Turtles and Snakes: Evidence for Molecular Shape-Selective Migration of Crude Oil Hydrocarbons in the Bakken Petroleum System

Abstract

Aromatic and aliphatic hydrocarbons were analyzed in extracts from 105 rock core samples collected from 16 wells across the Bakken Petroleum System in the Williston Basin, North Dakota (USA). Crude oil hydrocarbons recovered from the Upper (UBS) and Lower (LBS) Bakken shale source rocks had consistently higher proportions of aromatic hydrocarbons compared to aliphatic hydrocarbons than were found in the adjacent Middle Bakken (MB) and Three Forks (TF) target drilling zones. The higher aromatic/aliphatic (Ar/Al) ratios in the UBS and LBS source zones as compared to the adjacent MB and TF producing zones and the near molecular-sized nanometer pore throats in the UBS and LBS rocks suggest that primary migration of the aromatic hydrocarbons out of the source shale pores was (and still is) inhibited by the rounded, flat, and rigid "turtle" shape of the aromatic hydrocarbons significantly more than by the linear and flexible "snake" shape of the aliphatic hydrocarbons. Additional evidence for inhibited primary migration of aromatics versus aliphatics based on molecular shape was shown in laboratory experiments by comparing their recovery rates from the "as-received" UBS, LBS, and MB rock core samples from three wells using supercritical CO 2 and supercritical ethane under reservoir conditions of 34.5 MPa and 110 °C. CO 2 and ethane gave similar aromatic hydrocarbon recovery rates with all the rock samples regardless of whether recoveries from the individual rocks were slow or fast. Since CO 2 as a solvent can better disrupt polar interactions between aromatics and the shale organic matrix than ethane, the similarity in recovery rates comparing CO 2 and ethane indicates that sorption of the aromatics to the source shale organic matrix was not the primary reason for their inhibited migration. However, the recovery of aromatics as compared to the same-sized aliphatics (same carbon number) was much more inhibited from the UBS and LBS samples than from the MB samples, results which provide further evidence that shapeselective filtration during primary migration out of the source shale nanopores is the dominant mechanism for lower Ar/Al ratios in the oil-producing MB and TF zones than in the source UBS and LBS zones.

Status and Outlook of Oil Field Chemistry-Assisted Analysis during the Energy Transition Period

“…The data present are for the LBS; the UBS has a similar trend. Reproduced with permission from ref . Copyright 2021 American Chemical Society.…”

“…60,61 EOR performance should be estimated at the molecule or even atom level from analytical chemists' perspectives to truly understand the mechanisms of CO 2 and other types of gas. 62,63 Three series of fundamental experiments are proposed: minimum miscibility pressure (MMP) measurements 64 based on the standard of the vanishing interfacial tension between oil and gas; 65,66 the dissolving capacity using different kinds of solvent gas in the bulk phase without the porous media effect, 67 i.e., the vaporizing mechanism, 68,69 and how the residue oil composition changes at the molecule level; 70 and the extraction test with rigorous mass balance calculations using a natural core with initial oil saturations and GC analysis. 71,72 For the CO 2 EOR processes, and nowadays CO 2 "huff-npuff" applications in the unconventional shale oil reservoirs, 73,74 the philosophy of the reservoir engineer's treatment for the CO 2 issue is different from analytical chemists.…”

“…However, the aromatic recoveries are similar for carbon dioxide and ethane. Therefore, the adsorption caused by flow capacity inhibition is excluded or less potent than the molecular shape contribution on the basis of the judgment by Hawthorne et al 63 In other words, the conclusion is that the flow capacity of the "turtle"-shaped aromatics is much more inhibited than the "snake"-shaped alkanes, which is the primary reason why the aromatic has accumulated in the source rock of UBS and LBS. However, doubts exist about such conclusions.…”

“…Therefore, the conclusion is that more turtlelike aromatic portions reside in the source rocks that do not migrate into the more porous production zone, i.e., in the MB and Three Forks (Figure 8). 63 Besides, these formations use two kinds of solvents, CO 2 and ethane, for core extraction purposes. Through the use of GC, recovered crude oil differentiates the portion of C7−C10 nalkanes and aromatics with one ring, C10−C14 n-alkanes and aromatics with two rings, and C14−C18 n-alkanes and aromatics with three rings.…”

“…Theoretical development has already matured for the gas/crude oil phase behavior estimation system. , EOR performance should be estimated at the molecule or even atom level from analytical chemists’ perspectives to truly understand the mechanisms of CO 2 and other types of gas. , Three series of fundamental experiments are proposed: minimum miscibility pressure (MMP) measurements based on the standard of the vanishing interfacial tension between oil and gas; , the dissolving capacity using different kinds of solvent gas in the bulk phase without the porous media effect, i.e., the vaporizing mechanism, , and how the residue oil composition changes at the molecule level; and the extraction test with rigorous mass balance calculations using a natural core with initial oil saturations and GC analysis. , …”