Hamid Roshan

On wettability of shale rocks

The low recovery of hydraulic fracturing fluid in unconventional shale reservoirs has been in the centre of attention from both technical and environmental perspectives in the last decade. One explanation for the loss of hydraulic fracturing fluid is fluid uptake by the shale matrix; where capillarity is the dominant process controlling this uptake. Detailed understanding of the rock wettability is thus an essential step in analysis of loss of the hydraulic fracturing fluid in shale reservoirs, especially at reservoir conditions. We therefore performed a suit of contact angle measurements on a shale sample with oil and aqueous ionic solutions, and tested the influence of different ion types (NaCl, KCl, MgCl2, CaCl2), concentrations (0.1, 0.5 and 1M), pressures (0.1, 10 and 20MPa) and temperatures (35 and 70°C). Furthermore, a physical model was developed based on the diffuse double layer theory to provide a framework for the observed experimental data. Our results show that the water contact angle for bivalent ions is larger than for monovalent ions; and that the contact angle (of both oil and different aqueous ionic solutions) increases with increase in pressure and/or temperature; these increases are more pronounced at higher ionic concentrations. Finally, the developed model correctly predicted the influence of each tested variable on contact angle. Knowing contact angle and therefore wettability, the contribution of the capillary process in terms of water uptake into shale rocks and the possible impairment of hydrocarbon production due to such uptake can be quantified.

Dependence of quartz wettability on fluid density

Wettability is one of the most important parameters in multiphase flow through porous rocks. However, experimental measurements or theoretical predictions are difficult and open to large uncertainty. In this work we demonstrate that gas densities (which are much simpler to determine than wettability and typically well known) correlate remarkably well with wettability. This insight can significantly improve wettability predictions, thus derisking subsurface operations (e.g., CO2 geostorage or hydrocarbon recovery), and significantly enhance fundamental understanding of natural geological processes.

Evaporative cooling of speleothem drip water

This study describes the first use of concurrent high-precision temperature and drip rate monitoring to explore what controls the temperature of speleothem forming drip water. Two contrasting sites, one with fast transient and one with slow constant dripping, in a temperate semi-arid location (Wellington, NSW, Australia), exhibit drip water temperatures which deviate significantly from the cave air temperature. We confirm the hypothesis that evaporative cooling is the dominant, but so far unattributed, control causing significant disequilibrium between drip water and host rock/air temperatures. The amount of cooling is dependent on the drip rate, relative humidity and ventilation. Our results have implications for the interpretation of temperature-sensitive, speleothem climate proxies such as δ18O, cave microecology and the use of heat as a tracer in karst. Understanding the processes controlling the temperature of speleothem-forming cave drip waters is vital for assessing the reliability of such deposits as archives of climate change.

Analysis of Pore Pressure Distribution in Shale Formations under Hydraulic, Chemical, Thermal and Electrical Interactions

Change in pore pressure in chemically active rocks such as shale is caused by several mechanisms and numerous studies have been carried out to investigate these mechanisms. However, some important coupling terms or driving forces have been neglected in these studies due to simplifying assumptions. In this study, a hydro-chemo-thermo-electrical model based on finite element method is presented to investigate the change in pore pressure in shale formations resulted from thermal, hydraulic, chemical and electric potential gradients. The change in pore pressure is induced by hydraulic conduction, chemical, electrical and thermal osmotic flow. In order to solve the problem of ion transfer under the influence of an electrical field, the Nernst–Planck equation is used. In addition, ion advection is considered to investigate its possible effect on ion transfer for the range of shale permeability. All equations are derived based on the thermodynamics of irreversible processes in a discontinuous system. The numerical results are compared against existing and derived uncoupled analytical solutions and good agreement is observed. The numerical results showed that the ion transfer and pore pressure are considerably affected by the electric field in the vicinity of the wellbore. It was also found that advection can play a remarkable role in ion transfer in shale formations. It was further shown that the change in pore pressure in shale formation is characterized by the combined effect of hydraulic, chemical, thermal and electro osmotic flow.

A Non-Isothermal Constitutive Model for Chemically Active Elastoplastic Rocks

Hydration/dehydration of water-active rocks under non-isothermal conditions is encountered in many industries, in particular the drilling industry. Water-active rocks can adsorb water on their basal crystal surfaces on both the external, and, in the case of expanding lattice clays, the inter-layer surfaces. There are two main mechanisms in which the instability may occur for a structure built in water-active rocks. First mechanism is the change in stresses resulted from mechanical, hydraulic, chemical and thermal interaction, which we formulate based on the concept of non-equilibrium thermodynamics. Second mechanism illustrates the change in mechanical properties of the rock due to physic-chemical interaction between the rock and exposed fluid. We postulate the later mechanism in terms of chemically active plastic deformation by using the modified Cam Clay model. The complete governing equations for non-isothermal chemo-poro-elastoplastic rock are therefore presented. A three-dimensional finite element model is then developed to solve the obtained governing equations. From the numerical results, it was found that the effect of temperature coupling term on stress cannot be ignored when non-isothermal conditions are encountered. However, this effect is almost trivial on pore pressure. It was also revealed that the effect of change in the shale’s mechanical properties on stability due to physic-chemical interaction is as important as change in stresses due to mechanical, hydraulic, chemical and thermal interactions.

