Bakken Member Research Articles

Stress-Dependent Petrophysical Properties of the Bakken Unconventional Petroleum System: Insights from Elastic Wave Velocities and Permeability Measurements

The net-effective stress is a fundamental physical property that undergoes dynamic changes in response to variations in pore pressure during production and injection activities. Petrophysical properties, including porosity, permeability, and wave velocities, play a critical role and exhibit strong dependence on the mechanical stress state of the formation. The Williston basin’s Bakken Formation represents a significant reservoir of hydrocarbons within the United States. To investigate this formation, we extracted core plugs from three distinct Bakken members, namely Upper Bakken, Middle Bakken, and Lower Bakken. Subsequently, we conducted a series of measurements of ultrasonic compressional and shear wave velocities, as well as pulse decay permeabilities using nitrogen, under various confining pressures employing the Autolab-1500 apparatus. Our experimental observations revealed that the ultrasonic wave velocities and permeability display a significant sensitivity to stress changes. We investigated existing empirical relationships on velocity-effective stress, compressional-shear wave velocities, and permeability-effective stress, and proposed the best models and associated fitting parameters applicable to the current datasets. In conjunction with the acquired datasets, these models have considerable potential for use in time-lapse seismic monitoring and the study of production decline behavior. The best fitting models can be used to forecast the petrophysical and geomechanical property changes as the reservoir pore pressure is depleted due to the production, which is critical to the production forecast for unconventional reservoirs.

Effect of mineralogy on elastic properties of the Bakken formation: A lab scale rock physics modelling

The Bakken formation in the Williston Basin, is one of the largest tight oil reserves in the U.S.A. and is currently the second largest oil producer in the nation. The Bakken formation is divided into the upper and lower shale members which comprise the source rocks, while the middle Bakken, which is a fractured silicious-carbonate formation, is the main producer. The Bakken has a complex mineralogy, composed of mainly quartz and clay in the two shale members and quartz, calcite, dolomite, and anhydrite in the middle member, along with several other minerals with lower volume fractions. Past studies have reported the importance of mineral composition in development, completion, and production from shales.In this study, the effect of mineral composition of the three Bakken members on rock elastic and velocity properties are investigated. Rock physics models were developed at macro (core) scales from data in Dunn and Mountrail Counties. The minerals' volume fractions from core XRD data were used for estimation of rock stiffness and velocities. The calculations were performed for the upper and lower Bakken (UB, LB) shales as well as the clastic and carbonate intervals of the middle Bakken (MB). Hill averaging was used to mix the non-clay members. Clays were added using both Kuster-Toksoz (K-T) model and differential effective medium (DEM) theory. The effect of fluid phase (assuming both homogeneous and patchy fluids) was applied using K-T (Kuster and Toksöz, 1974) and Gassmann equations (Gassmann, 1951) for frequency-dependent velocity estimation. For shales, Backus averaging (Backus, 1962) captured the upper and lower bounds of the data, whereas the K-T model developed for parallel and perpendicular to the laminations, allowed accurate curve fits to the lab data by changing the pore's aspect ratios to capture the pores heterogeneity exhibited in Bakken. The results showed that, In overall, the UB and LB shales, due to the presence of clay minerals, show lower properties than the MB and composed mainly of microfracures (pores). The MB is shown to have more intraparticle pores with stiffer properties. The estimated stiffness and velocity properties are valuable input data for the development, completion and production planning from future wells.

Log scale rock physics modelling to study the effect of mineralogy on elastic properties of the Bakken Formation

