Improved/ enhanced oil recovery (IOR/EOR) has the potential to significantly enhance production from unconventional plays. A promising IOR/EOR method for tight oil bearing unconventional rocks is gas injection Huff and Puff. Huff and Puff is a sequential, cyclic injection and production process with an optional soak period in between. The method has been reported to increase production up to 70% over primary production in the field, and thus, has the capability of improving oil recovery by millions of oil barrels across unconventional basins. Despite such huge potential, the fundamental mechanistic understanding of the process is limited. Huff and Puff is often modeled using reservoir simulation, which inherently have uncertainties due to gridding choices and constraints. This work describes an experimental approach to evaluate the underlying mechanisms controlling the Huff and Puff process in unconventional rocks. Laboratory measurements using reservoir fluids at reservoir conditions were made on oil-bearing unconventional core samples with nanodarcy permeability. The rocks were saturated with live oil and hydrocarbon gases were injected (Huff) and produced (Puff). A Nuclear Magnetic Resonance (NMR) technique was used for monitoring the fluid saturation and distribution over time. Fluid analysis on the injected and produced fluids using gas chromatography was also employed to understand the thermodynamic behavior. The thermodynamic mechanisms of oil recovery, vaporization, viscosity reduction and swelling, and the impact of gas transport mechanism were analyzed. This work describes a novel approach to determining the factors impacting the Huff and Puff process. The quantification of these recovery mechanisms provides important data for designing field implementations and calibrating simulation models.
Oil production from unconventional reservoirs has significantly increased over the past fifteen years (EIA, 2019) due to widespread application of horizontal drilling and hydraulic fracturing technologies. Long horizontal wells coupled with large interfacial area between tight matrix and large hydraulic fractures enable large quantities of fluids to be produced from unconventional hydrocarbon formations. However, the recovery factors from unconventional wells are usually low (5 to 15%) as most of the production occurs via primary depletion, i.e., without any pressure support or fluid injection. In other words, a large majority of the oil and gas remains in-place after primary depletion. Unconventional wells also have a high decline rate, and surface facilities are often built to handle the high rates achieved over the first few months. As unconventional wells decline with time, facility capacity becomes underutilized. Low recovery factors coupled with underutilized facilities over most of the life of the wells underscore the need for improved / enhanced recovery from unconventional wells.