Use of geostatistical modeling to improve the understanding of permeability upscaling in isotropic and anisotropic burrowed reservoirs

doi:10.1016/j.marpetgeo.2021.105067

Marine and Petroleum Geology

Volume 129, July 2021, 105067

https://doi.org/10.1016/j.marpetgeo.2021.105067 Get rights and content

Highlights

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Computer models (75 models) of different burrow morphology and range of bioturbation intensity.
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The models provide a framework to investigate how burrow connectivity affect permeability anisotropy.
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Examples to select an averaging method for permeability in upscaling burrow-related reservoirs.

Abstract

Coarse materials filling burrows in reservoirs can provide permeable pathways through an otherwise impermeable host rock matrix —thus enhancing fluid flow properties. However, understanding permeability anisotropy and permeability-upscaling under such conditions represents a challenge in the exploration and development of reservoirs. Herein, we examined 75 hypothetical models that represent diverse types and amounts of bioturbation. The aim is to understand the impacts of burrows on petrophysical characteristics, including upscaling of permeability where the matrix is isotropic. These models were simulated in a high-resolution 1-m³ grid (8 × 10⁶ cells) using multipoint statistics (MPS) modeling techniques to systematically vary burrow morphology and bioturbation intensity in the same host rock matrix. The modeled burrow morphologies (vertical, horizontal, and boxwork morphologies) and bioturbation intensity―expressed as burrow percentage (BP, ranging from 2% to 50%)― show burrow connectivity trends increasing with increasing BP. In each trend, there is a burrow-morphology-dependent critical BP at which burrows develop connected permeability pathways. Accordingly, three unique patterns of burrow connectivity were identified among the 75 models. These include when burrows are: 1) just isolated volumes (isotropic system); 2) create unidirectional flow (anisotropic system); and 3) create omnidirectional flow (isotropic system). These connectivity patterns determine whether the permeability of the burrow-related reservoir is isotropic or anisotropic. Thus, identifying each of these connectivity patterns in burrowed reservoirs would help determine the appropriate averaging method to upscale permeability. The results of this study bridge an essential gap in reservoir modeling and simulation of hydrocarbon reservoirs and aquifers with burrowed strata.

Introduction

Burrowed strata occur worldwide and are associated with numerous hydrocarbon reservoirs and aquifers (e.g., Gingras et al., 1999; 2004; 2012; Cunningham, 2004; Cunningham et al., 2009; 2012; Hovikoski et al., 2007; Volkenborn et al., 2007; Tonkin et al., 2010; Baniak et al., 2013; 2014; 2015; La Croix et al., 2013). The reservoir quality in burrowed strata is mainly controlled by burrow fillings and their connections to one another (e.g., Gingras et al., 1999; Pemberton and Gingras, 2005; La Croix et al., 2012; Baniak et al., 2013, 2014, 2015; Eltom et al., 2019; Eltom et al., 2019). Thus, understanding the flow properties of bioturbated reservoirs requires consistent prediction of the spatial distribution of burrows and their effect on petrophysical properties.

Previous studies indicated that the degree of contrast between the permeabilities of the host rock matrix (matrix) and burrow filling is an important factor that determines the overall petrophysical characteristics and flow behavior in bioturbated strata (Gingras et al., 1999, 2004; Pemberton and Gingras, 2005; Baniak et al., 2013, 2014, 2015). Accordingly, these studies introduced the concept of dual-permeability and dual-porosity fluid flow media (D-PermFFM, and D-PorFFM, respectively) for bioturbated strata. The terms D-PermFFM and D-PorFFM are used in ichnological studies. They are fundamentally different from the previously understood dual-permeability in which matrix and fractures contribute to fluid flow (Baniak et al., 2015).

Based on previous ichnological studies (e.g., Gingras et al., 1999, 2004), D-PermFFM exhibits a high permeability contrast between matrix and burrow filling (experimentally estimated to be a difference of three orders of magnitude, whereas D-PorFFM exhibits a lower permeability contrast between burrow matrix and filling (experimentally estimated as a difference of one order of magnitude, Gingras et al., 1999, 2004). These two end-member conceptual models of bioturbated strata have served as guidance for permeability upscaling in burrow-related reservoirs (Gingras et al., 2012; Baniak et al., 2013, 2014, 2015; La Croix et al., 2013; Eltom et al., 2019; Eltom 2020).

The impact of burrow connectivity on permeability anisotropy in such burrow-related reservoirs remains poorly investigated. In particular, it remains uncertain what appropriate permeability-upscaling method (geometric, harmonic, or arithmetic) can be used for these types of reservoirs, considering that these reservoirs exhibit permeability anisotropy or isotropy. Answering such questions will provide guidelines for the characterization and modeling of hydrocarbon reservoirs and aquifers in bioturbated strata.