The Bakken Formation is a geographically extensive, organic-rich, tight formation in the Williston Basin and currently serves as the second largest producer of oil in the USA. With respect to stratigraphy, the Devonian–Mississippian-age Bakken Formation typically comprises three members: the Upper (UB), Middle (MB), and Lower Bakken (LB). Lithologically, the upper and lower members of the Bakken are dominated by shales rich in organic-carbon, which act as the source rock for oil reservoirs in the Middle Bakken. The lithology of the Middle Bakken varies widely from siliciclastic to carbonates, with multiple distinct lithofacies typically being identified in the North Dakota portion of the Williston Basin. Good knowledge of the rock properties of the various Bakken members and lithofacies are a key component of their production potential.This study is a continuation of earlier rock physics modelling, which was done at core scale. Data from one well in Mountrail County was used for estimation of rock stiffness and velocities. Rock physics models were developed at log scale to obtain knowledge of formations' elastic properties across the Bakken members, as a function of porosity and pores' fluid type and geometry. Furthermore, the effect of frequency on fluid mixing type was investigated using both homogeneous and patchy models, then fluids were incorporated into the inclusion system using K-T, DEM, or Gassmann's equations. Kerogen was added to the system as a part of the inclusion in a way that fluid was embedded into the organic-matter. Critical porosity of 30% was also applied during DEM modeling to investigate the effects of kerogen being the load-bearing and rock matrix being suspended into it for case of high kerogen content in thermally immature source-rocks. The UB and LB shales, due to the presence of clay minerals, showed lower properties than MB and composed mainly of microfracture pores. Furthermore, the conversion of kerogen into hydrocarbons lowers the overall rock properties of the Bakken shales. The MB siliciclastic intervals showed to have more of intraparticle pores (primary porosity) with stiffer properties, whereas carbonate zones exhibited fracturing (secondary porosity). The rock physics models at macro and log scale complement each other and present a full picture of formations' elastic properties. The estimated stiffness and velocity properties are valuable input data for the development of and production from future wells.

Evaluation of CO2 enhanced oil recovery in unconventional reservoirs: Experimental parametric study in the Bakken

Previous CO2 enhanced oil recovery (EOR) pilot tests performed in the Bakken Formation indicated that cyclic CO2 injection might be a promising technique for enhanced oil recovery; however, no clear consensus has been reached, and reported results have revealed that CO2 EOR mechanisms in unconventional reservoirs are still poorly understood. This research investigates and analyzes the effects of different parameters on CO2 performance using samples from the Middle Bakken member and Three Forks Formation. The factors studied include sample size, the CO2 Huff-n-Puff (HnP) injection schedule, water presence within the fractures, and the volume of CO2 in contact with the rock matrix during the HnP experiments. Results indicated that reducing the sample size could overestimate the oil recovery factor and water presence could impede the CO2 potential to permeate the rock. Lowering the CO2 volume in contact with the tested sample resulted in lower oil recovery for the same cores. We also observed that cyclic injection outperforms CO2 flooding in fractured Middle Bakken and Three Forks samples. The obtained results provide a better understanding of CO2 EOR performance in tight formations and enlighten future pilot tests and experimental CO2 injection studies in unconventional reservoirs.

The Bakken Formation—understanding the sequence stratigraphic record of low-gradient sedimentary systems, shale depositional environments, and sea-level changes in an icehouse world

The Bakken Formation is a major petroleum producer in the continental US. However, its deposition in an intracratonic, low-gradient setting has often been mistakenly described as “layer-cake”. This contribution is designed to highlight the time-transgressive nature of its main petroleum-producer, the middle Bakken member. Correlation of individual parasequences reveal the subtle nature of otherwise invisible low-angle stratigraphic geometries. Sequence stratigraphically-relevant surfaces occur throughout the unit and subdivide the entire Bakken into 5 third-order sequences; one of them is a hidden sequence at the base of the petroleum-producing middle Bakken indicating both a lowstand and a subsequent transgression. The organic-rich shales above and below the middle Bakken were deposited in an oxygen-deficient environment and show several burrow/fecal string types and indications of active currents during deposition. The Bakken records high amplitude sea-level changes during sequences compared to relative low amplitude sea-level changes of parasequences. This, coupled with a likely mismatch in timing of Bakken deposition relative to world-wide ice-age-induced cyclicity makes it unlikely that the Bakken sea-level fluctuations were dominated by glaciation.

