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This interactive overview explains how geoscience is used in commercial oil and gas exploration and production. Part 1 looks at some of the commonly used geoscience disciplines. Part 2 focuses on some specific examples of the role of geoscience in oil and gas exploration and production.
There are 35 slides; each one should take only about 30 seconds to read and digest. You can hide the caption if you want to see the full image and you can click on the link to learn more about each topic.
The words ‘seismic’ and ‘geoscience’ are often associated with the study of earthquakes and natural disasters. When applied to the commercial discovery and development of natural resources, geoscience typically refers to the disciplines of geophysics, geology, petrophysics, geomechanics and reservoir engineering, along with how these all integrate into the business of the energy industry.
Geoscience concerns the study of the Earth.
We can collect data from many different sources covering a vast range of scales, from nanometer-scale rock sample analysis through centimeter- and meter-scale for well log and geophysical methods to hundreds of kilometers for regional geological interpretations.
Seismic surveying is a geophysical tool that is widely used in the oil and gas E&P cycle. The seismic method uses mechanical (acoustic) waves to probe the subsurface. It records the reflections created by changes in acoustic impedance (the product of velocity and density) between different geological layers.
Seismic technology has evolved rapidly in the last decade and can provide stunning images of the subsurface. We’ll dedicate the next few slides to explaining the fundamentals of modern seismic.
Seismic acquisition involves the controlled generation of acoustic energy at the Earth’s surface. This energy propagates through the subsurface as seismic waves and is reflected from the geological layers. When it returns to the surface it is recorded by arrays of seismic receivers. The recorded wavefield contains all kinds of useful information about the structure and composition of the subsurface.
2D surveys are used for regional exploration while 3D surveys (acquired on dense grids) provide high-resolution 3D images which reveal fine-scale geological structure for exploration in more complex settings.
Survey vessels tow energy sources and receiver arrays across the survey area on a pre-defined grid. The energy sources release high-pressure air bubbles which generate the acoustic waves which travel down into the subsurface. Hydrophone receivers, which record the energy reflected from subsurface layers, are contained in cables called streamers which are towed at depths of up to 50 m and can be over 10 km long.
Modern seismic vessels can tow 14 or more streamers to efficiently acquire large surveys of tens of thousands of square kilometers.
Onshore, vibroseis vibrator trucks have replaced explosives as the preferred seismic source. They use a hydraulic shaker to emit a long sweep of acoustic energy into the ground. Geophone sensors record the tiny vibration caused by the reflected seismic waves returning to the surface.
Data are typically sent back to a central recording system via cables. The use of cable-free wireless systems where data is recorded, stored and then harvested at a later time are increasingly popular due to their flexibility in rough terrain and obstructed areas.
Multi-component seismic data can be recorded on the seabed using cables or stand-alone nodes. Ocean bottom cables (OBC) are often used in shallow water where it is not possible to operate a large marine streamer vessel.
Ocean bottom nodes (OBN) are increasingly being used in deep water for full-azimuth surveys to illuminate complex geological structures. They can also be used to record data in gaps left by streamer surveys around oilfield infrastructure.
In the presence of challenging geological features, such as complex salt bodies, or naturally fractured reservoirs, there is a need to take multiple measurements from different directions (azimuths) to properly illuminate the structure and characterize the rock.
Offshore streamer surveys have evolved to provide wide- and full-azimuth illumination by using several source and streamer vessels working together. Onshore surveys now provide full-azimuth coverage using high-capacity recording systems. Recently, large deployments of ocean bottom nodes (OBN) have also been used for full-azimuth surveys.
In the last decade, a variety of techniques have been developed to provide ’broadband‘ seismic which offers better resolution and more detailed information about the subsurface. There are a range of acquisition and processing techniques available which can significantly increase the usable bandwidth of marine and land seismic.
Broadband techniques have revolutionized marine streamer seismic data quality in particular. Methods include variable-depth steamer acquisition combined with source and receiver deghosting and the use of multi-sensor multi-component streamers.
Seismic energy is propagated as a vector wavefield composed of different wave types. These can be recorded using multi-component sensors. Shear waves are of particular interest as they are insensitive to reservoir fluid so provide more information about the rock matrix and fractures.
Multi-component seismic is used for specific applications, such as imaging through subsurface gas clouds, characterizing fractured reservoirs and for more accurate reservoir characterization.