Microstructural effects on mechanical properties of shaly sandstone

AbstractUnderstanding the mechanical properties of shaly sandstone is of great importance in reservoir geomechanics. Because of the lack of core data, measurements based on acoustic wave velocities...

Size-Dependent Hoek-Brown Failure Criterion

AbstractThe size dependency of intact rock is of importance to different disciplines, such as civil and mining engineering. One example relates to the design of structures on or within a rock mass for which an estimation of the strength of the intact rock blocks within the mass is essential. Despite a large number of studies on size effects in rock, less research has investigated size effect under triaxial conditions. Thus, a suite of advanced triaxial compressive experiments was conducted on Gosford sandstone samples with diameters of 96, 50, and 25 mm. A size-dependent Hoek-Brown failure criterion was developed by incorporating a unified size-effect law into the original Hoek-Brown failure criterion. The model was calibrated against the triaxial data obtained from Gosford sandstone. It was shown that there is good agreement between the proposed model prediction and the experimental results. Finally, an example of application of the size-dependent Hoek-Brown failure criterion was presented to demonstrate...

Chemo-poroelastic analysis of pore pressure and stress distribution around a wellbore in swelling shale: effect of undrained response and horizontal permeability anisotropy

The aim of this paper is to study the effect of undrained response and horizontal permeability anisotropy on transient changes in pore pressure and stress state of surrounding area to wellbores drilled in swelling shales. For this purpose a finite element model of coupled chemo-hydro-mechanical processes is developed and used. Super convergent patch recovery method is employed to accurately evaluate time dependent stress tensor. From the results of this study it was found that at any time and radial position around the wellbore, change in pore pressure, thus effective stress, depends on horizontal permeability anisotropy, which influences three main contributing processes namely hydraulic flow, chemical effect and undrained response. It was also revealed that at a given time after shale is exposed to the mud, maximum change in pore pressure, thus in effective stress, can take place at different radial distances from the wellbore along different directions due to permeability anisotropy.

Carbon dioxide/brine wettability of porous sandstone versus solid quartz: An experimental and theoretical investigation

HYPOTHESIS Wettability plays an important role in underground geological storage of carbon dioxide because the fluid flow and distribution mechanism within porous media is controlled by this phenomenon. CO2 pressure, temperature, brine composition, and mineral type have significant effects on wettability. Despite past research on this subject, the factors that control the wettability variation for CO2/water/minerals, particularly the effects of pores in the porous substrate on the contact angle at different pressures, temperatures, and salinities, as well as the physical processes involved are not fully understood. EXPERIMENTS We measured the contact angle of deionised water and brine/CO2/porous sandstone samples at different pressures, temperatures, and salinities. Then, we compared the results with those of pure quartz. Finally, we developed a physical model to explain the observed phenomena. FINDINGS The measured contact angle of sandstone was systematically greater than that of pure quartz because of the pores present in sandstone. Moreover, the effect of pressure and temperature on the contact angle of sandstone was similar to that of pure quartz. The results showed that the contact angle increases with increase in temperature and pressure and decreases with increase in salinity.

Effects of Ion Advection and Thermal Convection on Pore Pressure Changes in High Permeable Chemically Active Shale Formations

Wellbore instability problems are often encountered while drilling in water active shales due to changes in pore pressure. The change in pore pressure is caused by hydraulic, thermal, chemical, and electrical potential gradients. In all previous studies it has been found that the effects of ion advection and thermal convection have a negligible effect on changes in pore pressure for a range of very low permeable shale formations (>10−5 md). This is an appropriate assumption for very low permeable shale formations. For high permeable shale formations (e.g., shale with a disseminated microfissure network), however, thermal convection and ion advection can play a significant role. The authors present a hydro-chemo-thermo-electrical model based on finite element method to investigate the effect of advection on ion transfer and thermal convection on temperature and their combined effect on pore pressure in shale formations. All equations are based on the thermodynamics of irreversible processes in a discontinuous system. The characteristic Galerkin discretization method is used to stabilize the solution of advection and convection equations in the finite element approach. Results of this study revealed that ion and heat transfer are controlled primarily by permeability of the shale formations. Movement of fluid into or out of the formation is due to a combination of hydraulic, chemical, electrical, and thermal osmotic flow. Results have also shown that in high permeable shale formations the chemical potential gradient between the pore fluid and drilling fluid reaches equilibrium faster than in low permeable shale formations. This is mainly due to the advection of ion from drilling fluid to the shale formation.