Integrated petrophysical evaluation of the Lower Middle Bakken Member in the Viewfield Pool, southeastern Saskatchewan, Canada

The Viewfield Pool of the Lower Middle Bakken Member, the largest light oil pool in Saskatchewan, Canada has been described as an unconventional resource play. Reservoir evaluation in this pool has been challenging because of low permeability, complex lithology and reservoir heterogeneity. To address the difficulties, this study employed an integrated approach to derive reservoir parameters and provided a detailed petrophysical characterization of the Viewfield Middle Bakken play, which led to a division of Unit A, the Lower Middle Bakken Member into two subunits: A1 for the lower interval and A2 for the upper interval. Petrophysical models were developed based on thorough analyses of core measurements, conventional well logs and available advanced nuclear magnetic resonance (NMR) well log data. The proposed models were applied to determine reservoir properties including shale content, porosity, permeability and water saturation for the two subunits in 163 vertical wells among which three have NMR logs. The results reveal that subunits A1 and A2 differ significantly in petrophysical properties and reservoir quality. A2, the very fine-grained dolomitic sandstone subunit has larger pore size, higher porosity and permeability, lower shale content and water saturation, and thus represents the preferred interval for oil production across the Viewfield Bakken Pool area and has greater resource potential than the siltstone subunit A1. The mapped petrophysical properties show spatial trends that align well with the previously recognized regional geological features. Favourable potential high productivity zones can be outlined by the areas of intersection of all essential reservoir parameters above defined threshold values.

Analytical Study Examines Production of Multistage Horizontal Wells in the Bakken

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191455-18IHFT-MS, “Twelve Years and 12,000 Multistage Horizontal Wells in the Bakken: How Is Industry Continuing To Increase the Cumulative Production per Well?” by C. Mark Pearson, SPE, Larry G. Griffin, SPE, Stacy L. Strickland, SPE, and Paul M. Weddle, SPE, Liberty Resources, prepared for the 2018 SPE International Hydraulic Fracturing Technology Conference and Exhibition, Muscat, Oman, 16–18•October. The paper has not been peer reviewed. Multistage horizontal well designs were first implemented in the Bakken in 2007. Since then, more than 12,000 wells have been completed in either the Middle Bakken or Three Forks zones. Early-time production rates, as measured by 180-day state-reported cumulative production, have increased fourfold during this period, during which industry has pursued a program of innovation and continuous improvement in completions technology, with production per well increasing in 10 of 12 years. Through a big data analytical study comparing geological data, completions parameters, and state-reported production results, the authors have evaluated the fundamental changes that have guided industry to these production rates during the period discussed. Geology The Bakken Formation is Mississippian and Devonian in age and consists of Upper, Middle, and Lower members. The focus of this paper is to review work in both the Middle Bakken member and the lower Three Forks intervals contained within the Bakken Petroleum System. The Upper and Lower Bakken shales are the prolific source rocks for the petroleum system. The Middle member consists of distinct lithofacies that range from silty sandstone on the east flank of the basin to silty dolomite on the west flank. Porosities in the Middle member range from 4–10% and permeabilities are generally less than 0.1 md. Fig. 1 shows the basin depth structure map with a color fill of the thermal maturity of the Upper Bakken shale. The onset of oil generation occurred at a pyrolysis temperature (Tmax) of greater than 430°F as indicated in the figure by shades of green and warmer colors. Oil generation in this thermally mature window created overpressure and resulted in oil mobilization into less- mature and immature regions toward the extremities of the basin. The Bakken is a basin-centered system and thus the increased pore pressure that occurred with the process of kerogen conversion and the generation of oil has resulted in pore pressure gradients as high as 0.9•psi/ft in the deepest part of the basin. Unsurprisingly, in the deepest part of the basin, water cut is the lowest, with increasing water cut with distance from the areas of greatest thermal•maturity.• The complete paper provides a detailed description of the Bakken development history, which the authors divide into seven periods beginning with first production in 1953 to the present. Production Bakken production has grown historically, but as recently as early 2004 it yielded only approximately 25,000 BOPD. The increase in drilling activity then drove production growth, with an initial peak in December 2014 of 1.22 million BOPD. The rapid decline in industry activity then resulted in a significant production drop. However, the decline reversed, and new production records of 1.23 and 1.24 million BOPD were set in June and July of 2018. The return to peak production levels has been driven by companies using more-effective•completions.

