Abstract

Petroleum fractionation during migration and production is the progressive compositional change as petroleum fluids migrate through or are produced from porous media. This study employed Fourier Transform Ion Cyclotron Resonance mass spectrometry (FT-ICR-MS) coupled with atmospheric pressure photoionization (APPI) to study the heavy end compositional differences of in-situ oils across Bakken and Three Forks Formations at a single well location and nearby produced oils from Middle Bakken and Three Forks. Profound fractionation is evidenced in both hydrocarbons and non-hydrocarbons as migration and production proceed: 1) compositional complexity decreases; 2) non-hydrocarbon content decreases whilst hydrocarbon content increases correspondingly; 3) compounds with higher aromaticity lag behind; 4) co-variation patterns are discernible among different compounds and indicate polarity and aromaticity exert a larger influence on fractionation than molecular weight. Improved understanding of migration and production fractionation in the context of formation rock minerology, formation water and completion fluids chemistry, and reservoir PT conditions assists in optimizing completion and production and developing play-specific enhanced oil recovery technologies.

Introduction

Petroleum fractionation during migration and production, also known as geo-chromatographic effect, refers to the progressive compositional change as petroleum fluids migrate through or are produced from porous media. Such fractionation has long been observed yet remains challenging to predict, as it involves the interplay of a myriad of mechanisms, like differential partitioning of compounds between gas, oil and water phases and selective retention by minerals (Thompson, 1987, Thompson, 1988, Leythaeuser et al., 1988, Dzou, 1993, Later, 1996). To a certain degree, PVT simulation can be utilized to model the bulk compositional changes as fluids migrate to shallower strata or as a reservoir depletes, especially in the light ends (Larter et al., 1991, England, 2002). However, PVT modeling misses the majority of geochemical changes due to interactions of petroleum fluids with formation rocks, formation water, and completion fluids. While changes in fluids bulk properties, like API gravity, resulting from the fractionation are readily detectable, the underlying geochemical changes can be difficult to determine, particularly in the heavy end with a large portion of polar compounds not amenable to GC based analyses. This study employed Fourier Transform Ion Cyclotron Resonance mass spectrometry (FT-ICR-MS) coupled with atmospheric pressure photoionization (APPI) to investigate the heavy end compositional differences of in-situ oils (core extracts) across Bakken and Three Forks Formations at a single well location and produced oils from nearby horizontal wells landed in the Middle Bakken and Three Forks (Robb et al., 2000, Purcell et al., 2007, Marshall, 2008, Han et al., 2020). Profound fractionation is manifested in both hydrocarbons (compounds containing only H can C) and non-hydrocarbons (compounds containing heteroatoms like O, N, S besides H and C) as migration and production proceed. Left behind during migration and production, heavy and polar petroleum compounds modify formation rock wettability and permeability and affect subsequent migration and production efficacies, particularly in plays with low porosity and permeability. Improved understanding of migration and production fractionation in the context of formation rock minerology, formation water and completion fluids chemistry, and reservoir PT conditions assists in optimizing completion and production and developing play-specific enhanced oil recovery technologies.

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