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Static Modelling and Dynamic Simulation for Geological Co2 Storage: An Integrated Regional Scale Approach for the Bunter Sandstone Formation, Southern North Sea (UK)
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Seismic history matching using binary images
Get PDFThe ability to predict the flow of multiple fluids in a reservoir, and update the reservoir in a timely efficient manner is the dream of every reservoir engineer, and has been the bane of much research in the oil and gas industry. This is highly sought after because it enables efficient reservoir monitoring, management and planning. However this requires some level of skill and precision as well as the ability to interpret and input data from different sources into the model. In order to predict the flow of multiple fluids in a reservoir, the relative permeability of these fluids has to be determined. In this thesis, 4D seismic data is used to estimate some endpoints of the relative permeability curve ( and ), whereby multiple seismic surveys are employed in association with the production history, depletion mechanism, geological and structural effects as well as reservoir simulation predictions. The multiple survey 4D seismic data is interpreted so as to decipher instances of critical gas saturation as well as maximum gas saturation effects, these are then quantitatively analysed, and a relationship between the ratio of their amplitudes and gas saturation are used to estimate the values.
In addition, integrating 4D seismic data with production data in a quantitative manner in a reservoir model improves the model’s capability and reduces the uncertainty, however doing this is quite a challenging problem. This thesis addresses this challenge by utilising a binary approach which circumvents the full rock physics modelling approach. The binary approach is developed and tested where gas and water saturation from 4D seismic data and the reservoir simulation model are converted to binary indicators. Different metrics for quantifying the binary misfit in terms of their strengths and short comings are analysed, and the Current measurement metric and Hamming distance exhibit better capabilities than the Hausdorff distance and Mutual Information measurements. The binary approach is then tested on a synthetic model in order to validate its use, as well as show its functionality in a practical setting. In the synthetic study, three different scenarios are analysed – the gas exsolution scenarios, the water evolution scenarios, and a combination of gas exsolution and water evolution, and the results show that the binary approach provides a quick and efficient method of assessing reservoir parameters. The binary approach is then implemented on a real field data from the United Kingdom Continental Shelf (UKCS), where 104 uncertain reservoir parameters are initially assessed. An initial ensemble of fluid flow simulation models is created where the full range of uncertain parameters are acknowledged using experimental design methods, and an evolutionary algorithm is used for optimization in the history matching process. It is found that the primary control parameters for the binary seismic gas match are the permeability and critical gas saturation, while the volumetric parameters are important for the binary seismic water match in this particular reservoir. This binary approach is then compared to a conventional seismic modeling approximation approach, where the results show that the binary approach gives a good match to gas saturation distribution and water saturation distribution, and the reservoir parameters converge towards a solution. The conventional approach captures some signals of hardening and softening in the seismic data, however most parameters do not fully converge towards a solution, and hence in summary, the binary approach seems more suitable as a quick look reservoir management tool
Petroelastic approximations for quantitative 4D seismic interpretation
Get PDF4D seismic interpretation generally involves analysing changes in amplitude or travel times between a base-line survey (ideally pre-production) and subsequent monitor surveys. Amplitude analysis is commonly performed by calculating the 4D amplitude difference – given by the subtraction of time-shift corrected monitor - base amplitude volumes. This thesis focuses on the development of theoretical approximations that allow explaining, from a physics perspective, the relation between the 4D amplitude signal and production-related changes in pore pressure and fluid saturation. The main objective of finding such approximations is to aid 4D interpretation, by providing physical insights in a quick and intuitive manner that conventional modelling cannot offer.
The approximations developed here are expressed in terms of constants and variables with clear physical meaning. This helps in identifying the role that each parameter has in the resulting 4D seismic signal, allowing us to identify those terms with the largest impact and uncertainty. The first conclusion obtained from our derivations is that, except for porosity, the parameters involved act as groups of parameters rather than as individual parameters to form the 4D seismic signal. It is also possible to demonstrate how porosity plays a major role in the magnitude of the 4D signal (this is intuitive for saturation but not for pressure). It was found that, for low porosities, pressure is expected to dominate over saturation and vice-versa. Another important observation is that the magnitude of the rock stress sensitivity determines the polarity of 4D amplitude changes in places where pressure and saturation signals cancel each other (like around injectors). The formulations also allow the analysis of the angle dependence (4D AVO). We observe that, regardless of the type of AVO anomaly of the reservoir in the seismic base line, the 4D AVO signatures behave in a similar way and the role of the overlaying shale in the 4D amplitude variation with angle is minimal. Additionally, previous research has shown that the rock stress sensitivity carries the largest uncertainty and, using the formulations, a methodology was devised to determine a constraint, based on field data observations at injector wells from different fields.