Regionally Correlated Oxygen and Carbon Isotope Zonation in Diagenetic Carbonates of the Bakken Formation

ABSTRACT Diagenetic minerals preserve records of burial processes that overprint records of seawater chemistry and impact reservoir porosity. The Mississippian-Devonian aged Bakken Formation in the Williston Basin is a reservoir rock of economic importance whose productivity is affected by diagenetic carbonates, particularly dolomite-ankerite-series carbonates. To investigate how diagenetic carbonate alteration manifests in the Bakken and how that might change across the basin, a combined δ18O and δ13C isotope dataset for diagenetic carbonates was collected by in situ SIMS analysis of 10-μm spots from nine drill holes covering a ∼250 km transect of the middle Bakken member. Observed core-to-rim isotopic variability in these small Fe-zoned dolomites and calcites frequently exceeds 10‰ in both δ18O and δ13C, indicating significant changes in thermal and chemical conditions during cementation. Individual ankeritic growth bands can be correlated across the basin by systematic similarities in minor-element compositions and isotope ratios. In the central part of the basin, δ18O and δ13C trends at sub-mm-scale decline consistently towards the rims of ankerite-series carbonates, which is interpreted to reflect mineral growth coincident with rising temperatures and an increasing organic contribution to inorganic carbon during burial. The most abrupt shifts in δ18O and δ13C (changes as large as 12‰ for δ18O and 6.5‰ for δ13C within distances

Strontium residual salt analyses (SrRSA) and geochemistry of Bakken Formation core samples from Fleckton 1-20, North Dakota

Samples of Bakken Formation core from the Fleckton 1-20 well in Ward County, North Dakota, were analyzed using the Strontium Residual Salt Analysis (SrRSA) method to assess pore-water communication among the upper, middle, and lower sections of the unit by analyzing 87Sr/86Sr in pore-water salts leached from the core. Major and trace element analyses were also conducted on bulk-rock samples and leachates (Elemental Residual Salt Analysis or ERSA). The middle part of the Bakken Formation, a calcareous and dolomitic siltstone to fine sandstone, is encased between two black shales—the upper and lower sections of the formation. These units are informal and herein named the upper, middle, and lower Bakken members. Strontium isotope measurements and concentration of some solutes in leachates indicate that pore water in the black shale of the upper Bakken member has remained isolated from pore water in the middle Bakken member except for within a few feet immediately near the contact where elemental profiles indicate diffusive mixing. The SrRSA 87Sr/86Sr values from the middle Bakken member are consistent with produced water collected from 28 wells in Montana and North Dakota. In contrast to the similarity in 87Sr/86Sr values, ratios of concentrations such as K/Rb, Ca/Sr, Ca/Mg, and Na/Cl are slightly different between the pore-water leachates and produced water values. The differences in K/Rb and Ca/Sr are probably due to selective adsorption of Rb leading to larger K/Rb ratios and minor dissolution of carbonate minerals leading to larger Ca/Sr ratios in the leachates.

Image analysis of the pore structures: An intensive study for Middle Bakken

Pores with sizes ranging from nanometers to micrometers are widely distributed in shale gas and shale oil formations. These pores are the sites for hydrocarbon accumulation and provide the flow paths for hydrocarbons during production. The Middle Bakken member is the main production zone of the Bakken Formation in North Dakota. In order to reveal the pore structures of the Middle Bakken, we employed field emission scanning electron microscopy (FE-SEM). After segmentation of the SEM images, we calculated the surface area and shape of the pores using image analysis and then quantified the complexity and heterogeneity of the pore structures by applying both fractal and multifractal analyses. Finally, we employed the fractal permeability model to estimate the permeability of the samples. The results showed that different pore types, such as interparticle and intraparticle pores exist in the Middle Bakken samples. Even under the same scale of the same sample, the pore parameters could be different. Sample 2 has the largest average porosity, followed by Sample 1 and Sample 3. The mean pore size of these samples is less than 31 nm indicating that the pores in Middle Bakken samples are very small. The pore structures in the Middle Bakken exhibited fractal and multifractal behavior. The fractal dimension from the entire size range of pores is the largest compared with the fractal dimension of the subdivided groups. The pore size distribution in Sample 2 is the most heterogeneous.

