Subsurface Modeling#

Michael J. Pyrcz, Professor, The University of Texas at Austin

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Chapter of e-book “Applied Machine Learning in Python: a Hands-on Guide with Code”.

Cite this e-Book as:

Pyrcz, M.J., 2024, Applied Machine Learning in Python: A Hands-on Guide with Code [e-book]. Zenodo. doi:10.5281/zenodo.15169139 DOI

The workflows in this book and more are available here:

Cite the MachineLearningDemos GitHub Repository as:

Pyrcz, M.J., 2024, MachineLearningDemos: Python Machine Learning Demonstration Workflows Repository (0.0.3) [Software]. Zenodo. DOI: 10.5281/zenodo.13835312. GitHub repository: GeostatsGuy/MachineLearningDemos DOI

By Michael J. Pyrcz
© Copyright 2024.

This chapter is a summary of Subsurface Modeling including essential concepts:

  • What is Subsurface Modeling?

  • The Subsurface Data

  • Modeling Purpose

  • Strategies for Modeling

YouTube Lecture: check out my lecture on Spatial, Subsurface Concepts. For your convenience here’s a summary of salient points.

Motivation for Subsurface Modeling#

You could open a Jupyter notebook in Python and immediately start building machine learning models to solve subsurface problems.

But you would be building on a shaky foundation.

Successful data science begins with understanding the problem domain. Before selecting algorithms or writing code, you need to understand the subsurface, the available data, the objectives of the modeling effort, and the practical constraints under which the model will be developed and used.

This chapter introduces the fundamental concepts that provide this foundation, including,

  1. Subsurface Models — what they are, why they are built, and the different forms they can take.

  2. Subsurface Data — the major data types, their relationships, associated metadata, methods of collection, strengths, and limitations.

  3. Modeling Purpose — the goals of the model, its intended products and users, and ultimately the decisions the model is designed to support.

  4. Modeling Strategies — practical principles, workflows, trade-offs, and fit-for-purpose approaches for developing effective subsurface models.

Together, these concepts provide the context needed to develop robust data science workflows using the methods presented throughout this e-book.

Subsurface Modeling#

What is subsurface modeling? I start my second day lecture with this question and a blank slide on the screen. I wait for students to provide their feedback. I am fortunate to have a variety of departments represented in your courses, and this is a sumamry of what they often say,

  • engineering students - flow forecasting with a physics-based model, to learn how fast and how much will be recovered from our wells and to ensure this model matches historical, past, production rates and composition.

  • geoscience students - building a subsuface heterogeneity model that intergrates, basin analysis, well logs and well cores, inverted seismic attributes, to learn what is the resource in place.

  • data science students - a statistical data-driven model that is calibrated against withheld testing data.

  • business students - an uncertainty model that support optimum business development decision making.

  • environmental students - an accurate prediction model that allows for extraction while minimizing environmental disturbance, protecting health and safety of the employees and communities.

Which is subsurface modeling?

  • it is all of these.

This is one of the reasons that students from all over our campus enjoy and benefit from this content.

Subsurface Modeling Aspects#

Calculating a numerical representation of the subsurface:

  • over the interval (volume in space, or time) of interest

  • over the features of interest

  • integrating all information sources

  • integrating uncertainty through multiple realizations and scenarios

  • to support decision making

Oblique view of a fluvial reservoir model with facies model and plan section of seismic-derived feature (Pyrcz and Deutsch, 2014).

The Volume of Interest#

The volume of interest or area of interest is the volume of the subsurface modeled, and could be,

  • 2D map from a single reservoir to a basin to the entire Gulf of America (Gulf of Mexico)

  • 2D cross section of a single reservoir unit, to the entire reservoir to the entire sediment package from the mudline to the maximum depth of resolvability

  • 3D model of near bore at the scale from 1 cm3

  • 3D model of a single reservoir at the model cell scale of 50 x 50 x 1 m

For all of these volumes and areas, the scale and extent is determined by,

  • purpose of the model – e.g. forward seismic model requires the overburden be included

  • geologic interpretation – e.g. geological unit of interest onlaps uninteresting unit

  • engineering constraints – e.g. drainage radius of a well, flow boundaries

is often subset into a variety of regions that are modelled separately.

