A study on restoration and composite arrangement of core samples for SCAL experiments

Sripal, Edison Amirtharaj (2023) A study on restoration and composite arrangement of core samples for SCAL experiments. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Special core analysis (SCAL) programs are designed to meet explicit requirements for the specific reservoir type and lithology. The success of a SCAL program relies on qualitative determination of the petrophysical parameters (density logs, resistivity logs, etc.) and dynamic parameters (wettability, relative permeability, etc.). Since wettability is critical for capillary pressure and relative permeability experiments, coreflood tests cannot be performed until wettability tests have been completed and analyzed. This research develops an optimal method for wettability restoration in reservoir rocks and validation of the appropriate arrangement of the aged core plugs in a composite core arrangement for a successful SCAL program. An important aspect of preliminary core preparation for SCAL experiments is the restoration of the core sample to its original wettability. The prevalent method for restoration of a core sample, to either strongly oil-wet or weakly oil-wet, is largely dictated by increasing or decreasing the aging time at reservoir temperature. There is no consistent or reliable method ascribed for a specific sedimentary core sample to restore it to its original state that is obtained from preserved core samples. In this study, we identify brine salinity, restoration temperature, and restoration time (age in number of days) as important parameters contributing to wettability. The objective of this work is to determine the optimum level of these independent variables (brine salinity, temperature, and age) for restoring wettability. Additionally, the Box Behnken model of surface response methodology was applied to analyze the effects of these parameters on wettability restoration. Wettability was experimentally validated using a combination of contact angle measurement, USBM tests and SEM imaging (at low vacuum conditions). A seminal effort in applying SEM-MLA image analysis for wettability determination was also explored. The study shows a comprehensive influence of brine salinity, aging time, and temperature towards wettability restoration. Further, a systematic approach was applied to quantify the degree of uncertainty linked to a) wettability estimation and b) the aging procedure to control wettability in Berea, Silurian dolomite, and chalk. With comprehensive experimental work, we were able to alter the wettability of chalk samples. Core samples from different depths of an exploration well are often used as composite cores for routine and special core analysis to evaluate the potential of a reservoir. The question is whether or not the order of the core plugs in the composite core makes a difference in the absolute and relative permeability measurements. The seminal work by Huppler (1969) proposed ordering individual core samples harmonically in a composite core in order to match the overall permeability. Langaas (1998) proposed ordering the core samples in decreasing permeability for effective relative permeability measurements, based on a theoretical framework of North Sea sandstones. In this work, we tested the methods proposed by Huppler and Langaas experimentally. The orientation of the core samples was tested for permeability and relative permeability and compared with the theoretical model developed by Langaas. In addition to the experimental work, simulations were performed with four different composite cores and the representative recovery factor was compared to provide the appropriate composite core arrangement.

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