Rock Physics of Reservoir Rocks with Varying Pore Water Saturation and Pore Water Salinity

摘要

Advanced waterflooding (injection of water with selective ions in reservoirs) is amethod of enhanced oil recovery (EOR) that has attracted the interest of oil and gas companies that exploit the Danish oil and gas reservoirs. This method has been applied successfully in oil reservoirs and in the Smart Water project performed in a laboratory scale in order to evaluate the EOR processes in selected core plugs. A major step towards this evaluation is to identify the composition of the injected water that leads to increased oil recovery in reservoirs and to define changes in the petrophysical properties of the rock due to the water injection. During advanced waterflooding of reservoirs, or in the Smart Water project, during core flooding experiments, several chemical and petrophysical processes occur in the grains and pore space due to rock, brine and oil interactions. These processes may affect the rate and amount of oil recovered.Advanced waterflooding experiments of reservoir rocks are performed on laboratory scale, but the mechanisms that describe the effects of water injection on the rock minerals are poorly understood. After many decades, a methodology on how this technique should be performed on specific geological structures and why it is sometimes successful; has yet to be established. The presence of both oil and water in the pore space, several different ions present in the injected water that contact the pore walls, possible changes in the fluid wetting the surface of the grains and high stress applied on the minerals, comprise the complex system of waterflooding. These parameters affect the fluid/fluid, solid/fluid and solid/solid interfaces. The changes of the petrophysical and mechanical properties of the core affected from waterflooding are the main topic of research in the present study. In an effort to simplify the complex system of waterflooding, the parameters that affect the solid/fluid interfaces simultaneously, during the experiments, are studied individually. Many chemical and petrophysical phenomena have been documented in previous studies that may affect either the mechanical or physical properties of the rock during waterflooding experiments. The phenomena include decreased pore stiffness and subsequent compaction and can be related to a variety of parameters; including precipitation and dissolution reactions, as well as adsorption reactions and changes in wettability. In order to understand the potential mechanisms under the action of water injection, the present study investigates the effect of the selected ions on the solid/fluid interface of the porous medium under reservoir conditions by studying the following conditions separately: 1) during coreflooding experiments, the rock is subjected to high external stresses that resemble the reservoir stresses; 2) the fluid distribution within the pore space changes during the flow through experiments and wettability alterations may occur; 3) different ions, present in the salt water injected in the core, interact with the surface of the mineral.This study aims to improve the theoretical understanding of the detailed mechanisms involved in waterflooding, using advanced and sensitive tools on a laboratory scale to illustrate the potential mechanisms behind the action of water injection on oil and brine bearing rocks. In order to investigate the action of pore water with selective ions on the solid/fluid interface, low field nuclear magnetic resonance (NMR) spectrometry, ultrasonic velocities, electrical resistivity and mineralogical characterization are performed on quarry and reservoirs cores. The rocks are saturated with fluids similar to the ones used in the core flooding experiments. Ultrasonic velocities and electrical resistivity data are collected to detect changes with respect to strength and pore geometry of the rock. Low field NMR spectrometry is used to detect changes in texture, wettability and pore‐fluid distribution. While investigating the petrophysical properties of reservoir rocks, information concerning the mineralogy is an important factor for the establishment of a rock physical model. Therefore, additional experiments are performed; X-ray diffraction (XRD), backscatter electron microscopy images (BSEM), mercury injection capillary pressure (MICP) curves and specific surface analysis (BET) illustrate the mineralogy and texture of the rock samples.Chalk from Stevns Klint near Copenhagen, Denmark, (a rock analogue to reservoir chalk from the North Sea) was used for rock mechanical testing in order to understand the potential mechanisms behind the action of ions in high concentration on the chalk surface; such as precipitation and dissolution. The effect of the divalent ions on the elasticity and pore collapse of this rock was observed and validated from the ultrasonic velocity data. Low field NMR was used to detect any precipitation that may occur in the pore space of chalk saturated with divalent ions. Precipitation occurred only in single cases; therefore, it is doubtful whether it is the responsible mechanism for the water weakening of chalk. The same rock material was used to illustrate the use of low field NMR to detect differences in the texture of chalk; in our case a carbonate mudstone and a carbonate wackestone as previously observed by electron microscopy.The solid-fluid affinity of chalk from the Gorm field, Berea sandstone and chlorite bearing greensand from the Solsort field was defined from low field NMR data. Longitudinal relaxation time (T1), transverse relaxation time (T2) and self-diffusion coefficient of the fluids within the core plugs were measured at different saturation states; water, reservoir oil and oil and water at irreducible water saturation. T1/T2 ratio proved a non-destructive and fast way to determine the solid-fluid affinity and fluid distribution within the pore space of the selected rocks.Finally, supplementary experimental work includes the determination of small amounts of oil in water samples from the NMR T2 distribution. Low field NMR spectrometry was able to accurately determine the oil and water volume in effluents. This is found very useful, because when the oil reaches residual saturation during core flooding experiments, the produced oil is very small and the quantification of these fluids is often difficult.iv