Cross section of subsurface model volume of interest.

The Features of Interest#

The feature of interest generally include, measures of rock and fluid properties, for example,

  • porosity

  • permeability

  • saturations

  • lithology

  • acoustic impedance

informative for the resolving subsurface forecasts, for example,

  • resource volumes

  • flow rates

  • recovery factor

Based on direct or indirect measures of the subsurface,

  • direct measures often have limited coverage, small scale, but high accuracy, for example, porosity from well cores

  • indirect measures often have exhaustive coverage, but with larger scale and lower accuracy, for example, seismic information

Cross section for a feature in the subsurface model.

Information Sources#

The subsurface data come from a variety of information sources, including,

  • well or drill hole-based - for local conditioning data, spatial statistics, and high resolution local trend models

  • remote sensing with seismic or gravity information - to support model extents determination, trend models and stationary domain segmentation

  • basin or deposit analysis - large scale modeling to support model choice, model parameter inference and uncertainty modeling

  • production or run of mine samples - output from the extraction opperations that support inverse modeling workflows to estimate the feature models between the data

subsurface modeling is a data integration challenge

Cross section of subsurface model with well and seismic data added.

Subsurface Uncertainty Models#

Subsurface uncertainty is modeled and communicated through the deisgn and calculation of multiple models, including,

  • scenarios – based on changing the model assumptions and decisions, to capture measurement uncertainty and error along with model parameter inference uncertainties

  • realizations - while holding model assumptions and decisions constant, apply new random seeds that cause the models to fluctuate away from the data and under the constraint of model choices and model parameters, to capture spatial uncertainty

Cross section of subsurface model with heterogeneity realizations.

Support Decision Making#

Often subsurface modeling is applied as support for decision making,

  • modeling only adds value when it impacts a decision

  • subsurface modeling is decision support, the actually development decision has many other constraint, including contracts, resources, safety and multiproject company-wide strategies

  • decisions are made considering all scenarios and realizations simultaneously

For example, where to place these injector wells for a water flood, secondary recovery?

Cross section of subsurface model with heterogeneity realizations, production wells and proposed injection wells (red).

Numerical Model#

A subsurface model can be generalized as a numerical model that is based on,

  • quantification - summaries of data, analog inforamtion and expert judgement to calculate summary spatial statistics and trends over the subsurface volume of interest

  • reproduction – impose spatial reservoir property distributions over the subsurface volume of interest that reproduce the quantification to support decision making

This approach of quantification and reproduction is critical to,

  • subsurface model construction based on multi-displinary teams

  • model checking through closing the loops, i.e., do the models reproduce the quantifications that were applied as inputs

  • model diagnostics and sensitivity studies that evaluate value of information and opportunity to improve model accuracy and reduce uncertainty

Cross section of subsurface model with heterogeneity realizations, production wells and proposed injection wells (red).

Subsurface Modeling Caveats#

While this e-book is focused on the subsurface for estimation and forecasting, methods taught in this e-book could be applied,

  • to any spatial problem, petroleum, mining, climate modeling, optimum EV charge station placement, etc.

  • to many multivariate and temporal problems

Yes, the context of subsurface resources is in many of the demonstrations, but the,

  • accessible descriptions of machine learning methods could be applied to any data science challenge.

Data for Subsurface Modeling#

Subsurface data are collected from a variety of sources. Each data source may be described by three fundamental characteristics:

  • Property – What property of the subsurface does the data measure?

  • Resolution – At what spatial or temporal scale is the property measured?

  • Coverage – Over what proportion of the subsurface is the property available?

Property – What property of the subsurface does the data measure? Examples include:

  • static properties that describe the reservoir at a point in time, including facies, porosity, permeability, mineralogy and saturation

  • dynamic properties that describe reservoir behavior through time, including pressure, flow rate, velocity, water cut and fluid composition

  • the most useful data are always related to the modeling objective

Resolution – At what spatial or temporal scale is the property measured? Examples include:

  • a 2 cm core plug

  • a 20 cm whole-core measurement

  • a gamma ray measurement that samples approximately 1 m into the formation every 20 cm along the well

  • seismic acoustic impedance with approximately 10 m vertical resolution

Coverage – Over what proportion of the subsurface is the property available? Examples include:

  • sparse core measurements

  • well logs available for every well

  • 3D seismic covering the entire reservoir and overburden

Commonly there is a coverage and resolution trade-off.

Data Dimensionality#

Subsurface data are represented in one to four dimensions depending on how the property varies in space and time.

1D - data recorded in a sequence of distance or time

  • variation in vertical trend of mineralogy or mechanical properties

  • variation in a single well log, gamma ray (shale indicator)

  • production data rate or composition measured at a production well

2D - often used to support spatial interpretation

  • geologic maps with depositional systems, stationary regions and trends

  • core images with facies and sedimentary structures interpreted

  • correlated well logs with interpolation model between well along a section

  • exploration seismic sections with interpreted reservoir and non-reservoir within a stratgraphic framework

3D - micro, sector, reservoir and basin models

  • seismic volumes with inverted reservoir attributes

  • core tomography with pore-scale models

  • near well bore micro models

  • injection and production paired wells sector model

  • comprehensive reservoir model for production forecasting

4D – repeated measurements through time

  • time-lapse (4D) seismic

  • repeated pressure surveys

  • evolving saturation models

  • reservoir simulation predictions through time

Quantitative and Qualitative Data#

In subsurface modeling, data are often classified as quantitative or qualitative.

  • Quantitative data are numerical measurements that describe the magnitude of a property, such as porosity, permeability, pressure, temperature, or production rate.

Density and neutron log for measuring porosity from a sandstone unit, in Alberta, Canada, http://wiki.aapg.org/Density-neutron_log_porosity.
  • Qualitative data require interpretation before they can be represented in a subsurface model. Examples include lithology, facies, depositional environment, fault interpretations, and sequence stratigraphic units. Image below from sepmstrata.org.

Interpretation of lagoon carbonate setting from Pomar (2004), from Society for Sedimentary Geology. .

Terminology: In geostatistics and reservoir modeling, qualitative commonly refers to interpreted geological information. In machine learning and statistics, these data are more commonly referred to as categorical variables because they are represented as discrete classes (nominal or ordinal).

Hard and Soft Data#

In subsurface modeling, data are often classified as hard or soft based on the degree of measurement certainty and uncertainty.

Hard Data – measurements with a high degree of confidence that are treated as known values at their sample locations. Hard data are typically obtained from direct measurements. Examples include,

  • core porosity measurements

  • well log measurements

  • lithofacies interpreted from core or high-confidence well log analysis

Soft Data – indirect measurements or interpretations that provide information about the property of interest with an associated degree of uncertainty. Soft data are often represented as probabilities, distributions, or calibrated estimates. Examples include,

  • probability distribution of porosity inferred from seismic acoustic impedance

  • probability of lithofacies from seismic attributes

  • interpreted geologic trends and depositional environments

Primary and Secondary Data#

Subsurface features are also commonly classified as primary or scondary data based on their role in modeling.

Primary Data – the property of interest that is modeled directly.

Examples include,

  • porosity measurements used to build a 3D porosity model

  • permeability measurements used to model reservoir flow properties

  • facies observations used to construct a facies model

Secondary Data – another property that is correlated with the primary data and provides additional spatial information through a calibrated relationship.

Examples include,

  • acoustic impedance used to support porosity modeling

  • porosity used to support permeability modeling

  • seismic attributes used to support facies modeling

Relationship between primary data (permeability) and secondary data (porosity) used to improve subsurface modeling (Pyrcz and Deutsch, 2014).

General Types of Subsurface Features#

Subsurface features are commonly represented using one of the following measurement scales.

  • the measurement scale determines which mathematical and statistical operations are meaningful for a feature.

Categorical Nominal Feature – discrete categories with no meaningful ordering, including,

  • Mineral type: quartz, feldspar, mica

  • Lithology: sandstone, shale, limestone

  • Depositional environment

Categorical Ordinal Feature – discrete categories with a meaningful ordering, but differences between categories are not necessarily equal, including,

  • Geologic age (youngest → oldest)

  • Mohs hardness scale

  • Reservoir quality ranking (poor, fair, good, excellent)

Continuous Interval Feature – numerical values with equal intervals, but an arbitrary zero, so ratios are not meaningful, including,

  • Temperature on the Celsius or Fahrenheit scale

Continuous Ratio Feature – numerical values with equal intervals and a true zero, so both differences and ratios are meaningful, including,

  • Temperature on the Kelvin scale

  • Porosity

  • Permeability

  • Fluid saturation

  • Pressure

  • Thickness

  • Production rate

General measurement scales for subsurface features.

Now a convenient table to summarize these data aspects,

Classification

Question

Quantitative vs. Qualitative

What type of information is available?

Hard vs. Soft

How certain is the information?

Primary vs. Secondary

What role does the information play in modeling?

Categorical (Nominal/Ordinal) vs. Continuous (Interval/Ratio)

How is the information represented?

Now we are ready for a short summary of the various subsurface data sources.

  • this is a concise, high level summary of salient points

Core Data#

Let’s summarize core data using the concepts of property, resolution, and coverage.

Sectioned core photographs of the Cook Formation, a shallow marine sandstone reservoir from the North Sea. The core has been interpreted as representing a mixed fluvial and deltaic depositional setting with general upward progradation (Folkestad et al., 2012).

Core Data Property#

Core provides direct measurements and geological observations that anchor many other subsurface data sources, including,

  • petrophysical properties such as porosity and permeability measured directly from core samples, assuming limited core disturbance during extraction and handling

  • sedimentary structures that support interpretation of lithofacies, depositional facies, and depositional environment

  • fractures that may be directly observed in core, although observations are biased by fracture orientation relative to the core axis

  • essential support for petrology, mineralogy, and stratigraphic analysis

Core Data Resolution#

Core provides very high spatial resolution compared with most subsurface data sources, although the effective resolution depends on the measurement method.

Core samples can be analyzed through,

  1. Routine Core Analysis (RCA) resulting in,

  • porosity, permeability, and fluid saturation measurements

  • core gamma logging for calibration to well logs

  • core tomography (CT) scans to assess pore structure and heterogeneity

  1. Special Core Analysis (SCAL) resulting in,

  • electrical measurements for calibration of spontaneous potential (SP) and nuclear magnetic resonance (NMR) well logs

  • mercury injection capillary pressure for pore throat size distributions

  • relative permeability measurements for multiphase flow characterization

The scale or size of the core data is typically,

  • whole cores are commonly cut with diameters of 36–85 mm (BQ, NQ, HQ, and PQ) for mining and 2.5–4 inches for oil and gas applications

  • typical core runs include lengths of 1.5–3.0 m for mining and 9–30 m for oil and gas

Core Data Coverage#

Core coverage varies significantly between mining and oil and gas applications.

  • In mining, diamond drilling cores are commonly collected extensively for exploration and grade control, providing relatively dense spatial coverage.

  • In oil and gas, core acquisition significantly reduces drilling efficiency and increases cost. Therefore, cores are collected infrequently and are typically targeted to specific intervals of geological or reservoir interest.

Well Log Data#

Well logs are commonly applied in oil and gas to augment the limited availability of core samples and provide continuous measurements along the wellbore.

Suite of Well Logs with Interpreted Structures from the Core Data and Stratigraphic Units Form the Cook Formation, a Shallow Marine Sandstone Reservoir from the North Sea. The core data have been interpreted as a fluvial / deltaic depositional setting with general progradation upward Folkestad et al. (2012). .

Well Log Data Property#

Well logs provide indirect measurements and geological observations that support many subsurface interpretations, including,

  • petrophysical properties such as porosity, saturation, and shale volume estimated from log responses

  • sedimentary structures that support interpretation of lithofacies, depositional facies, and depositional environment using borehole image logs

  • fluid types and important fluid contacts, including oil-water contacts and oil-gas contacts

  • stratigraphic analysis by integrating well log markers with seismic data to establish large-scale compartments, trends, and reservoir extent

Common well logs and interpreted subsurface properties include,

  • gamma ray – shale volume and facies interpretation

  • density – porosity estimation and facies interpretation

  • neutron porosity – porosity estimation and fluid effects

  • resistivity – fluid type, water saturation, and facies interpretation

Advanced well logs may also be acquired in selected wells, including,

  • borehole image logs – sedimentary structures, fractures, facies, and depositional setting

  • nuclear magnetic resonance (NMR) – porosity, pore-size distribution, fluid typing, and permeability estimation

  • formation pressure measurements – in situ pressure, fluid gradients, and reservoir compartmentalization

Well Log Data Resolution#

Well logs provide continuous measurements along the wellbore with high vertical resolution compared with most subsurface data sources. The effective resolution depends on the logging tool, formation properties, and logging conditions.

Conventional well logs commonly have,

  • vertical sampling intervals of approximately 15–30 cm (6–12 inches)

  • effective vertical resolution typically ranging from approximately 0.3–1 m depending on the logging tool and formation conditions

Borehole image logs provide higher-resolution observations of the near-wellbore environment, including,

  • electrical or ultrasonic images with millimeter-scale resolution

  • detailed interpretation of thin bedding, fine fractures, breakouts, and sedimentary structures

  • azimuthal information supporting structural and geomechanical interpretation

Well log measurements provide excellent vertical resolution along the wellbore. Image below from Schlumberger.

FMI Image Log examples from Schlumberger.

Well Log Coverage#

Well logs are relatively inexpensive compared with drilling and core extraction and are commonly acquired in oil and gas wells.

Well logs provide excellent one-dimensional coverage along wells, but poor total volumetric coverage due to commonly large well spacings.

  • prediction between wells requires integration with seismic, geological interpretation, and spatial modeling methods.

Remote Sensing-based Data#

In subsurface modeling, remote sensing refers to geophysical measurements that infer subsurface properties from measurements acquired at or near the surface. In oil and gas, the primary remote sensing method is reflection seismic, while in mining common remote sensing methods include gravity, magnetic, and electromagnetic surveys. Image below from Jafari et al., 2017

Acoustic-impedance section result of model-based inversion on the seismic section in A-1 well location. The black well-log curve is the sonic log (Jafari et al., 2017).

Remote Sensing Property#

Remote sensing measurements provide indirect observations of the subsurface and must be interpreted or inverted to estimate rock properties, including,

  • seismic acoustic impedance, a measure of resistance to seismic transmission (the product of density and seismic velocity)

  • seismic attributes related to lithology, fluids, and reservoir heterogeneity

  • density anomalies associated with mineralization and ore grade

Due to uncertainty in the depth location of remote sensing responses,

  • remote sensing interpretations must be positionally calibrated using well log and core data

Due to the non-uniqueness of geophysical inversion,

  • remote sensing-derived properties require calibration to distributions observed in wells and core data while accounting for differences in measurement support

After processing and calibration, remote sensing data provide the regional subsurface framework, including,

  • structural horizons and fault interpretation defining resource extent, geometry, stationary regions, and geological trends

  • acoustic impedance and seismic attributes supporting soft data models for reservoir properties such as porosity and facies, and providing stratigraphic correlation between wells

  • density anomalies supporting mining geological models, including rock type and mineral zone interpretation

Remote Sensing Data Resolution#

Remote sensing methods provide indirect measurements of subsurface properties over large spatial areas. The effective resolution depends on the physical measurement process, acquisition geometry, and processing methods.

Oil and Gas Reflection Seismic - reflection seismic surveys provide broad spatial coverage and are the primary geophysical tool for imaging subsurface structure and reservoir-scale features. Typical resolution includes,

  • Vertical resolution: approximately 10–50 m, controlled by seismic wavelength, frequency, and processing

  • Horizontal resolution: approximately 25–100 m, controlled by acquisition geometry, wave propagation, and depth

Reflection seismic provides excellent lateral coverage across fields and basins, but measurements are indirect and require calibration using wells and geological interpretation.

Mining Gravity Surveys - gravity surveys measure variations in the Earth’s gravitational field caused by density contrasts in the subsurface. Typical resolution includes,

  • Regional gravity surveys: kilometers to tens of kilometers

  • Exploration-scale gravity surveys: hundreds of meters to kilometers

  • Detailed ground gravity surveys: tens to hundreds of meters

Gravity surveys provide very large spatial coverage but have relatively low spatial resolution and significant non-uniqueness, requiring integration with geological, geochemical, and drilling data.

Remote Sensing Coverage#

Remote sensing coverage depends on the development stage and survey objectives. For oil and gas,

  • Exploration – 2D seismic lines are commonly acquired over large areas to identify structural and stratigraphic opportunities

  • Appraisal – 3D seismic volumes commonly cover discovered fields and surrounding areas to characterize reservoir architecture

  • Development – 4D time-lapse seismic surveys may be acquired to monitor fluid movement and optimize recovery

and for mining,

  • Exploration – regional gravity, magnetic, and electromagnetic surveys identify geological structures and potential mineral systems

  • Resource definition – detailed surveys are integrated with drilling and geological interpretation to define ore bodies

Production Data#

Production data represent the observed output response from a reservoir or ore body. Examples include,

  • oil and gas – production rate, fluid composition, water cut, pressure, and temperature by well

  • mining – run-of-mine or plant feed rate and grade measured from material delivered from the mine to the processing plant

Production data integration requires inverse modeling to understand the subsurface conditions that result in the observed response.

Salient questions include,

  • What subsurface state could produce the observed production response?

  • What does the production history reveal about reservoir or ore body behavior?

  • How can this information improve future production forecasts?

Production Data Property#

The subsurface resource industry challenge is to economically and safely extract resources while minimizing impacts to people and the environment.

Unlike other subsurface datasets, production data directly measure the economic response of the resource. Production properties include,

  • oil and gas production rates and fluid compositions

  • mining production rates and material grades

These observations provide the ultimate validation of subsurface models.

Production Data Resolution#

Production data resolution is highly variable and often challenging.

Oil and gas examples include,

  • Lowest resolution: production is commingled over multiple producing intervals or wells, representing the response of a large portion of the reservoir

  • Highest resolution: production logging tools (PLT) measure contributions from individual producing intervals within a well

Image below is from the American Association of Petroleum Geology Wiki

Production log from a producing logging tool (PLT) of a well from AAPG Wiki.

Mining examples include,

  • production data commonly require reconciliation between mine surveys, grade-control drilling, and plant measurements

  • typical production resolution may include weekly production from individual benches, stopes, or mining blocks

Production Data Coverage#

Production data are available throughout actively producing regions of the subsurface resource.

Unlike core, well logs, and seismic data, production data are continuously collected during resource extraction and provide a time-dependent record of system behavior.

