Article pubs.acs.org/EF
Tracing the Impact of Fluid Retention on Bulk Petroleum Properties Using Nitrogen-Containing Compounds Nicolaj Mahlstedt,*,† Brian Horsfield,† Heinz Wilkes,‡ and Stefanie Poetz† †
GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany Carl von Ossietzky University Oldenburg, Institute for Chemistry and Biology of the Marine Environment, Carl-von-Ossietzky-Straße 9-11, 26129 Oldenburg, Germany
‡
ABSTRACT: Predicting gas-to-oil ratio (GOR) and bulk petroleum properties is of paramount concern when developing production strategies for unconventional resource plays. Subtle changes in bulk fluid composition can result in large differences in phase envelope geometry and predicted fluid types, which can cause unfavorable pressure drawdown during production. Here we present new insights into the thermal evolution of petroleum compositions in conventional and unconventional reservoirs, focusing on polar compounds, i.e., nitrogen-, sulfur-, and oxygen-containing (NSO) compounds. These compounds feature functional groups that strongly influence the sorption, solubility, and partitioning behavior of petroleum constituents in unconventional shale systems. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is a powerful tool to rapidly characterize NSOcompounds in complex mixtures and was used to compare the polar compounds in (1) extracts of six Posidonia Shale source rock samples with maturity levels between 0.48 and 1.45% vitrinite reflectance (Ro), (2) open-system pyrolysates of those six source rocks, and (3) four Posidonia Shale-sourced medium-gravity conventional crude oils. The aromaticity and degree of condensation were found to increase much more pronouncedly with increasing maturity for retained than for expelled oil NSO compounds. Pyrolysate NSO compounds have compositions intermediate between those of retained and expelled NSO compounds, pointing to preferential expulsion of smaller compounds in the crudes and enhanced cyclization and aromatization within retained fluids. Aromatization was shown to occur at the expense of aliphatic carbon. A genetic link between the fluids as well as the likely timing of petroleum expulsion is revealed by comparing the carbon number distributions of compounds from the N1 elemental class, that is, compounds that contain one nitrogen atom (e.g., carbazoles). The here documented chemical differences between the investigated fluid types are significant and need to be taken into account when formulating production strategies.
1. INTRODUCTION “If you want to understand function, study structure” is the axiom of Francis Crick,1 who discovered the double helix structure of DNA together with his colleagues James Watson and Rosalind Franklin. Mullins2 describes this quote as motivation to decipher the petroleomethe analogue of the genomeof crude oil. According to the author, the complete listing and quantification of all chemical constituents within a given crude oil would allow prediction of its physical properties and thus its behavior (e.g., phase separation) under various PVT conditions. Although many technical and practical hurdles remain, the main prerequisite equipment is now in place, especially since the development of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). FT-ICR MS extends the analytical window for complex fluid mixtures up to >10 000 Da, while achieving resolving power in excess of one million. As discussed by Marshall and Rodgers,3 the opportunity now exists to comprehensively assess the chemical composition of the least studied bulk fraction of different petroleum types, i.e., high molecular-weight, medium to high polarity NSO compounds. It is well-known that the structural diversity of these compounds, including very large species in the asphaltene and resin fractions, causes significant variability in the physicochemical properties of fluids within the source−carrier−reservoir system.4,5 Nevertheless, to date, only very few non-hydrocarbons have been used as proxies to assess petroleum origin or processes such as expulsion and migration © 2016 American Chemical Society
