Utpalendu Kuila

Specific surface area and pore-size distribution in clays and shales

One of the biggest challenges in estimating the elastic, transport and storage properties of shales has been a lack of understanding of their complete pore structure. The shale matrix is predominantly composed of micropores (pores less than 2 nm diameter) and mesopores (pores with 2–50 nm diameter). These small pores in the shale matrix are mainly associated with clay minerals and organic matter and comprehending the controls of these clays and organic matter on the pore-size distribution is critical to understand the shale pore network. Historically, mercury intrusion techniques are used for pore-size analysis of conventional reservoirs. However, for unconventional shale reservoirs, very high pressures (> 414 MPa (60 000 psi)) would be required for mercury to access the full pore structure, which has potential pitfalls. Current instrumental limitations do not allow reliable measurement of significant portions of the total pore volume in shales. Nitrogen gas-adsorption techniques can be used to characterize materials dominated by micro- and mesopores (2–50 nm). A limitation of this technique is that it fails to measure large pores (diameter >200 nm). We use a nitrogen gas-adsorption technique to study the micro- and mesopores in shales and clays and compare the results from conventional mercury porosimetry techniques. Our results on pure clay minerals and natural shales show that (i) they have a multiscale pore structure at different dimensions (ii) fine mesopores, with a characteristic 3 nm pore size obtained with N2 gas-adsorption are associated with an illite-smectite group of clays but not with kaolinite; (iii) compaction results in a decrease of pore volume and a reduction of pore size in the ‘inter-aggregate’ macropores of the illitesmectite clays while the fine ‘intra-tachoid’ mesopores are shielded from compaction; (iv) for natural shales, mineralogy controls the pore-size distributions for shales and the presence of micropores and fine mesopores in natural shales can be correlated with the dominance of the illite-smectite type of clays in the rock. Our assessment of incompressible 3 nm sized pores associated with illite-smectite clays provides an important building block for their mineral modulus.

Stress-dependent elastic properties of shales: measurement and modeling

Despite decades of research, current understanding of elastic properties of shales is insufficient as it is based on a limited number of observations caused by the time-consuming nature of testing resulting from their low permeability. Though it is well known that shales are highly anisotropic and assumed to be transversely isotropic (TI) media, few laboratory experiments have been carried out for measuring the five elastic constants that define TI media on well-preserved shales. Many previous measurements were made without control of pore pressure, which is crucial for the determination of shale elastic properties.

Pore Size Distribution And Ultrasonic Velocities of Compacted Na-montmorillonite Clays

Shale exhibits dual-porosity structure and has more complex pore-structure than that of sandstones and limestones. The predominant gas flow occurs through the interconnected fracture network system and this system is recharged by the gas flowing through the matrix. Shale matrix has predominantly microto meso-pores (pore-size below 50 nm as per IUPAC classification). Another unique aspect of these sediments is the high pore surface area to pore volume ratio. Clays are layered–sheet silicate minerals of extremely small grain size which contributes to the large surface areas. The gas molecules attach themselves to these large surface areas by adsorption.

Chapter 7: A Comparison of Measurement Techniques for Porosity and Pore Size Distribution in Shales (Mudrocks): A Case Study of Haynesville, Eastern European Silurian, Niobrara, and Monterey Formations

Abstract Porosity and pore size distribution (PSD) are required to calculate reservoir quality and volume. Numerous inconsistencies have been reported in measurements of these properties in shales (mudrocks). We investigate these inconsistencies by evaluating the effects of fine grains, small pores, high clay content, swelling clay minerals and pores hosted in organic content. Using mudrocks from the Haynesville, Eastern European Silurian, Niobrara, and Monterey formations, we measured porosity and pore or throat size distribution using subcritical nitrogen (N2) gas adsorption at 77.3 K, mercury intrusion, water immersion, and helium porosimetry based on Gas Research Institute standard methodology. We used scanning electron microscope (SEM) images to understand the pore structure at a microscopic scale. We separated the samples from each formation into groups based on their clay and total organic carbon (TOC) contents and further investigated the effects of geochemical and mineralogical variations on porosity and PSD. We find that differences in the porosity and PSD measurement techniques can be explained with thermal maturity, texture, and mineralogy, specifically clay content and type and TOC variations. We find that porosity and PSD measurement techniques can provide complementary information within each group provided the comparison is made between methods appropriate for that group. Our intent is to provide a better understanding of the inconsistencies in porosity measurements when different techniques are used.

Measuring and Modeling the Elastic Moduli of Clay Minerals

Summary The P-wave velocities of naturally occurring recent sediments are measured as a function of water content. The samples are dewatered by centrifuging to stimulate compaction environment. The velocities increase with decreasing water-content and there is a sharp rise in velocity below 30% water-content.

Strength Prediction and Evolution in Shales

Shale strength is an important parameter for wellbore stability, trap integrity, hydrofracturing and various types of subsurface geological storage. However, strength of shales is not well constrained due to limited available data. This study is in two parts: 1. Empirical correlations between physical and petrophysical properties of a range of shales from basins worldwide to static and dynamic elastic properties. Here, good correlations were found between porosity and normalised cation exchange capacity (CEC) to cohesion and unconfined compressive strength. CEC normalised to density proved a useful parameter in these empirical correlations. Friction was harder to derive an empirical relationship for, although a complex relationship was found with good correlation although of dubious statistical merit. 2. Evolution of static and dynamic mechanical properties with compaction and diagenesis. Investigations of a sequence of shales from a well in the Officer Basin in Western Australia and showed significant increases in static mechanical properties with chemical compaction likely the dominant mechanism. P- and S-wave velocities both parallel and normal to bedding also increase with depth and the anisotropy of velocity decreases. While these results are somewhat intuitive, this is believed to be the first laboratory-based study to show such a result.