Abstract
In shale oil reservoirs, more than 90% of the oil remains in reservoirs after the primary production. The goal of this work is to study the effectiveness of gas huff-n-puff in conjunction with a chemical blend to improve oil recovery. The chemical blend constitutes of an oxidizing agent and a surfactant in a low salinity brine. Outcrop Mancos core plugs were saturated with kerosene and CO2 and chemical blend huff-n-puff experiments were conducted. Liquid volumes in the core plugs were evaluated using NMR, before and after huff-n-puff cycles. CO2 Huff-n-Puff recovered 41% of incremental oil. CB improved oil recovery by 23% over that for pure CO2. First injecting gas and then pressurizing the wells with the CB also improved the oil recovery significantly. It can also reduce the gas compression cost.
Unconventional resources in the United States contribute about 60% of the oil production. According to EIA, US has nearly 345 billion barrels of oil and 7299 trillion cubic feet of gas that can be technically recovered (Kuuskraa et al., 2013). After primary production, more than 90% of the oil is left behind in shale oil reservoirs due to ultra-low permeability. Traditional enhanced oil recovery techniques such as gas and surfactant floods are not applicable in unconventional reservoirs due to nD range permeability. The gas huff-n-puff have been tested in shale reservoirs (Alfarge et al., 2018; Louk et al., 2017; Yu et al., 2017). The gases used in the huff-n-puff process are carbon dioxide, nitrogen, flue gases (N2 + CO2), and produced gas (Lashgari et al., 2019; Sanchez-Rivera et al., 2015; Wan et. al., 2018; Chen et al., 2014).
Using greenhouse gases such as carbon dioxide and methane can not only increase the oil production but also benefit the carbon sequestration effort (Alharthy el al., 2018; Jia et al., 2019; Jin, 2017; Hawthorne et al., 2017; Winter, 1993, Lashgari et al., 2019, Bardon et al., 1994). The majority of produced gas contains more than 90% of methane which would be inexpensive to use in the gas huff-n-puff process. CO2 is the most commonly used gas in the huff-n-puff process with great success. According to a study conducted (Li et al. 2019), the huff-n-puff process using CO2 above 1900 psi, under miscible conditions, produced 10% more oil than the immiscible condition with a shorter soaking time. Pressure change from 1900 psi to 2900 psi did not show any significant improvement in oil production. CO2 huff-n-puff was successfully used in the Big Sinking Field, a conventional reservoir. The process was used from 1986-1994 until liquid CO2 price increased by 50%. Then the process was continued with rich gas and exhaust gas (80% N2 and 20% CO2). Using both methods the oil recovery efficiency increased significantly compared to CO2 huff-n-puff (Miller et al., 1998, Charles et al., 1995; Miller et al., 1994).