fractionation, water washing, or biodegradation. This is due to limitations associated with the analytical window of the most common analytical method, gas chromatography coupled with mass spectrometry (GC-MS). For example, nitrogen-containing alkylcarbazoles and alkylbenzocarbazoles have been used as indicators for migration distance6−8 or maturity.9−11 Oxygencontaining aromatic carbonyl compounds12 and carboxylic acids,13,14 as well as alkylated thioaromatics,15,16 have been employed to assess thermal maturity in petroleum systems. In contrast, Orr17 assessed relationships between sulfur-rich Monterey oils and related kerogen and asphaltenes on a rather broad geochemical basis only using elemental composition (C, H, O, N, S) and atomic ratios. The purpose of the present paper is to trace the compositional pathways taken by polar compounds during conventional and unconventional petroleum formation using FT-ICR MS, and thence to infer changing fluid properties. The role played by source rock kerogen type and maturity in controlling the physical properties of fluids expelled into conventional reservoirs is already largely established. For example, excellent reconstructions of natural fluid composition have been demonstrated in the Sonda de Campeche, Mexico,18 Snorre Field, North Sea Viking Graben,19,20 Reconcavo Basin, Brazil,21 and Received: April 25, 2016 Revised: June 30, 2016 Published: July 27, 2016 6290
DOI: 10.1021/acs.energyfuels.6b00994 Energy Fuels 2016, 30, 6290−6305
Article
Energy & Fuels Jeanne d’Arc Basin offshore Newfoundland.22 Establishing genetic relationships between kerogen moieties and major compound classes in petroleum is the basis for those compositional modeling studies. In contrast, the effects of retention on the chemical composition of fluids sorbed on organic matter and minerals, or stored as a free phase in the pore structure of mudstones, are less well understood, largely because of the difficulty in resolving the nature of the polar compounds. What is already known, at least since Brenneman and Smith Jr.’s work,23 is that source rock bitumen is much richer in polar compounds, occurring largely in the asphaltene and resin fractions, than is oil in conventional reservoirs, which is enriched in saturated hydrocarbons.5 This compositional difference is generally accepted to be a consequence of expulsion fractionation, i.e., chemical fractionation due to the polarity of compounds.24,25 For instance, NSO-containing compounds have functional groups that strongly enhance sorption, in contrast to saturated hydrocarbons. Thus, Leythaeuser et al.25 proposed a preferential expulsion sequence (saturated hydrocarbons > aromatic hydrocarbons > polar compounds), which was later experimentally reproduced26,27 and theoretically modeled.28,29 Interestingly, intraformational migration over only a few centimeters within the source rock itself also seems to bring about fractionation and enrichment of saturated hydrocarbons within the migrated fluids.30,31 Not only does the bulk composition of unconventional and conventional fluids differ, i.e., saturates, aromatics, resins, and asphaltenes (SARA), but the molecular composition within those fractions also differs. Preferential expulsion of smaller rather than larger compounds has been documented for conventional settings,32,33 although the phenomenon is not always readily discernible.34,35 Small differences in polarity and shielding of functional groups within differently shaped carbazole and benzocarbazole species might also cause expulsion fractionation.8,36 The ratios of those compound groups can be applied to assess oil migration distances when maturity and source facies effects are well-constrained. In the present paper we place the focus on unravelling compositional differences between retained and expelled fluids and on establishing relationships between precursors and products. In order to decipher the effects of expulsion fractionation/ retention on petroleum in conventional and unconventional reservoirs, we contrast the evolving polar compound compositions of extracts37 and pyrolysates of six Posidonia Shale samples with those of four related natural crude oils using FT-ICR MS with electrospray ionization (ESI) in the negative ion mode (−). ESI-(−) FT-ICR MS covers a much wider molecular mass range up to m/z 2000 than GC-MS, while retaining sensitivity in the lower mass range. It has already been used to characterize the NSO fraction in various petroleum fluids,38−49 i.e., acidic constituents that are able to deprotonate,
including carboxylic acids, phenolic compounds, and pyrroles. Poetz et al.37 have already proposed reaction pathways to account for changes in the acidic NSO compounds fraction of the Posidonia Shale extracts with increasing maturity (0.48− 1.45% Ro). Building on their work, the use of open-system pyrolysates is the key to defining the geochemistry of the first-formed petroleum, upon which all further processes within the source−carrier−reservoir system act. We consider how the compositional differences we have observed might influence the physical properties of petroleum fluids, and we discuss how compositional similarities at various maturity levels might be used to assess the timing of petroleum expulsion.