Oxygen Isotope Microanalysis By Secondary Ion Mass Spectrometry Suggests Continuous 300-million-year History of Calcite Cementation and Dolomitization in the Devonian Bakken Formation

Calcite cementation and dolomitization are key diagenetic processes in many sedimentary systems. Unravelling detailed histories and timescales of cementation and replacement is, however, often compromised by the limited spatial resolution of many analytical techniques; in some cases multiple grains are co-analyzed so that diagenetic histories are blurred and reaction periods are difficult to establish. In this study we have used 10-micrometer-resolution, in situ secondary ion mass spectrometry to determine the oxygen isotope composition of 197 individual, 10-50-micrometer-size crystals of dolomite and calcite from six samples in a single core of Upper Devonian Middle Bakken Member siltstones and sandstones, the major tight oil formation of the Williston Basin, USA. This amount of data places important constraints on the range of temperatures and times that carbonate cementation and replacement occurred. Petrographic data show that microcrystalline calcite cement is an early phase, and combined with mineralogical data suggest that much of the dolomite replaces calcite. Over spatial scales of less than a centimeter, analyses of individual calcite crystals have a range of 5‰ for 18O in the group of crystals, and for the group of individual dolomite crystals, 10‰. These sub-centimeter ranges are as great as those observed in previous studies of carbonate cements sampled over many meters and remind us that previous low-resolution studies may have inadvertently analyzed mixed phases. There is no relationship between dolomite texture and isotopic composition at this spatial scale; microscale backscattered electron imagery and scanning electron microscopy cathodoluminescence zoning is seen, but cannot be resolved with a 10 micrometer spot size. Assuming, since it is an early cement, that calcite precipitated from seawater (δ18O = -1.5‰), it formed at ca. 15-40°C, mainly at the lower temperatures. Present-day formation waters in Devonian rocks in this region have oxygen isotope compositions of 7-8‰ VSMOW. Using these values as a likely dolomitizing fluid, we suggest that dolomitization occurred continuously between 40 and 140°C over 150 – 200 million years, most likely in a fluid with a high Mg/Ca ratio resulting from gypsum formation in local evaporites. We suggest that this exceptionally low rate of dolomitization was controlled by the low rate of supply of Mg in a very sluggish flow regime; dolomitization is incomplete because of a limited supply of Mg.

Fault detection using principal component analysis of seismic attributes in the Bakken Formation, Williston Basin, North Dakota, USA

Seismic fault detection using principal component analysis (PCA) is an effective method for interpreting fault distribution and orientations in the Bakken Formation. The PCA fault attribute indicates significantly different, and geologically more plausible, 3D fault distributions than conventional seismic attributes, such as curvature. The PCA fault attribute has identified different fault patterns in the Upper, Middle, and Lower Bakken members and the Three Forks Formation. Two distinct fault trends in approximately 40°–50° northeast–southwest and 50°–60° northwest–southeast directions are observed in the Bakken Formation in the study area, and they are apparent on the strike and dip attributes derived from the PCA fault attribute. Fault cuts interpreted from missing well-log sections correlate well with the PCA fault attribute. Seismically derived fault orientations correlate with borehole image log data in the horizontal wells. Crossing conjugate faults observed on the fault dip attribute may result in the widening of the faulted area and localized thinning of the rock sequence where the faults intersect, and this could potentially enhance permeability along the fault strike.

Application of micro-FTIR mapping and SEM to study compositional heterogeneity of siltstones: Example from the Late Devonian-Early Mississippian Middle Bakken Member.

This paper explores the applicability of micro-FTIR mapping to study heterogeneity of organic matter-lean siltstones. Closely spaced samples of Late Devonian dolomitic siltstones of the Middle Bakken Member were analysed with micro-FTIR, powder X-ray diffraction, and scanning electron microscopy (SEM) to explore the distribution and chemical properties of organic matter (OM), muscovite/feldspar/clay group, carbonates, and quartz, and their influence on porosity and permeability of these rocks. Our results show that quartz is the dominant component of the samples, and the main mineralogical differences between the samples are reflected in the abundance of carbonate minerals. Organic matter content is usually far below 1 wt. % and dominantly represented by terrestrially derived vitrinite and inertinite. Micro-FTIR mapping demonstrates that the more spatially connected quartz and muscovite/feldspar/clays become, the larger permeability in the rock develops, and these correlations are especially strong for planes parallel to bedding. In contrast, carbonate connectivity shows a strong negative correlation with permeability. No correlations between connectivity of components and porosity have been detected. These observations suggest that micro-FTIR not only can document compositional heterogeneity of siltstones, but also has potential to help understanding their permeability systems.