The first approximation developed considers a single interface applicable for non-compacting oil reservoirs with no free gas. Subsequent approximations were developed to define the 4D response of fluid contacts, which are applicable to non-compacting reservoirs, including those where free gas is present. The fluid contact equations for the oil-water system were used to develop a technique to estimate residual oil saturation from 4D seismic amplitudes and the method was tested in two North Sea datasets. Finally, an incidental outcome of the new formulations is their potential to be used for 4D seismic inversion to compute pressure and saturation changes. We provide a basic (map-based) implementation of 4D inversion applicable to oil reservoirs with no free gas. The inversion scheme has a straight-forward implementation and requires a minimal amount of a priori information and constraints. It was found that, three angle stacks are required as a minimum to obtain a meaningful inversion result
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Integration of well data into dynamic reservoir interpretation using multiple seismic surveys
Get PDFThis thesis develops and tests a new technique which integrates information from well production and 4D seismic data directly in the data domain. This method is of value when seismic data are acquired by multiple surveys over the same area of a hydrocarbon reservoir. Sequences of 4D seismic changes can then be extracted over different time intervals from multiply repeated seismic surveys and these are cross correlated with identical time sequences of cumulative fluid volumes produced or injected from wells. The technique is applied to frequently repeated seismic surveys from three North Sea fields, including two compartmentalised reservoirs: the Schiehallion and Norne field, and a compacting reservoir: the Valhall field. Maps of well to seismic cross-correlations are proven to produce a strong, localised and stable signal in the connected neighbourhood of individual wells.
The correlation signatures from the Schiehallion and Norne application investigated in this thesis are the consequence of pressure performance due to reservoir compartmentalisation. In the Schiehallion study, the mapped results help identify the production signal related only to individual wells, thus leading to a better delineation of reservoir compartments. In the Norne study in particular, an extra reservoir volume connected to the original segment is highlighted by the technique. The reservoir simulation model is subsequently updated and a better match between the observed and simulated data can be achieved. The application to the compacting Valhall field involves using data from the Life of Field Seismic project, for which the 4D signature is dominated by compaction-assisted pressure depletion. For these data, both AI and time-shift attributes are found to have a remarkably consistent correlation with the well activity for selected groups of wells. Further, maps of these results possess sufficient fine scale detail to resolve and disentangle interfering seismic responses generated by closely spaced wells and localised zones of gas breakout along long horizontal producers. These case studies indicate our proposed methodology of uniting well data and 4D seismic and confirm that this does indeed provide an insightful product for dynamic interpretation of the producing reservoir
Pressure estimation using time-lapse seismic in compacting reservoirs
Get PDFThis thesis focuses on developing a new approach to estimate pressure changes from 4D amplitude
attributes in compacting reservoirs. The time-lapse seismic signal in these types of reservoirs results
from the combination of pressure depletion, rock compaction and stress redistribution within the
reservoir and throughout the surrounding rocks. Simulations using iterative coupling are performed
to understand the link between geomechanics, fuid fow and the seismic response. The analysis
of synthetic data defines a power law equation (eq.1) which relates pressure changes (¢P) to 4D
amplitude attributes (¢A). The coefficients (C1 and C2) are a function of initial porosity. The
pressure predictions show an agreement with the output from the reservoir modelling. However the
results indicate that the rock compaction has considerable effect on the normal average stress, and
the 4D seismic response shows a stronger correlation with effective stress than with pore pressure.
¢A = C1 ¤ (¢PC2) (1)
The technique is applied to the south east flank of the Valhall Field, Norwegian North Sea. The areas
where the initial porosities are above 38 % show a good correlation between the pressure changes
predicted from 4D amplitude and the pressure changes from the reservoir model. However, major
differences between both outputs occur in areas where no 4D signal is observed; these areas are
correlated with low porosity zones where the porosity reduction has not been significant enough to
enhance the 4D signal. Furthermore, the pressure predictions from the 4D seismic identify areas
where the reservoir has not been properly drained.
