Sai Wang

Date of Award

January 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Kegang Ling

Abstract

The low recovery of oil from the tight liquid-rich formations is still a main challenge for the tight reservoir. Thus, in order to break the chains and remove the obstacle such as the low recovery factor in the Bakken tight formation, even though the horizontal drilling and hydraulic fracturing technologies were already well applied in this field, the supercritical CO2 flooding was proposed as an immense potential recovery method for the production improvement.

In this research, we conducted a series of CO2 flooding experiments under various injection pressure (2500psi, 2800psi, 3000psi, 3500psi), to investigate the recovery potential of the core sample from Bakken tight formation. Also, the NMR analysis was processed of the core samples flooded with CO2 agent under the above injection pressure variables. The result comparison demonstrates that, with the supercritical CO2 injection pressure increase, the recovery factor gets incremental trend from 8.8% up to 33% recovery. Also, the macro pore and natural fracture system were proved to contribute more on the recovery potential. After reaching the miscible phase between the CO2 and oil in the sample, the hydrocarbon existed in the micro pores start the contribution to the recovery potential. Thus, The CO2 was identified as a potential recovery agent and the supercritical CO2 EOR method was proposed as the potential recovery technology due to the high recovery factor obtained in the immiscible and miscible processes.

Up to date, rare study was put forward to account for the formation properties variation during the CO2 EOR process, especially the investigation at the micro-scale. This work conducted a series of measurements to evaluate the rock mechanical change, mineral alteration and the pore structure properties variation through the supercritical CO2 (Sc-CO2) injection process. Corresponding to the time variation (0 days, 10 days, 20 days, 30 days and 40 days), the rock mechanical properties were analyzed properly through the nano-indentation test, and the mineralogical alterations were quantified through the X-Ray diffraction (XRD). In addition, pore structures of the samples were measured through the low-temperature N2 adsorption tests. The results showed that, after Sc-CO2 injection, Young’s modulus of the samples decreases. The nitrogen adsorption results demonstrated that, after the CO2 injection, the mesopore volume of the sample changes as well as the specific surface area (BET) which rises from the chemical reactions between the CO2 and some authigenic minerals. XRD analysis results also indicated that mesopores were altered due to the chemical reaction between the injected Sc-CO2 and the minerals.

Also, with existence of the natural fracture and the hydraulic fracture in the Bakken formation, it becomes more and more crucial to quantify the fracture system precisely and reconstruct the structure system in order to identify the preferential flow channels for the fluid flow, such as figuring out the domain pass way of the supercritical CO2. Up to date, the imaging technologies for the fracture quantization and reconstruction of the tight rock is still challenged by the ineffective segmentation, thus significantly influence the fracture properties calculation, including the porosity, aperture, openness, etc.

In this research, a novel image processing method was proposed and certificated. By definition, the multi-stage image segmentation (MSS) method can separate the fracture system from the background by combining the global information and local information of the X- ray CT image. Through employing the entropy function and indicator kriging method, the generated three-dimensional model overcome the over– and in-sufficient segmentation due to the tiny fracture apertures and could provide visualization of the fracture systems existing in the core. Also, the important parameters of the fractures can be obtained, including aperture, length, tortuosity, and porosity. All the obtained parameters are beneficial to fracture identification during the CO2 EOR process in the Bakken tight formation.

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