The lost Devonian sequence - Sequence stratigraphy of the middle Bakken member, and the importance of clastic dykes in the lower Bakken member shale, North Dakota, USA

The Late Devonian to Early Mississippian Bakken Formation in the Williston basin of North Dakota, USA, shows a tri-partite subdivision: a middle mixed carbonate-siliciclastic member is sandwiched in-between two black siliciclastic mudstones, the lower and upper Bakken member shales. However, the transition from the lower shale member to the middle member does not represent a gradual coarsening but contains in places several millimeter - to centimeter-thick siliciclastic mudstones and carbonates that consist of three facies: (1) a glauconitic carbonate-rich siliciclastic mudstone, (2) a carbonate mud-to wackestone, and (3) an echinoderm wacke-to packstone with shell fragments. These three facies are present in many (all?) of the cores close and directly in the basin center in Mountrail County, North Dakota. At least one of these three facies is present in all 23 cores included in this study.This thin carbonate unit at the transition between the lower and the middle Bakken members is interpreted as representing the remnants of the transgressive systems tract. It is assumed that relative sea-level fell before deposition of the middle Bakken member establishing a proximal coarse-grained to distal fine-grained depositional transect that successively migrated into the basin. During the subsequent transgression, the siliciclastic input was low to absent, and the entire sedimentary system switched to depositing carbonates. The proximal to distal transect during this time showed coarse-grained packstones (and grainstones?) close to the shoreline, and a fining outwards towards the distal parts of the basin. This transgression also eroded what remained of the regressive and most of the subsequent transgressive sediments, leaving only the thin carbonate layer behind. Evidence for the regression, even though no sediment is directly preserved along the lower to middle Bakken member contact, comes from the fill of clastic dykes that cut through the lower Bakken member shale. The fill of the clastic dykes is partly siliciclastic and partly carbonate and not similar to any of the surrounding sediment. This indicates that these dykes must have originated before the middle Bakken member was deposited, yet the overlying sediment must have been carbonate at some point and siliciclastic another time. As it is not present anymore, this sediment must have been entirely removed by erosion.The here presented model suggests that the Bakken Formation reflects two entire sea-level oscillations. The first encompasses the lower Bakken member shale and the siliciclastic regressive portion of the lowstand only preserved as infill of the clastic dykes. The subsequent transgression deposited the carbonates now blanketing the lower to middle Bakken member transition, and the highstand and subsequent regression plus lowstand are represented by the middle Bakken member. The transgressive surface and therewith the onset of the topmost Bakken transgression is marked by the transition from the middle to the upper Bakken shale member.

Improving Oil Recovery by Use of Carbon Dioxide in the Bakken Unconventional System: A Laboratory Investigation