In this context, the objective of this study is to explore the connectivity pattern of burrows with different morphologies to allow accurate prediction of the upscaled permeability of burrow-related reservoirs. To achieve this objective, this study tests the hypotheses that burrow connectivity varies with burrow morphology and bioturbation intensity. If correct, the upscaled permeability (which is a function of burrow connectivity) depends on these two variables (bioturbation intensity and burrow morphology). The results suggest that constraining the upscaling process of permeability with the burrow morphology, bioturbation intensity, and matrix properties provides reservoir modeling guidelines for bioturbated reservoirs.

Section snippets

Methods

To test the hypotheses of this study, three variables were modeled: 1) burrow morphology (three end-member burrow morphologies― boxwork, vertical and horizontal); 2) bioturbation intensity expressed as burrow volume percentage (BP) (25 intensities, ranging from 2% to 50%); and 3) matrix properties held consistent for all models. These variables were simulated in a high-resolution 1-m³ geocellular grid (8 × 10⁶ cells), resulting in 75 models. Details about the construction of these 75 models are

Variation of burrow connectivity with BP and morphology

In the MPS models, boxwork, vertical and horizontal burrows (Thalassinoides, Skolithos, and Planolites, respectively; Fig. 1) show various changes in connectivity when BP is increased (Fig. 2, Fig. 3, Fig. 4, Fig. 5). In each MPS model we progressively increase BP and track the change in the volume of the largest connected burrow volume (LCBV) and the second-largest connected burrow volume (SLCBV) to help understand connectivity patterns of the burrows (Fig. 2, Fig. 3, Fig. 4).

In runs with low

Permeability anisotropy in bioturbated strata

Unlike the permeability in bedded reservoirs, the permeability in burrowed strata are not easy to predict (Baniak et al., 2013, 2014; Ben-Awuah and Padmanabhan, 2014). In part because of the various factors that control the distribution of the burrow porosity, such as the burrow morphology, burrow filling, and bioturbation intensity, among others. Hence, direct lab measurements of the permeability of bioturbated strata provide little insight into the underlying relationship between the pore

Conclusions

A total of 75 MPS models with different BPs and burrow morphologies allowed assessing the impact of burrow connectivity on permeability upscaling in burrow-related reservoirs. These models assume that the burrow filling is more porous than the matrix and that the burrow filling provides more permeable flow pathways in an otherwise less permeable medium. Analysis of the results revealed how burrow connectivity increases with increasing burrow percentage. Accordingly, this study defined critical

Credit author statement

Hassan A. Eltom: Conceptualization, Investigation, Methodology, Writing – original draft, Supervision, Data curation, Funding acquisition, Project administration, Resources, Validation, Visualization, Writing – review & editing, Robert H. Goldstein: Conceptualization, Investigation, Methodology, Writing – original draft, Supervision, Data curation, Funding acquisition, Project administration, Resources, Validation, Visualization, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study is part of a research project (SF19031) funded by the College of Petroleum Engineering and Geosciences at King Fahd University of Petroleum and Minerals, Saudi Arabia. The authors would like to thank Dr. Luis Buatois, Associate Editor, Dr. Greg Baniak and the anonymous reviewer for their comments to enhance the scientific content of this article. Planolites specimen was provided by the KU Division of Invertebrate Paleontology. We thank Bruce Lieberman (Curator) and Natalia Lopez

