Øyvind Sylta (-> Abstract, Content, Migris)
Hydrocarbon migration modelling and exploration risk.
A thesis accepted by
The Norwegian University of Science and Technology (NTNU),
Trondheim, Norway.
For the defence of the academic degree
Dr. philos.
2004
Preface
This dissertation is the result of my involvement in scientific investigations within the topics of basin modelling, hydrocarbon migration modelling and prospect risking for more than 10 years. It is a result of a long term fascination with trying to understand geology and geological processes by the use of quantitative methods. Modelling and the use of computerized numerical techniques have therefore become key elements in the work. During this work I have had the pleasure of cooperating with many geologists in different companies. These have always inspired me, and some of the most critical comments have turned out to be extremely helpful in helping to determine the direction of my research.
The topics covered in this dissertation are demanding in that several large research topics are touched upon. These include quantitative geology, numerical modelling, hydrocarbon generation and modelling, fluid flow modelling, reservoir technology and risking techniques. I am in no way an expert on all of these topics. Rather, it is the mixture of elements from all of these topics that I here try to combine into a contribution that it is hoped can influence the future success of petroleum exploration.
There are a great number of references that are relevant to the topics touched upon in this thesis. I have chosen to refer only to those papers that are most relevant for the work. Each chapter is a self-contained paper, except for Chapters 1 and 22 which tie together the work and concludes, respectively. Published papers are referred to by author(s) and chapter numbers to facilitate the navigation within the volume for the reader.
The work presented here would not have been possible without the support from Karl Oscar Sandvik during the early days, when the concept of modelling secondary migration was still new to the petroleum industry. Karl Oscar was always supportive and established contacts with many future clients in the oil companies. Atle Mørk provided the inspiration to write this dissertation. Wenche Krokstad, Olav Selle, Odd Steve Hustad, Hans Borge, Ane Lothe, Oddmund Frigård and Frode Vassenden have been important discussion partners in different research topics. I am grateful for the many discussions with all my co-authors. Berit Fossum has helped with the drafting of figures in many of the papers.
Early support from Conoco, and in particular Tony Dore, was extremely important for me. The cooperation with Arild Skjervøy and Kate Weissenburger was rewarding because of their excellent understanding of the petroleum exploration process and willingness to try new and unproven techniques in Norwegian licensing rounds. During recent years, Agip and Norsk Hydro are particularly thanked for their support of my work. The many discussions with Domenico Grigo, Nils Telnes, Christian Zwach and Stuart Burley (BG) have enabled me to further develop the concepts discussed in this dissertation.
The Norwegian Research Council (NFR) has played a key role as an enabler: The first version of the Semi secondary migration simulator was coded with basic funding from NFR, and the final compilation of the dissertation was funded through a grant from NFR (Petroforsk). I am grateful for this funding. All the work included has been performed while I was an employee with IKU, which later became SINTEF Petroleumsforskning AS, and working on oil company funded research projects. SINTEF is thanked for making this possible.
My wife Wenche and our children Torstein and Inga have always been supportive. Without their support, I would not have been able to complete this work.
-technique,
and show secondary migration be a focused process. Migration of oil
and gas occurs in the uppermost parts of carrier systems, and
flow-paths are typically less than 1 m thick. The hydrocarbon
saturation and relative permeabilities are low, but migration
velocities approach 1000 km/My in many carrier systems. Full Darcy
fluid flow modelling using computational cells with vertical sizes of
more than a few centimetres, and average hydrocarbon saturations
within the computing nodes will, in general, not provide a good
description of the secondary migration process, and can result in too
large losses being modelled. Methods for quantifying secondary
migration and computing resulting hydrocarbon saturations, velocities
and losses are presented. These methods can be used for quick-look
assessments, and can be utilized in ray-tracing simulation
techniques.