Summary Compared with a conventional reservoir, the ultralow permeability in the Bakken Formation makes it very challenging to perform normal waterflooding or gasflooding operations. “Permeability-jail” effects cause low injectivity and prevent injected fluids from sweeping oil out of the matrix efficiently. Two distinguishable flow regimes have been identified in fractured, hydrocarbon-rich shale formations: viscous flow in high-permeability fracture networks and diffusion-dominated flow in the low-permeability matrix with high oil saturation. Improving hydrocarbon transport (and technically recoverable resources) in unconventional reservoirs relies on our ability to enhance diffusion-dominated flow from the oil-saturated matrix to the natural- or induced-fracture network, which is the focus of this study. To unlock the unproduced Bakken and Three Forks oil, high-pressure carbon dioxide (CO2) may be used to enhance the diffusion-dominated flow in the matrix and keep the viscous flow in the fractures under reservoir temperature and pressure conditions (e.g., 230°F and 5,000 psi). Core samples were collected from two Bakken wells, including all oil-bearing intervals: Upper Bakken (UB), Middle Bakken (MB), and Lower Bakken (LB) Members and the Three Forks (TF) Formation. Detailed core analyses were performed to measure petrophysical properties and characterize these units. Ten samples were selected for pore-size-distribution measurement and 21 samples (11-mm-diameter rods) were used for 24-hour CO2 exposures and hydrocarbon-recovery experiments. These experiments were conducted as CO2 “bathing” at reservoir conditions (rather than “flow through” tests) and were aimed at increasing our understanding of the microstructure and diffusion-dominated-flow ability within these tight geologic formations. CO2-exposure and hydrocarbon-extraction experimental results clearly showed the improvement of diffusion-dominated flow in all the Bakken members. The UB and LB samples, characterized by generally high total-organic-carbon (TOC) content (10–15 wt%) and small pore size (approximately 3–7 nm), yielded approximately 60% of the present mature hydrocarbon at the end of the 24-hour exposure. The MB and TF samples, characterized by lower TOC content (<0.5 wt.%) and moderate pore size (approximately 8–80 nm), provided more-favorable flow conditions for CO2 and hydrocarbons, yielding approximately 90% of the mature-hydrocarbon content. Because all experiments were conducted at reservoir conditions, the results demonstrate that diffusion plays a significant role in the mobilization of oil in tight reservoirs. CO2 greatly enhances the diffusion process to improve hydrocarbon transport in the tight matrix. This observation is especially useful for densely fractured shale-oil formations (high surface-area/volume ratio) where CO2 has greater areal contact with the reservoir, enabling CO2 diffusion into the matrix and hydrocarbon diffusion out of the matrix to occur more efficiently (increasing recoverable reserves), and where the fracture networks assist in alleviating potential injectivity challenges.

Production Performance in the In-Fill Development of Unconventional Resources

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 175963, “Production Performance in the In-Fill Development of Unconventional Resources,” by Bilu V. Cherian, Sanjel; Matthew McCleary, Samuel Fluckiger, Nathan Nieswiadomy, Brent Bundy, and Sarah Edwards, SPE, SM Energy; and Rafif Rifia, Kristina Kublik, Santhosh Narasimhan, James Gray, Olubiyi Olaoye, and Hamza Shaikh, Sanjel, prepared for the 2015 SPE/CSUR Unconventional Resources Conference, Calgary, 20–22 October. The paper has not been peer reviewed. Data now show that the behavior of unconventional wells to in-fill drilling varies significantly across basins. A key influence may be changes in pore pressure and saturation (saturation history). This paper presents results from the analysis of the effect of in-fill drilling on parent-well performance, and describes a simplistic approach to understanding the effect of the quest for operational efficiencies and economic cycles on development strategies. Methodology This study focuses on two unconventional plays, the Eagle Ford and the Bakken. The objective was to model the well performance of the parent wells with the aim of matching and predicting in-fill-well performance. Because the two assets are at two significantly different portions of the development cycle (Eagle Ford is very early in its cycle), the Bakken data set has the luxury of modeling and matching the performance of the parent and in-fill, whereas the Eagle Ford portion of this study focuses on forward modeling and optimizing in-fillwell completions. The methodologies used in this study (parent-well modeling, petrophysical models, geomechanics, fracture modeling, production modeling, and in-fillwell/ depletion modeling) are discussed in detail in the complete paper. Bakken System Parent-Well Modeling Petrophysics. The Middle Bakken member was divided into three main facies. Petrophysical evaluation indicates that average porosity and average water saturation were 8 and 50%, respectively, for the Middle Bakken. Average Klinkenberg permeability for the entire Middle Bakken is approximately 0.02 md. Mercury-injection capillary pressure curves indicate that irreducible water saturation is between 30 and 40% for rock with porosity between 1 and 7%. Residual oil is between 30 and 40%. Accordingly, the moveable fluid is low (20–40%), and hydraulic fracturing was recommended to stimulate more production. The Three Forks formation was divided into five facies. Petrophysical evaluation suggests that the upper part of the Three Forks, Facies TF 23, has oil potential. Most facies are strongly affected by calcite and dolomite diagenesis, which allows alternating porosity development in some cases. In this area, the Middle and Lower Three Forks have higher water saturation, with very-low-permeability streaks (less than 0.007 md).

