverting the raw NMR signal measured at subsurface conditions or in the laboratory. This paper aims to cast some light into the best practices for the T2 data acquisition and inversion in shales, with a focus on the suitability of different inversion methods. For this purpose, the sensitivity to various signal acquisition parameters was evaluated from T2 experiments using a real shale core plug. Then, four of the most common inversion methods were tested on synthetic T2 decays, simulating components often associated with shales, and their performance was evaluated. These inversion algorithms were finally applied to real T2 data from laboratory NMR measurements in brine-saturated shale samples. Methods using a unique regularization parameter were found to produce solutions with a good balance between the level of misfit and bias, but could not resolve adjacent fast T2 components. In contrast, methods applying variable regularization – based on the noise level of the data – returned T2 distributions with better accuracy at short times, in exchange of larger bias in the overall solution. When it comes to reproducing individual T2 components characteristic of shales, the Butler-Reeds-Dawson (BRD) algorithm was found to have the best performance. In addition, our findings suggest that threshold T2 cut-offs may be derived analytically, upon visual inspection of the T2 distributions obtained by two different NMR inversion methods

Pore characterization and clay bound water assessment in shale with a combination of NMR and low-pressure nitrogen gas adsorption

© 2018 Elsevier B.V. Pore size distribution (PSD) and the volume of clay bound water (CBW) are crucial parameters for gas shale reservoirs formation evaluation. Low-field nuclear magnetic resonance (LF-NMR) has been extensively applied to characterize petrophysical properties of reservoirs. However, limited understanding remains for unconventional shales. Defining NMR T 2 cutoff to differentiate CBW from free water is a challenge in shales since conventional approach, such as using centrifuge, is not feasible to completely remove free water in tight shales. Thermal treatment is therefore suggested for further extraction of movable pore water, however, the influence of temperature on nanoscale pore structure and clay mineralogical composition has been underestimated in previous studies and thus requires further investigation. This paper re-defines the critical dehydration temperature for accurate PSD interpretation in Permian Carynginia shale, Western Australia to determine T 2 cutoff for CBW. By using low-pressure N 2 gas adsorption (LP-N 2 -GA) in parallel with LF-NMR, we identified a striking anomalous PSD consistency for critical temperature detection and verification. Our results shows that movable pore water can be maximally removed around 80 °C (75 °C), while the sensitive clay, CBW and microstructure are well-preserved for accurate petrophysical evaluation. Clay mineral conversion would occur when temperatures are higher than 80 °C, while temperatures lower than 75 °C would induce large misinterpretations for nanopore structure. Our recommended scheme could provide a potential adaptability for the formation evaluation of Permian Carynginia shale in the downhole practices