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This study investigates the potential of bioturbated strata within the Jubaila Formation in Saudi Arabia to serve as water aquifers with exceptional storage and flow capacities. Additionally, it explores scenarios related to remediating oil as a possible contaminant in these aquifers. The study employs multipoint statistics modeling to model the burrow attributes of the dominant burrow, Thalassinoides, aiming to understand how the burrow network impacts the flow capacity of the water aquifer in these strata. The modeling results demonstrate that Thalassinoides can generate a connected burrow network responsible for permeability that may exceeds 3.5 Darcey, indicating potential for super permeable aquifers. The study also modeled a hypothetical aquifer for the bioturbated strata in the Jubaila Formation and conducted fluid flow simulation to understand the behavior of oil as a contaminant in such highly permeable strata. The simulation results suggest challenges in extracting oil from aquifers with such permeability, as more than 50% of the oil remains in the aquifer despite the simulation being run for ten times the duration it took for a water breakthrough occurred. This study provides valuable insights into the potential for bioturbated strata in carbonate succession to form water aquifers with exceptional storage and flow capacities. The study also highlighted the challenges associated with remediating oil contamination in such environments.
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Recent evaluations of burrow connectivity have shown that burrow morphology, abundance, and size impact the sampling scheme required for faithfully representing permeability in the laboratory. Quantifying these controls to design appropriate sampling is essential. This study illustrates a new approach using multipoint statistics modeling and applies it to Thalassinoides, to simulate a range of burrow abundances and sizes, and then interrogates the outcome to develop a statistical model that allows sampling strategies to be designed. Our workflow started with digital models that were known to have open or permeable Thalassinoides that connected one side of the model volume to another, (i.e., high permeability). We cut the model volumes into 540 digital samples with a range of burrow abundance, burrow size and sample cross section. The Thalassinoides showed no cross-sample connectivity in 173 (32%) samples; the remaining 367 (68%) showed cross-sample Thalassinoides connectivity. The data on burrow abundance, burrow size, and sample dimensions (as independent variables) and the result of samples representing Thalassinoides connectivity or not (as a dependent variable), was analyzed using logistic regression modeling. An important outcome from the logistic regression model is an equation that provides a probability of Thalassinoides connectivity across samples, given burrow abundance, burrow size, and sample dimension inputs. This equation has practical applications in determining the minimum sample cross-sections and sample lengths needed for samples to adequately represent the permeability. As examples, for samples with cross sections similar to core plugs and full diameter core, given various burrow percentages and sizes, only 37% yield burrow connectivity across any and all sample lengths. For the rest, our data on the largest connected burrow length allows for calculation of the probabilities of samples of typical lengths being representative of the burrow connectivity. Thus, researchers can assess the likelihood of their samples representing Thalassinoides connectivity given a range of burrow abundance, burrow size, sample cross section and sample length. The spreadsheet to make these calculations and lookup tables are provided as digital accessory files. The example using Thalassinoides provides proof-of-concept of a methodology that should be broadly applicable to any given burrow morphology. An algorithm and lookup table that can be modeled for each given burrow morphology with an appropriate training image.
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Citation Excerpt :
It is evident from these observations that in the case discussed herein, bioturbation index of Taylor and Goldring (1993) is not particularly useful in predicting burrow connectivity, and therefore permeability. The shortcomings in predicting burrow connectivity from bioturbation index was discussed by Eltom and Goldstein (2021) who stated that “The classical classification schemes of bioturbation seem to have shortcomings in predicting the patterns of burrow connectivity. To capture the difference in connectivity pattern of burrows, there is a need for classification schemes to include finer grades than the traditional classification schemes of bioturbation intensity (e.g., Taylor and Goldring, 1993), or to not use grades at all but to quantify BP and morphology.”
Subsurface reservoirs with Thalassinoides are well known. Using a CT scan of an 18.5 cm-diameter core (top cross section, 267.21 cm²; length, 26.5 cm) with Thalassinoides, this study demonstrates the scale dependence of sampling, and variables important in selecting representative sample dimensions for modeling these reservoirs. In the CT scan, 40 vertical square prism subsamples were extracted with cross sectional area ranging from 1.44 cm² to 92.16 cm². The results illustrate the scale dependence and sampling biases on Thalassinoides morphology, burrow percentage (BP), and connectivity of burrows from the top to bottom of subsamples (a proxy for permeability). Analysis of the entire scan shows a BP of ∼38%. The 40 subsamples vary in BP from 7% to 44% with the smallest cross sections showing the expected highest variance, but also showing the lowest mean BP. This illustrates that small-diameter vertical subsamples are likely to under sample Thalassinoides networks. In the full core, Thalassinoides connects across the top and bottom. Subsample cross sections >51.84 cm² have 100% probability of showing this, whereas subsample cross sections <12.96 cm² have 0% probability of showing this. To find the dimensions of representative samples to model Thalassinoides-bearing reservoirs, burrow diameter, BP, vertical length of sample, and core diameter must all be considered.
Bioturbation enhanced petrophysical properties in the Ordovician carbonate reservoir of the Tahe oilfield, Tarim Basin, NW China
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Citation Excerpt :
The organism–sediment interaction alters the primary sedimentary fabrics through excavation, overturn and backfill, resulting in variations in petrophysical properties between the burrow-fills and the host sediments, thus decreasing or enhancing the porosity and permeability of the primary strata, influencing reservoir petrophysical properties and fluid flow characteristics (Gingras et al., 2004a, 2012; Corlett and Jones, 2012; Baniak et al., 2014a; Ben-Awuah and Padmanabhan, 2014; Dey and Sen, 2017; Golab et al., 2017a). In general, the texture of burrow-fills are more unconsolidated than that in the host sediments and will serve as a preferential pathway for subsequent diagenetic fluids or hydrocarbons (Gingras et al., 2004a, 2004b, 2014; Baniak et al., 2014b, 2014c; Golab et al., 2017a; Niu et al., 2020a), resulting in a much more variant or different diagenetic transition of the burrow-fills than the host sediments and amplifying the initial textural and fabric distinctions, which is very important in enhancing the reservoir capacity of tight reservoirs (Pemberton and Gingras, 2005; Cunningham et al., 2009; Gordon et al., 2010; Tonkin et al., 2010; Bednarz and McIlroy, 2012, 2015; Baniak et al., 2013, 2014b, 2015; La Croix et al., 2013; Ben-Awuah and Eswaran, 2015; Eltom et al., 2019, 2020, 2021a, 2021b; Liu et al., 2019; Knaust et al., 2020; Oliveira De Araújo et al., 2021). Whereby, carbonate reservoirs with porosity and permeability below 4% and 10✕10-3 μm2, respectively, are referred to as tight reservoirs according to the industry standard of “Evaluating methods of oil and gas reservoirs” of the People's Republic of China (SY/T 6285-2011).
Bioturbation plays an important role in enhancing the reservoir capacity of tight reservoirs. This study aims to understand the alteration mechanism and effects of bioturbation on oil and gas reservoirs, to clarify the key control factors and constraints influencing the production of bioturbation. The petrophysical characteristics of bioturbation and host sediments in carbonate rocks, such as mineral composition, pore spaces, porosity and permeability, were studied based on the detailed observation and description of the Ordovician cores from the Tahe oilfield. The effect of bioturbation on petrophysical properties of carbonate rocks were carefully analyzed. The results show: (1) Two types of bioturbation, Thalassinoides-like burrows and Planolites-like burrows, mainly occur in the Ordovician cores of the Tahe oilfield, ranging from the Lower–Middle Ordovician Yingshan Formation to the Middle Ordovician Yijianfang Formation. The burrow-fills are mainly composed of dolomite with subhedral and euhedral crystals. The host sediments mainly consist of micrite. (2) The pores in the host sediments are poorly developed and are incapable of forming effective reservoir spaces. However, well-developed intercrystalline pores among dolomites and microfractures in bioturbated sediments with better connectivity can form effective pore spaces. The results of porosity and permeability show that the host sediments without bioturbation are characterized by poor porosity and permeability. However, with the increase of bioturbation intensity, the porosity of the bioturbated sediments firstly increases and then decreases, whereas the permeability increases all the time. (3) Multiple burrows overprinted in the Ordovician carbonate rocks, forming a large-scale bioturbated carbonate rocks with lateral continuity and vertical connectivity, due to the suitable sedimentary setting, ecological conditions, favorable spatiotemporal sediment matching, and abundant organism–substrate interaction. Subsequently, diagenesis (particularly dolomitization and dissolution) has played a positive role in altering the rock fabric and improving the petrophysical properties of bioturbated carbonates.
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Research paperUse of geostatistical modeling to improve the understanding of permeability upscaling in isotropic and anisotropic burrowed reservoirs