Vertical leakage through a cap-rock has been studied in a visual laboratory experiment, and the results are interpreted to show that capillary leakage is a dynamic process that will typically build hydrocarbon columns that exceed the entry pressures of the cap-rocks. When supply from a source is stopped, the column will shrink. The column may shrink to a snap-off pressure that is less than the cap-rock entry pressure column height. The result is a dynamic trap that can continue to leak hydrocarbons millions of years after the filling has stopped. Leakage can therefore continue also during periods of uplift and erosion. Vertical capillary leakage will typically occur at low saturations and over a large area. Percolation modelling does not provide a good process description of this process, while multi-phase Darcy fluid flow modelling can provide an accurate description if the vertical resolution of the cap-rock is sufficient to accommodate the vertical flow property variability and hydrocarbon saturations.
The effects of faults as barriers and conduits to fluid flow are investigated. A pressure compartment technique is suggested for modelling the average single-phase flow properties of faults. The modelling is thereby simplified and speeded up to allow for sensitivity studies. This description has allowed for studying the time dependency of hydraulic cap-rock leakage and the influence on hydrocarbon trapping. Clay-smearing within fault planes is considered to be a good method for describing the fault seal properties for the modelling of multi-phase oil and gas migration across and along fault planes. This approach is integrated into a ray-tracing secondary migration simulator.
Migration and trapping of hydrocarbons are affected by many geological processes. The effects of burial reconstruction techniques and palaeo-water depths on secondary migration are investigated. The results suggest that traps may be sensitive to these processes.
Case studies demonstrate that secondary migration modelling can be used to study the dynamic history of trap filling and changes in trapped hydrocarbon phases. The good matches of the migration models that are achieved to calibration data suggest, but cannot prove, that migration modelling is a valid method for providing input to prospect risk assessment in the exploration process.
A Monte Carlo simulation technique that uses (pseudo) 3D basin modelling and migration modelling to compute probability distributions of accumulated volumes of oil and gas in prospects is presented. Important input variables are described using probability distributions. The method uses existing (drilled) calibration wells to weight simulation runs by the difference between modelled and measured oil and gas volumes or column heights. The methods can be used to compute maps of oil and gas finding probabilities and maps of most likely oil and gas columns. Standard deviations can also be plotted to study the uncertainties of model predictions. The quality and accuracy of the geologic knowledge can be investigated by construction of a-posteriori probability distributions of the geologic variables used to define the hydrocarbon generation and migration system. This is possible when large amounts of simulations are compiled.
Hydrocarbon migration, entrapment and preservation: Processes and Evaluation.
Experimental verification of low-dip, low-rate two-phase (secondary) migration by means of gamma-ray absorption.
On the vertical and lateral distribution of hydrocarbon migration velocities during secondary migration.
Quantifying secondary migration efficiencies.
New techniques and their application in the analysis of secondary migration.
Modelling of secondary migration and entrapment of a multicomponent hydrocarbon mixture using equation-of-state modelling techniques.
Secondary migration in a 2D visual laboratory model.
On the dynamics of capillary gas trapping: implications for the charging and leakage of gas reservoirs.
On the use of modelling techniques for hydrocarbon migration in carriers and seals.
3D Modelling of fault bounded pressure compartments in the North Viking Graben.
Evaluation of late cap-rock failure and hydrocarbon trapping using a linked pressure and stress simulator.
A method for including the capillary properties of faults in hydrocarbon migration models.
Structural restoration techniques in 3D basin modelling: Implications for hydrocarbon migration and accumulation.
Application of quantitative palaeobathymetry in basin modelling, with reference to the northern North Sea.
Modelling the hydrocarbon system of the North Viking Graben: a case study.
Modelling of expulsion and secondary migration along the southwestern margin of the Horda Platform.
From basin modelling to basin management: reuse of basin-scale simulations.
Oil and gas migration in the Sherwood sandstone of the East Irish Sea Basin.
Risk assessment using volumetrics from secondary migration modelling: assessing uncertainties in source rock yields and trapped hydrocarbons.
Estimation of Oil and Gas Column Heights in Prospects Using Probabilistic Basin Modelling Methods.
A probabilistic approach to improved geological knowledge and reduced exploration risks using hydrocarbon migration modeling.