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Effect of capillary condensation on the geomechanical and acoustic properties of tight formations, The
Albannay, Aamer
Albannay, Aamer
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2019
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2021-01-07
Abstract
The main objective of this research is to experimentally assess how capillary condensation affects the mechanical and acoustic properties of tight rocks. In order to do so, a new facility was built to experimentally investigate the changes in the mechanical and acoustic properties due to capillary condensation. The experimental set-up is capable of the simultaneous acquisition of coupled stress, strain, resistivity, acoustic and flow data. Carbon dioxide was used as the pore pressure fluid in these experiments. Capillary condensation is the condensation of the gas inside nanopore space at a pressure lower than the bulk dew point pressure as a result of multilayer adsorption. Capillary condensation occurs due to the high capillary pressure inside the small pore throat of unconventional rocks. This condensation affects the phase behavior of the pore fluid, which in turn significantly impacts hydrocarbon-in-place evaluation and assessment. Due to condensation, the mechanical and acoustic properties of the rock may change. Acoustic properties variation due to capillary condensation provides us a tool to monitor phase change in reservoir as a result of nano-confinement as well as mapping the area where phase change occurs. This is particularly important in tight formations where confinement has a strong effect on phase behavior that is challenging to measure. Acoustic data provides an indirect tool for this purpose. It can also be used to characterize pore size distribution. Theoretical studies have examined the effects of capillary condensation, however, these findings have not been verified experimentally. The main components of the experimental facility are: triaxial load cell, pore fluid injection system, back pressure system, vacuum system, axial and confining pressure system and a temperature control system. The axial and confining pressure system is capable of simultaneously applying overburden pressure (axial) and isotropic horizontal stress (confining pressure). The facility can handle stress and pore pressure up to 10,000 psi with temperature up to 100\lyxmathsym{\textdegreeC. Both top and bottom axial pistons are equipped with 1 MHz resonant frequency compressional (P) and shear (S) wave transducers. A serial digital communication protocol acquires and transfers pressure and syringe volume data from pump controllers. The system temperature fluctuation of 0.1°C ensures reliable high-quality data due to minimal temperature disturbance since pore volume change due to capillary condensation and permeability measurements are very sensitive to temperature changes. In this research, I conducted a series of experimental investigations to study the changes in the acoustic and geomechanical properties using core samples from the Diyab and Austin Chalk formations with and without capillary condensation. Carbon dioxide was used as the pore fluid in these experiments. Nitrogen adsorption experiments were also conducted to characterize the pore size distribution of the core samples. A grain-contact model was developed to predict the mechanical and acoustic changes of the samples during the experiments. The model is capable of predicting changes in acoustic and mechanical properties with temperature, phase saturation, frequency, pore pressure and effective stress. Results of the model prediction show a good match to the experimental data. Experimental data on core samples tested from the Austin Chalk and the Diyab formations show a 5% increase in Young's Modulus as carbon dioxide condensation occurs. This increase is attributed to the increase in pore stiffness as condensation occurs reinforcing the grain contact. We also observed a noticeable increase in shear velocity when capillary condensation occurs. This is because of the confined fluid's lower mobility and higher resistance to shear relative to the gas phase. These geomechanical and acoustic signatures were observed at around 750-800 psi at 27\lyxmathsym{\textdegreeC which is lower than the unconfined \mathrm{CO_{2}} bulk dew point pressure of 977 psi. These experimental findings are the first observation of the signature of capillary condensation on the acoustic and mechanical properties of tight samples. Therefore, it is recommended to further investigate this phenomenon in field-scale and to use acoustic data as a tool for monitoring condensation during the lifecycle of the reservoir.
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