Jin Zhao

Date of Award

December 2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Kegang Ling

Abstract

Currently, fossil fuels (including oil, natural gas, and coal) are still the dominant energy sources in the United States. However, their extraction and utilization generate a significant volume of carbon dioxide (CO2) and associated gases, such as methane (CH4), ethane (C2H6), propane (C3H8), etc. Injecting these gases back into deep oil reservoirs offers a practical means to enhance oil recovery while reducing air pollution. The target formations are usually characterized by good porosity and permeability to ensure sufficient injectivity and storage capacity. However, recent unconventional reservoir development indicated that tight formations could also be potential targets for enhanced oil recovery (EOR) using gas injection despite their relatively low matrix porosity and permeability. Abundant fractures provide flow paths for the injected gas to flow through the fractured tight reservoirs. The goal of this research is to extend the gas EOR technologies from conventional reservoirs to unconventional reservoirs and fill the gap between theoretical studies and field implementations for unconventional reservoirs. Given the availability of research resources and data, this study mainly focuses on gas injection EOR in both naturally fractured conventional and hydraulically fractured unconventional reservoirs in North Dakota. The first reservoir studied is Dickinson Lodgepole Mounds (DLM) which is beneath the city of Dickinson, North Dakota. The mound structures have abundant natural fractures but relatively low matrix porosity (2%–6%) and permeability (0.1–10 md). Core samples from the reservoir are used in laboratory investigations (Chapter 3). Contact angle measurements show the reservoir is strongly oil-wet. Interfacial tension (IFT) and minimum miscibility pressure (MMP) measurements illustrate the interactions between oil, water, and CO2. Results show that there is a strong IFT between oil and water regardless of reservoir pressure while CO2 can reduce the IFT effectively. Core samples are saturated with oil and then flooded with water to mimic the water flooding mechanisms in the reservoir. Both miscible CO2 and rich gas flooding tests are then conducted to observe the EOR response under reservoir conditions. Experimental results show that water could only displace a minor amount of oil from the rock matrix while CO2 and rich gas components could recover as much as 44% and 80% of oil in the cores, respectively. Based on the literature review presented in this study, the implementation of EOR in unconventional reservoirs is still in an early stage compared to their deployment in conventional reservoirs. Although numerous laboratory experiments and modeling efforts have been conducted to explore EOR in unconventional reservoir plays, only limited research has been reported to investigate field-level EOR implementations and the corresponding monitoring strategies. Aimed at advancing gas injection EOR technologies in unconventional reservoirs, this study comprised a series of field case analysis activities to bridge the gap between theoretical study and actual field applications. Twenty-four EOR pilot tests are collected from the major unconventional plays in North America to evaluate the performance of different EOR technologies (Chapter 4). Detailed EOR performance evaluation and reservoir monitoring studies are performed to reveal the key variables involved in actual EOR implementations in unconventional reservoirs (Chapters 5 and 6). Fit-for-purpose experiments and simulations are performed (Chapter 7) to investigate the effects of injection rate and pressure on EOR performance, as well as to reveal the effectiveness of huff ‘n’ puff (HnP) cycles in actual field operations. The selection of injection rate and pressure as key parameters for investigation is based on field observations and communications with oil and gas operators because these two parameters play critical roles in both facility design and overall cost for an EOR project. Results show that miscible EOR with high gas injection rate and pressure is required for effective field operations because the injected gas needs to penetrate and extract oil from the tight matrix. Experimental results indicate that there is a correlation between oil recovery and the logarithm of core volume for miscible EOR. Immiscible gas EOR could not yield a satisfactory EOR response in depleted unconventional reservoirs where not much oil remains in fractures. This is because the injected gas tends to flow through fractures instead of penetrating the matrix to interact with oil. Results also show that reaching minimum miscibility pressure does not guarantee an optimum EOR operation in unconventional reservoirs. Pressure higher than MMP is preferred in field operations. When designed properly, up to a tenfold oil production rate boost is achievable in field applications within a short period. However, such a high-performance operation is only effective in the first several HnP cycles due to the limited gas penetration depth into the rock matrix.

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