2. METHODS AND SAMPLES 2.1. Samples. The Posidonia Shale in the Hils Syncline has been the subject of many publications on petroleum formation over the last 30 years.50,51 The most important reported bulk source rock characteristics are summarized in Table 1. Total organic carbon contents (TOC) and hydrogen index values (HI) decrease with increasing maturity from 12.2% to 6.9% and 663 to 77 mg HC/g TOC, respectively. Vitrinite reflectance values (Ro) increase from 0.48% to 1.45%, which causes increasing Tmax values (432 to 457 °C). Although facies variability is relatively small for a given maturity, we have chosen to analyze the same six Posidonia Shale (Lias ε) samples from wells Wenzen (WE), Wickensen (WI), Dielmissen (DI), Dohnsen (DO), Harderode (HAR), and Haddessen (HAD) described in Poetz et al.37 for the sake of continuity. Open-system pyrolysates were prepared from each sample using crushed and finely ground, unextracted core material and a modified Quantum MSSV-2 Thermal Analyzer©. The temperature controlled Glass Lined Tubing (GLT) between the pyrolyser and the GC column was replaced by a simple, unheated glass-liner to trap higher molecular-weight pyrolysis products. The condensed pyrolysate was flushed from the glass liner using 2 mL of dichloromethane. Four Posidonia Shale-sourced medium gravity conventional crude oils from The Netherlands were provided by Erdem Idiz (Shell). The distance of ca. 400 km between source rock samples and crude oils does not pose a problem as the Posidonia Shale has a homogeneous facies across Western Europe, comprising Type II kerogen deposited in an epicontinental sea of moderate depth in a restricted environment with prevailing anoxic conditions.52−56 2.2. Sample Preparation for FT-ICR MS Analysis. Sample preparation for FT-ICR MS analysis, mass calibration and data analysis were performed as described in Poetz et al.37 A stock solution with a concentration of 1 mg/mL in methanol and toluene (v/v = 1:1) was prepared for each sample, i.e., the pyrolysates, the six rock extracts and the four crude oils. Each stock solution was diluted with the same solvent mixture to give a final concentration of 100 μg/mL. A concentrated aqueous NH3 solution (10 μL) was added to 1 mL sample solution to facilitate the deprotonation of the sample constituents. 2.3. FT-ICR MS Analysis. Mass analyses were performed in negative ion ESI mode with a 12 T FT-ICR mass spectrometer equipped with an Apollo II ESI source, both from Bruker Daltonik GmbH (Bremen, Germany). Nitrogen was used as drying gas at a flow
Table 1. Bulk Source Rock Characteristics of Posidonia Shale Samples from Six Well Locations in the Hils Syncline, Northern Germanya Well Location abbreviation Ro [%] Tmax [°C] HI [mg HC/g TOC] TOC [%]
Wenzen WE 0.48 423 663 12.2
Wickensen WI 0.53 427 617 13.4
Dielmissen DI 0.68 438 574 8.1
Dohnsen DO 0.73 445 429 9.4
Harderode HAR 0.88 444 363 6.3
Haddessen HAD 1.45 457 77 6.9
a
Data are average values for a different number of samples per well from Rullkötter et al.51 (WI, DI, HAR, HAD) and Wilkes et al.11 (WE, DO). Ro: measured vitrinite reflectance in oil. Tmax: temperature of maximum hydrocarbon generation during Rock−Eval pyrolysis. TOC: total organic carbon. HI: Hydrogen Index = S2 * 100/TOC. 6291
DOI: 10.1021/acs.energyfuels.6b00994 Energy Fuels 2016, 30, 6290−6305
Article
Energy & Fuels
Table 2. General Characteristics of the ESI-(−) FT-ICR Mass Spectra for Posidonia Shale Extracts and Pyrolysates and Posidonia Shale-Sourced Crude Oilsa
a
Fluid
Well Abbr.
Sample Nr.