Quality control is vital, not just during seismic recording, but at every stage in a survey. Recording systems provide advanced QC for every aspect of a survey from source monitoring, positioning and navigation to the seismic data quality itself. To meet industry demand for faster delivery of seismic results to support drilling decisions, on-board (or in-field) processing is becoming increasingly advanced.
Creating an accurate image of the subsurface requires considerable work. Data are first processed to remove noise and unwanted “echoes” known as multiples. To reconstruct an accurate image of the subsurface, the conditioned data containing reflection energy are “migrated” back to the subsurface points of origin.
An accurate velocity model built using tomographic methods and well data is required for migration. These models can also take into account some of the complexities of wavefield propagation, such as anisotropy and absorption.
3D seismic image volumes contain a huge amount of information. They need to be interpreted to understand the geology and its implications for exploration and development. Automated tools allow interpreters to quickly pick the tops of geological layers (horizons) and faults, analyze structures and look at variations in seismic amplitudes which can indicate lithology and the presence of oil and gas.
Information from well logs and other geoscience data are used alongside the seismic to build a geological interpretation which will guide E&P decisions.
Multi-physics is an approach for characterizing the subsurface where measurements from several different geophysical techniques are combined to provide a better understanding. Typically, multi-physics refers to potential and induced field methods, such as gravity, magnetics and electromagnetics.
Surveys can be performed using specially adapted aircraft for rapid and efficient coverage of large areas. These methods can detect anomalies in a range of different subsurface properties, such as density, magnetic susceptibility and conductivity.
Multi-physics can be used in the exploration for a wide range of natural resources, such as minerals, ground water and oil and gas. It is often used to support seismic and well data interpretation.
Specialized software is used to interpret the data and create models of subsurface properties. These models can be focused on near-surface local property characterization or deep regional structural interpretation, depending on the survey method.
Petroleum geology is the study of the origin, generation, migration, accumulation, preservation, exploration and exploitation of oil and gas. It explores the entire petroleum system from the humble beginnings of source rocks through to the present-day preservation of oil and gas in reservoirs and traps.
The end-result of a petroleum geology study is an earth model defining the full geologic characteristics of a basin, play, potential prospect or reservoir.
Geological studies provide an essential understanding of basins and petroleum systems. Stratigraphy and structural geology study the evolution of basins, highlighting architecture, potential traps and migration pathways, depositional settings and chronology.
Different methods can be used to establish sequence stratigraphy. For example, biostratigraphy uses micro- and nano-fossils of organisms and pollen to establish the age of sediments and correlate between sequences of rocks.
Automated quantitative mineralogical analysis of rock samples and drill cuttings provides a ready source of geological data at the well site. This can be used to accurately geosteer well bores and determine reservoir quality and mechanical properties for well completion design.
Geochemical analysis of oil and gas samples from wells or naturally occurring surface seeps assesses the maturity, source and viability of petroleum systems for production, giving insight into expected volumes and likely distribution of oil and gas.
Petrophysical analysis establishes relationships between core, wireline log and seismic measurements. This allows lithologies and facies (rock units) to be identified and mapped across a field or basin using well logs. It also provides a way to estimate reservoir properties, such as water saturation, porosity, and permeability from geophysical measurements.
Petro-elastic models are used to define the relationship between rock properties (e.g. porosity) and elastic properties measured by seismic data and well logs, such as velocity. These provide the link between seismic data and reservoir properties needed for seismic reservoir characterization workflows.
When a prospective field has been discovered a decision will be taken as to whether or not to develop it. Reservoir engineering and petroleum economics expertise is required to build a reservoir model, determine recoverable oil and gas volumes, estimate production capital expenditure and model cash flow and return on investment.
There are uncertainties to be considered at every stage in this assessment, from reservoir quality and oil recovery factors to the fiscal terms of the host country and long-term oil prices.
Geoscience data help us build an understanding of the structure and properties of the subsurface and the natural resources it holds.
From identifying prospective new oil and gas basins in frontier areas and drilling the first exploration well to managing production and eventually abandoning mature oil & gas fields, geoscience data guide decisions at all stages in the E&P cycle.
When exploration companies are looking at new regions they can generally benefit from databases of modern geoscience information that have been recently acquired and interpreted in that area. This can include seismic, well and geological data sets as well as interpretative products and reports.
This off-the-shelf information, referred to in the industry as “multi-client” or “non-exclusive” data, can accelerate decision-making and reduce exploration risk.
Basins can be overlooked following unsuccessful exploration efforts in previous decades. New geological concepts and advances in geoscience and oil and gas production technology can make these basins worth a second look.