Summary of Subsurface Data#

Now we can summarize all these data types in a single table,

  • For completeness, I have included analog types of data sources, analog reservoirs, outcrops, geomorphology, shallow seismic, experimental stratigraphy and numerical process models.

Data Source

Resolution

Coverage

Property

Certainty

Core

Very high (mm–cm scale)

Well locations only

Lithology, pore structure, sedimentary structures, direct rock properties

Hard

Well Log

cm–m scale

Along wells

Facies, porosity, mineralogy, saturation, petrophysical properties

Hard-calibraded

Image Well Log

mm–cm scale

Near-wellbore

Sedimentary structures, bedding, fractures, faults, geomechanical features

Hard-calibrated

Seismic

~10 m scale

Field to basin scale

Structural framework, stratigraphic trends, seismic attributes, facies and property trends

Soft

Production Data

10–100 m (drainage scale)

Producing reservoir volume

Volumes, connectivity, effective permeability, dynamic reservoir response

Hard

Analog Reservoirs

Variable

Analog field/outcrop

Validation, geological priors, conceptual models

Soft

Outcrop

Very high (mm–cm scale)

Local exposure

Geological concepts, depositional processes, input statistics

Soft

Geomorphology

Very high (surface scale)

Modern analog locations

Depositional concepts and spatial patterns

Soft

Shallow Seismic

Element-scale resolution

Analog locations

Geological concepts, input statistics

Soft

Experimental Stratigraphy

Laboratory scale

Experimental systems

Depositional processes and geological concepts

Soft

Numerical Process Models

Process-dependent

Model domain

Geological concepts and process understanding

Soft

Example Subsurface Models#

Subsurface models are developed for a wide variety of purposes, resulting in very different model scales, properties, and workflows. The following examples illustrate how modeling objectives influence the design of the subsurface model.

For additional discussion, see the cited references or the more comprehensive treatment in Pyrcz and Deutsch (2014).

2D Mapping for Volumetrics#

Goal: Estimate the remaining resource in place.

Typical properties:

  • net reservoir thickness

  • vertically averaged porosity

  • vertically averaged saturation

  • seismic-derived attributes

Typical model:

A two-dimensional estimation model is used to interpolate smoothly varying properties between wells while honoring geological trends and, where appropriate, geophysical information.

The oil in place (OIP) is estimated by summing the oil volume within each grid cell,

\[ \mathrm{OIP} = \sum_{\alpha=1}^{n} V(\mathbf{u}_{\alpha}) \bar{\phi}(\mathbf{u}_{\alpha}) \overline{S_o}(\mathbf{u}_{\alpha}) \]

where,

  • \(\mathrm{OIP}\) = total oil in place

  • \(n\) = number of grid cells

  • \(\mathbf{u}_{\alpha}\) = location of grid cell \(\alpha\)

  • \(V(\mathbf{u}_{\alpha})\) = bulk rock volume of grid cell \(\alpha\)

  • \(\bar{\phi}(\mathbf{u}_{\alpha})\) = block-average porosity

  • \(\overline{S_o}(\mathbf{u}_{\alpha})\) = block-average oil saturation

For regular grids, the cell volume may be constant and omitted from the summation.

Image below from Geological Survey of Canada.

Kriged map of remaining oil in place for the Utica Shale, Québec (Geological Survey of Canada).

Regional Modeling#

Goal: Support strategic planning by understanding the large-scale spatial distribution of the resource.

Typical properties:

  • reservoir thickness

  • permeability-thickness

  • structural elevation

  • resource quality

Typical model:

Regional models cover hundreds to thousands of square kilometers using relatively large grid cells. Fine-scale geological variability is simplified while preserving large-scale trends important for field development, lease evaluation, and infrastructure planning.

Regional map of bitumen pay thickness in northwest Alberta (Hein, 2015).

Micro- and Mini-Scale Modeling#

Goal: Understand pore-scale processes and transfer their effects to larger-scale reservoir models.