Brittleness investigation of producing units in Three Forks and bakken formations, williston basin

Brittleness is a key factor in identifying intervals and areas for fracking in tight oil and gas reservoirs. The brittleness index of reservoir rocks can be calculated by elastic properties, but laboratory measurement of elastic properties is usually expensive and time consuming. In this paper, a method is proposed to predict the brittleness from known mineralogy and applied to assess the upper Three Forks formation in the Williston basin. In this method, a correlation between the elastic properties of the formation rock and mineralogy was first established based on rule of mixture. This correlation was verified by the measured elastic properties of the middle Bakken member. Then the correlating approach was used to predict the elastic property-based brittleness index of the upper Three Forks formation. The results indicate that the upper Three Forks formation should be conducive to reservoir stimulation as its predicted brittleness index ranges from 53 to 67 in the direction parallel to the bedding plane and 58 to 70 in the direction perpendicular to the bedding plane. Its brittleness is comparable to that of the middle Bakken member, which has been already successfully stimulated. The correlating method presented in this paper is also applicable to other major tight reservoirs.

Three-dimensional modeling study of the low-permeability petroleum system of the Bakken Formation

The Bakken Formation of the Williston Basin is a prime example of an unconventionally produced petroleum system, with a low-permeability reservoir that requires application of advanced technologies for commercial production. A three-dimensional model of the Williston Basin was constructed to integrate and assess the parameters that influence the generation and migration of hydrocarbons in the Bakken Formation. This study tests the applicability of available basin and petroleum system modeling technology on an unusually low-permeability petroleum system. The model is based on nine surfaces constructed from log tops of thousands of wells in the study area and additional depth and isopach maps of the Bakken Formation members. These were integrated with the established basin evolution in line with published research. Temperature and thermal maturity were calibrated during model construction from well temperature and geochemistry data. The resulting heat-flow map supports the existence of a heat-flow anomaly along longitude 103W, discussed controversially in the literature. Furthermore, the results indicate that the invasion-percolation migration approach best describes the distribution of petroleum accumulation and the saturated areas in the Bakken members. The volume of generated hydrocarbons was calculated, and the extent of the highly saturated accumulation beyond the area of the high-mature source was mapped. Furthermore, it was demonstrated that petroleum accumulations beyond the high-saturation zones have to be related to stratigraphic pinch-outs, lateral variability in permeability of the Bakken members, or smaller structural influences that were lost because of the resolution applied in the model.

Late Devonian and Early Mississippian Bakken and Exshaw Black Shale Source Rocks, Western Canada Sedimentary Basin: A Sequence Stratigraphic Interpretation

The Late Devonian and Early Mississippian Bakken and Exshaw formations are a continuum of regionally correlated, organic-rich (up to 35% total organic carbon), black shale source rocks covering much of the Western Canada sedimentary basin. The Bakken Formation is composed of (1) a black mudstone lower member, (2) a gray mudstone/sandstone middle member, and (3) a black mudstone upper member. The Exshaw Formation, beneath the Alberta Plains and in exposures in the Foothills and Front Ranges of the Rocky Mountains, is composed of (1) a lower black shale member and (2) an upper siltstone member. The basal black shale unit of the Lower Mississippian Banff Formation, overlying the Exshaw Formation, is a second organic-rich interval. These black shales are regionally significant hydrocarbon source rocks and local reservoirs. The middle Bakken member is a locally important reservoir rock with substantial economic potential. The Bakken and Exshaw formations and the basal Banff black shale are divisible into three systems tracts: (1) a transgressive systems tract, (2) a lowstand systems tract, and (3) a second transgressive systems tract. Lodgepole and Banff formation carbonates, overlying the Bakken and Exshaw formations, are part of a highstand systems tract. A sequence boundary occurs between the lower and middle Bakken members. The conformable equivalent of this sequence boundary is within the Exshaw black shale member. Variations in the internal composition of these systems tracts imply that two depocenters, (1) the Williston basin and (2) the Prophet trough and the western margin of the North American craton, were affected differently by relative sea level rise and fall during Bakken and Exshaw deposition because of differences in water depth and sediment accommodation. Spatial and temporal changes in black shale and gray mudstone/sandstone, as highlighted by this sequence stratigraphic interpretation, may have significant impacts on source rock potential and hydrocarbon reservoir size, location, and quality.