Nanopore Compositional Modeling in Unconventional Shale Reservoirs

Abstract

Understanding the mechanism of multicomponent mass transport in the nanopores of unconventional reservoirs, such as Eagle Ford, Niobrara, Woodford, and Bakken, is of great interest because it influences long-term economic development of such reservoirs. Thus, we began to examine the phase behavior and flow characteristics of multicomponent flow in primary production in nanoporous reservoirs. Besides primary recovery, our long-term objectives included enhanced oil production from such reservoirs. The first step was to evaluate the phase behavior in nanopores on the basis of pore-size distribution. This was motivated because the physical properties of hydrocarbon components are affected by wall proximity in nanopores as a result of van der Waals molecular interactions with the pore walls. For instance, critical pressure and temperature of hydrocarbon components shift to lower values as the nanopore walls become closer. In our research, we applied this kind of critical property shift to the hydrocarbon components of two Eagle Ford fluid samples. Then, we used the shifted phase characteristics in dual-porosity compositional modeling to determine the pore-to-pore flow characteristics, and, eventually, the flow behavior of hydrocarbons to the wells. In the simulation, we assigned three levels of phase behavior in the matrix and fracture pore spaces. In addition, the flow hierarchy included flow from matrix (nano-, meso-, and macropores) to macrofractures, from macrofractures to a hydraulic fracture (HF), and through the HF to the production well.From the simulation study, we determined why hydrocarbon fluids flow so effectively in ultralow-permeability shale reservoirs. The simulation also gave credence to the intuitive notion that favorable phase behavior (phase split) in the nanopores is one of the major reasons for production of commercial quantities of light oil and gas from shale reservoirs. It was determined that the implementation of confined-pore and midconfined-pore phase behavior lowers the bubblepoint pressure, and this, in turn, leads to a slightly higher oil recovery and lesser gas recovery. Also it was determined that the implementation of midconfined-pore and confined-pore phase-behavior shift reduces the retrograde liquidcondensation region, which in turn, leads to lower liquid yield while maintaining the same gas-production quantity. Finally, the important reason that we are able to produce shale reservoirs economically is "rubblizing" the reservoir matrix near HFs, which creates favorable permeability pathways to improve reservoir drainage. This is why multistage hydraulic fracturing is so critical for successful development of shale reservoirs. IntroductionThis paper sheds light on the physics of flow and the relevant hydrocarbon-recovery mechanisms from unconventional shale reservoirs. A shale reservoir is a fissile mudrock with fine-grain rock fragments consisting of silt (4 to 60 mm) and clay-size fragments (less than 4 mm) of variable mineralogy. Such reservoirs exhibit hydrocarbon storage and fl...

Phase Behavior of Methane/n-Butane Binary Mixtures in Organic Nanopores under Bulk Vapor Conditions

“…55 To better understand liquid-rich shales, hydrocarbon mixtures with heavier hydrocarbons should be considered. 27,65,69 To this end, the methane/n-butane (CH 4 /n-C 4 H 10 ) mixture would be an ideal choice, as it can represent both the bubble-point and gas-condensate dew-point fluid systems.…”

“…The critical pressure of the confined fluid decreases and the supercritical region expands to a lower pressure than that of the corresponding bulk system. − Recently, it was reported that the bubble point in nanoporous media is almost the same as the dew-point pressure, both of which are lower than the dew-point line of the corresponding bulk mixture . There are three commonly used methods to model the phase behavior in shale nanopores: (1) consider the influence of the capillary pressure; ,,,,, (2) modify the critical temperature and pressure in the equation of state (EoS) models; ,− and (3) modify the equation of state. − Besides, researchers have considered the adsorption effect, ,− but details differ: Dong et al emphasized the importance to subtract the thickness of adsorption film from the pore diameter, the equation of state was modified to mimic the adsorption effect in other groups, − and Sandoval et al considered the adsorption effect with the multicomponent Langmuir model, of which the parameters were determined by the multicomponent potential theory of adsorption . However, these methods remain to be testified and improved .…”

“…DFT has also been developed to reduce computational costs. − ,, Pitakbunkate et al studied the phase behavior of methane/ethane mixtures in nanopores by GCMC simulations . To better understand liquid-rich shales, hydrocarbon mixtures with heavier hydrocarbons should be considered. ,, To this end, the methane/ n -butane (CH 4 / n -C 4 H 10 ) mixture would be an ideal choice, as it can represent both the bubble-point and gas-condensate dew-point fluid systems.…”

The Synthesis of Polymeric Nanospheres and the Application as High-Temperature Nano-Plugging Agent in Water Based Drilling Fluid

“…Shale formations account for about 75% of drilled formations worldwide (Osisanya and Chenevert, 1994). The shale formation has a large amount of micropores (Alharthy et al, 2013(Alharthy et al, , 2016). On the one hand, the invasion of drilling fluids into micropores during the drilling process causes hydration and expansion of clay minerals (Luo et al, 2017;Huang et al, 2018a).…”

Isobaric Vapor–Liquid Phase Diagrams for Multicomponent Systems with Nanoscale Radii of Curvature

“…In the natural environment, multicomponent systems with highly curved interfaces can be found from 4 km below the Earth’s crust in hydrocarbon reservoirs to up to 15 km above the surface in the troposphere. , In the subsurface, porous structures can have pore throat diameters in the nanometer range. − These nanopores influence phase equilibrium, and if their effect is unaccounted for, predictions for unconventional hydrocarbon recovery operations may be inaccurate. − Phase diagrams are vital tools in recovering oil and natural gas from shale formations, and the effect of nanoscale curvature on phase diagrams has been calculated for specific compositions of hydrocarbons as a function of temperature and pressure. ,,− In the atmosphere, clouds can form when water condenses on highly curved nuclei with diameters in the nanometer range. , The number, size, and composition of these nuclei determine the number and size of water droplets, which dictate the size and lifetime of clouds . Thus, nanoscale processes can have a significant role in macroscopic properties and phenomena.…”