The impact of geological structure and gas saturation on the technique is assessed. Strong thickness
variations within the reservoir interval increase the errors on the pressure prediction. This is mitigated
if relative values instead of absolute values are used to estimate pressure changes, i.e. equation 1
becomes equation 2. Furthermore, the presence of gas on the reservoir requires a modification of the
i
equation 1 in order to accurately predict the pressure changes and account for the presence of free
gas on the reservoir (eq. 3).
¢A
A1
= C1 ¤ (
¢P
P1
)C2 (2)
¢A = (C3 ¤ ¢P) + C4 (3)
Finally, sensitivity analysis suggests that uncertainties in the elastic properties of the overburden
(rarely measured with accuracy) have little impact on the reservoir 4D amplitude response. Synthetic
models show that variations between 10 % to 15 % in the Poisson's ratio and Young's modulus of the
layer immediately above the reservoir causes negligible changes (less than 4 %) in the 4D amplitude
response
The influence of overburden on quantitive time-lapse seismic interpretation
Get PDFTime-lapse seismic data quality has improved over the past decade, which makes
dynamic interpretation of the reservoir changes possible. To push the limits of this
technique further, this thesis studies the time-lapse seismic noise generated by overburden
heterogeneities, as well as its influence on quantitative seismic interpretation.
This is done by testing the accuracy of a multi-attribute pressure and saturation
inversion method in this context to gain insight into its performance in the case
of seismic acquisitions not being perfectly repeated. Extensive seismic modelling
studies are conducted in order to quantify the accumulated error for three different
overburden complexities.
Channels in the overburden above the Nelson Field, North Sea, are found to cause
errors in the time-lapse amplitudes. The magnitude of these amplitude errors decreases
with increased repeatability of the monitor survey’s source and receiver
positions. On average, saturation change is estimated to an accuracy of less than
6% when affected by amplitude errors only. However, these mean errors significantly
increase to more than 20% if the residual time shifts caused by the channels
are not removed from the seismic data. Moreover, the maximum saturation change
estimation error can exceed the production induced signal locally. In addition, a
major finding of this study is that the shape of the channel in conjunction with
the acquisition direction has a significant impact on the spatial distribution of the
errors at the reservoir level. It is also shown that the commonly used repeatability
measures of NRMS or Δsource+ΔReceiver do not correlate well with the spatial
distribution of areas with increased saturation change estimation error.
Consequently, a layer stripping method is presented which reduces the amplitude
errors caused by the overburden channel and the acquisition non-repeatability by a
factor of two. Nevertheless, the limits of using post-stack data to invert for timelapse
changes become apparent and, as a result, it is strongly advised to do further
research into applying this method to pre-stack seismic data.
Production-induced amplitude changes inside the stacked reservoirs of a deepwater
West of Africa field constitute the second overburden complexity studied. These
changes imprint on the lower reservoir channel and reduce the time-lapse amplitude
change locally by up to 42%. Furthermore, time-lapse amplitude errors are as large
as 38% in case that the velocity change inside the upper reservoir is not included in
the monitor migration velocity model. In addition, an important conclusion of this
study is that due to the high frequency assumption ray-tracing based seismic modelling
does not perform well for cellular models such as this West of Africa example.
Finite-difference modelling methods are strongly advised to be used instead.
Finally, the effect of overburden changes above the highly compacting Ekofisk chalk
reservoir, North Sea, is investigated by combining reservoir simulation, geomechanical
and ray-tracing models. The velocity change of the overburden rocks reduces
the time-lapse amplitudes at the top reservoir predominantly in the zone of vertical
displacements greater than six metres. In this zone, the mean time-lapse amplitude
errors in the full and far offset stack data are 9.4% and 4.23%, respectively. These
errors decrease below 2.3% in areas of less than six metres vertical displacement.
Consequently, the full and far offset stack amplitudes are not suited for quantitative
time-lapse interpretation. The time-lapse amplitudes for the near and mid
offset stacks are significantly less affected and the mean errors are smaller than 1.5%
across the entire reservoir. Therefore, these two partial stacks are recommended for
quantitative time-lapse interpretation.
Three different overburden complexities in the North Sea and West of Africa are
studied and prove to have a measurable impact on the time-lapse amplitudes. It is
shown that these errors affect the ability to estimate the saturation change and in a
way that is not entirely predictable from inferences using commonly used repeatability
measures
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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