Highlights

Abstract

Introduction

Section snippets

Methods

Variation of burrow connectivity with BP and morphology

Permeability anisotropy in bioturbated strata

Conclusions

Credit author statement

Declaration of competing interest

Acknowledgements

Mar. Petrol. Geol.

J. Appl. Geophys.

Mar. Petrol. Geol.

Mar. Petrol. Geol.

Mar. Petrol. Geol.

An example of a highly bioturbated, storm‐influenced shoreface deposit: upper Jurassic Ula Formation, Norwegian North Sea

Sedimentology

Petrophysical characterization of bioturbated sandstone reservoir facies in the upper Jurassic Ula formation, Norwegian North Sea, Europe

J. Sediment. Res.

Impact of bioturbation on reservoir quality: a case study of biogenically reduced permeabilities of reservoir sandstones of the Baram Delta, Sarawak, Malaysia

J. Appl. Sci.

Prominence of ichnologically influenced macroporosity in the karst Biscayne aquifer: stratiform “super-K” zones

Geol. Soc. Am. Bull.

A semiquantitative field classification of ichnofabric

J. Sediment. Res.

Limitation of laboratory measurements in evaluating rock properties of bioturbated strata: a case study of the Upper Jubaila Member in central Saudi Arabia

Sediment. Geol.

Research paper
Use of geostatistical modeling to improve the understanding of permeability upscaling in isotropic and anisotropic burrowed reservoirs