Typical properties:

  • pore geometry

  • mineralogy

  • fluid distribution

  • relative permeability

  • capillary pressure

Typical model:

Micro-models typically represent a single core plug or rock sample, while mini-models represent the scale of a single reservoir simulation cell. These models are commonly used to derive effective flow properties for larger-scale simulation models.

Image from Digital Rocks Portal.

Direct pore-scale simulation of residual fluid trapped within Berea Sandstone (Digital Rocks Portal).

Reservoir Modeling#

Goal: Predict fluid flow and evaluate development scenarios.

Typical properties:

  • facies

  • porosity

  • permeability

  • saturation

  • pressure

  • production history

  • seismic attributes

Typical model:

Reservoir models typically contain millions of grid cells with horizontal dimensions of tens of meters and vertical thicknesses of approximately 0.25–1.0 m. They integrate geological, geophysical, and engineering information to support flow simulation, history matching, and production forecasting.

Image from SEG Wiki.

Reservoir model colored by oil saturation, illustrating spatial variation in remaining hydrocarbons (SEG Wiki).

A Continuum of Models#

These examples illustrate that there is no single “subsurface model”. Instead,

  • models span a continuum of scales and objectives—from pore-scale physics, to reservoir flow simulation, to regional resource assessment.

The appropriate model is determined not by the available software, but by the questions the model is intended to answer.

Comments#

This was a basic description of subsurface modeling. This is all critical prerequisites for anyone working in data science, data analytics, geostatistics and machine learning in the subsurface. Much more could be done and discussed, I have many more resources. Also check out my textbook with Professor Clayton V. Deutsch Geostatistical Reservoir Modeling for much more details on this topic.

Check out my shared resource inventory and the YouTube lecture links at the start of this chapter with resource links in the videos’ descriptions.

I hope this was helpful,

Michael

About the Author#

Professor Michael Pyrcz in his office on the 40 acres, campus of The University of Texas at Austin.

Michael Pyrcz is a professor in the Cockrell School of Engineering, and the Jackson School of Geosciences, at The University of Texas at Austin, where he researches and teaches subsurface, spatial data analytics, geostatistics, and machine learning. Michael is also,

  • the principal investigator of the Energy Analytics freshmen research initiative and a core faculty in the Machine Learn Laboratory in the College of Natural Sciences, The University of Texas at Austin

  • an associate editor for Computers and Geosciences, and a board member for Mathematical Geosciences, the International Association for Mathematical Geosciences.

Michael has written over 100 peer-reviewed publications, a Python package for spatial data analytics, co-authored a textbook on spatial data analytics, Geostatistical Reservoir Modeling and author of two recently released e-books, Applied Geostatistics in Python: a Hands-on Guide with GeostatsPy and Applied Machine Learning in Python: a Hands-on Guide with Code.

All of Michael’s university lectures are available on his YouTube Channel with links to 100s of Python interactive dashboards and well-documented workflows in over 40 repositories on his GitHub account, to support any interested students and working professionals with evergreen content. To find out more about Michael’s work and shared educational resources visit his Website.

Want to Work Together?#

I hope this content is helpful to those that want to learn more about subsurface modeling, data analytics and machine learning. Students and working professionals are welcome to participate.

  • Want to invite me to visit your company for training, mentoring, project review, workflow design and / or consulting? I’d be happy to drop by and work with you!

  • Interested in partnering, supporting my graduate student research or my Subsurface Data Analytics and Machine Learning consortium (co-PIs including Profs. Foster, Torres-Verdin and van Oort)? My research combines data analytics, stochastic modeling and machine learning theory with practice to develop novel methods and workflows to add value. We are solving challenging subsurface problems!

  • I can be reached at mpyrcz@austin.utexas.edu.

I’m always happy to discuss,

Michael

Michael Pyrcz, Ph.D., P.Eng. Professor, Cockrell School of Engineering and The Jackson School of Geosciences, The University of Texas at Austin

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