Ro [%]
no. signals total
Mn total
Mw total
Ox TMIA [%]
Ny TMIA [%]
OxNy TMIA [%]
Sz-containing TMIA [%]
other TMIA [%]
Extract Extract Extract Extract Extract Extract Pyrolysate Pyrolysate Pyrolysate Pyrolysate Pyrolysate Pyrolysate Crude Crude Crude Crude
WE WI DI DO HAR HAD WE WI DI DO HAR HAD 42 43 44 45
G000420 G007144 G000427 G000445 G007042 G007085 G000420 G007144 G000427 G000445 G007042 G007085 G013742 G013743 G013744 G013745
0.48 0.53 0.68 0.73 0.88 1.45 0.48 0.53 0.68 0.73 0.88 1.45
4635 3945 3164 3234 2759 2307 3050 3253 2424 2049 1584 254 1093 1242 1516 1233
382 366 385 392 380 374 363 385 397 385 367 316 406 412 411 416
407 387 412 419 401 392 380 401 415 404 385 330 425 432 434 438
49.7 37.8 29.0 30.0 8.3 11.8 36.0 32.9 13.3 16.1 14.2 55.4 43.4 41.7 40.4 39.0
1.9 10.4 37.4 31.9 48.3 64.6 18.5 27.8 65.3 69.2 72.3 30.0 36.9 40.5 41.4 43.9
30.1 40.4 25.9 31.4 34.4 14.7 25.7 18.4 6.2 4.1 3.5 0.8 9.9 6.6 9.0 4.8
18.2 11.3 7.2 6.7 8.9 8.8 19.8 19.5 14.8 9.8 8.3 4.5 8.4 8.3 6.5 10.5
0.0 0.0 0.4 0.2 0.1 0.1 0.0 1.4 0.4 0.7 1.6 9.3 1.3 2.9 0.3 1.8
Mn: number-average molecular weight. Mw: weight-average molecular weight. TMIA: total monoisotopic ion abundance.
rate of 4.0 L/min and a temperature of 220 °C and as nebulizing gas at 1.4 bar. The sample solutions were infused at a flow rate of 150 μL/h. The capillary voltage was set to −3000 V and an additional collisioninduced dissociation voltage of 70 V in the source was applied to avoid cluster and adduct formation. Ions were accumulated in the collision cell for 0.05 s and transferred to the ICR cell within 1 ms. Spectra were recorded in broadband mode using 4 megaword data sets. For each mass spectrum, 200 scans were accumulated in a mass range from m/z 147 to 1000. Sine-bell apodization was applied before the Fourier transformation was used to produce the frequency domain data, which were then converted to the final mass spectrum. 2.4. Mass Calibration and Data Analysis. An external calibration was done using an in-house calibration mixture for ESI negative mode containing fatty acids and modified polyethylene glycols.57 Subsequently, each mass spectrum was internally recalibrated using the most prominent peaks. For example, saturated fatty acids were used for extracts and pyrolysates of samples of lowest maturity, whereas homologues series of different carbazole derivatives were used for the crude oils as well as extracts and pyrolysates of shale samples with maturity >0.53% Ro. A quadratic calibration mode was chosen for all samples. The root-mean-square (RMS) errors of the six calibrations were between 0.001 and 0.031 ppm. Elemental formulas were assigned to the recalibrated m/z values with a maximum error of 0.5 ppm allowing 0−100 C, 0−200 H, 0−10 O, 0−4 N, 0−2 S and 0−1 Na atoms. Only single charged species were found in the spectra; therefore, m/z and mass are used synonymously in this paper. Data evaluation was done using the software packages Data Analysis 4.0 SP5 (Bruker Daltonik GmbH, Germany) and Excel 2010 (Microsoft Corporation, Redmont, WA). Elemental formulas were sorted according to the type of heteroatoms (elemental class), their number of heteroatoms (compound class), and the number of double bond equivalents (DBE class). The DBE is a measure of the degree of unsaturation (or hydrogen deficiency) in a molecule and expresses the number of double bonds (with at least one carbon atom) and rings. The DBE is calculated for a molecule containing carbon, hydrogen, oxygen, sulfur, nitrogen and monovalent halogens CcHhOoNnSsXx according to the formula DBE = c − 0.5h + 0.5n − 0.5x + 1. Note that the numbers of oxygen and sulfur atoms are not included in the formula.
3000 to ∼1500 at Ro = 0.88% and to