There will often be legacy data available which, although decades old, can still be valuable, especially once reprocessed with the most recent technology and expertise.
In this overview we simplify the natural resource exploration and production cycle into three stages and show where specific types of geoscience data are used. Some types of data, such as seismic, will be used at every stage, but for different purposes.
Since each stage has a different set of challenges associated with it, the requirements for information to support decision-making are also different.
Although oil and gas have already been discovered in most regions of the world, there are still many frontier areas to explore. With limited data available, underexplored frontier sedimentary basins can be high-risk opportunities.
Global geological data and models based on paleo-earth simulations can provide a good starting point for New Ventures teams to assess or “screen” frontier areas.
All natural oil and gas reservoirs leak. In some cases, they produce observable seeps at the surface, both onshore and offshore. If there is uncertainty as to whether a basin contains an active petroleum system, the presence of oil and gas seeps increases confidence.
Offshore seeps can be detected using tell-tale signatures on satellite radar images. The seeps can be sampled and their geochemistry analyzed to reveal their origin and maturity.
Seismic data sets contain much more information than is visible in a 3D image volume. Seismic gathers (before they are stacked together in the image) reveal the behavior of horizons in more detail. The amplitudes can vary with source-receiver offset or azimuth, for example.
Amplitude-versus-offset (AVO) analysis is a commonly used reservoir reconnaissance tool. Boundaries between different fluids and lithologies produce characteristic AVO signatures which can indicate the presence of oil and gas.
Reservoir characterization brings together geological and geophysical information to produce a model of reservoir properties, facies (rock type) and oil and gas distribution. It generally involves the “inversion” of AVO information in prestack seismic data into elastic rock properties as the basis for this model.
Petrophysics, rock physics modeling and geological constraints are used to establish the relationship at well locations between rock properties and seismic attributes to predict facies and fluid distribution. These models allow drilling locations to be accurately selected and support field development decisions.
Building an engineering model of a reservoir is an important element of field development planning. Geoscientists integrate available geophysical, petrophysical and geological data to build a representative model.
Reservoir Engineers then populate the reservoir model with properties, such as porosity and permeability, to allow them to simulate production effects and design the most effective development plan.
While reservoir characterization can provide detailed models to plan the best well locations and trajectories, keeping a directional or horizontal wellbore in the right reservoir interval during drilling can be challenging.
Geosteering services help drillers to navigate and stay on track. These services include a range of real-time geological analyses performed on drill cutting samples at the well site, such as biostratigraphy and automated mineralogy, to identify the stratigraphic interval.
Completion is the process of making a well ready for production. In deviated and horizontal wells targeting unconventional resources, installation of downhole completion equipment is complex and costly. To ensure that the investment is well spent, accurate subsurface information is required. This would normally come from wireline or measurement while drilling (MWD) logs, but in many horizontal wells these are not acquired.
Well-site cuttings analysis and workflows that combine this with seismic reservoir characterization and geomechanical modeling provide valuable insight. This can allow engineers to fine-tune completion plans and optimize production.
Once wells have been drilled and oil is being produced, the challenge for engineers is to understand the behavior of the reservoir, identify locations for infill drilling and design enhanced oil recovery programs. Estimated changes in fluid distribution indicate how effectively oil and gas have been produced and if there are any “unswept” zones.
4D seismic, obtained by repeating 3D seismic surveys at regular time intervals and looking for differences in the reservoir response, allows changes to be monitored at high resolution across the whole field.
When oil becomes more challenging to produce from mature reservoirs or from heavy oil fields, enhanced oil recovery (EOR) strategies, such as water and steam injection, are used to boost production. EOR-induced changes in the reservoir can be subtle and rapid, calling for more sensitive monitoring solutions.
Permanent Reservoir Monitoring (PRM) systems feature permanently installed seismic equipment (such as buried receiver cables) and tailored 4D processing workflows that provide a much higher sensitivity to changes in the reservoir and faster delivery of results. (Image courtesy of Shell BV and NAM).
Production strategies for a field are based on flow simulations using the reservoir model. During the life of a field, production data are gathered from gauges and from 4D seismic surveys. These data may reveal inaccuracies in the reservoir model which cause mismatches between simulations and observed measurements.
Reservoir engineers perform a “history match” to update the model so that the simulations fit the observed results better. The challenge with history matching is to use all the available geoscience data in the best way possible to make geologically plausible updates to the reservoir model.
We hope our online geoscience 'guided tour' has given you an introduction to what we do at CGG. If you'd like to learn more, we have some additional resources that we think you will find interesting: