Date of Award

January 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Hui Pu

Abstract

The recent advances in horizontal drilling and hydraulic fracturing have enabled a profitable oil and gas recovery from unconventional geologic plays. The Bakken is one of the largest oil-bearing tight formations in North America, with an estimated original oil in place of 600 billion barrels; however, only a small fraction (7% to 12%) of this oil is recoverable using currently available technologies.CO2 injection can be an effective technique to enhance oil recovery from unconventional reservoirs. It can assist with extracting residual oil and overcoming injectivity problems in tight formations. Previous CO2 enhanced oil recovery (EOR) pilot tests performed in the Bakken Formation indicated that cyclic CO2 injection might be a promising technique for enhanced oil recovery; however, no clear consensus has been reached, and the reported results have revealed that CO2 EOR mechanisms in unconventional reservoirs are still poorly understood. This study addresses the knowledge gap related to CO2 EOR in unconventional reservoirs, investigates the side effects of CO2 injection, and compares the EOR performance of different gases to determine the optimum EOR scheme in tight formations. We investigated and analyzed the effects of different parameters on CO2 performance using samples from the Middle Bakken member and Three Forks Formation. The factors studied include CO2 Huff-n-Puff (HnP) injection parameters, sample size, water presence within the fractures, and the volume of CO2 in contact with the rock matrix during the HnP experiments. The injected CO2 can interact with the in-situ reservoir fluids and rock minerals, which can impact and alter several reservoir attributes. The potential changes in rock wettability, pore size distribution, and effective porosity before and after exposure to CO2 were evaluated. The results indicate that CO2 can alter wettability and increase the hydrophilicity of the rock. The nuclear magnetic resonance spectroscopy technique was used to determine fluid distribution before and after CO2 injection. The results confirm that carbonic acid can dissolve portions of the dolomite, calcite, and feldspar in the rock and create new micro- and nanopores. We compared the EOR performance of CO2 and hydrocarbon gases to determine the most effective gases. Then we introduced a novel gas EOR scheme to boost oil mobilization and achieve higher recovery factors.

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