Supervisors: Professor Mike Lovell and Dr Sarah Davies (University of Leicester), Dr Tim Pritchard and Mr Robert Webber (BG-group)

This studentship is fully funded by BG-group (Reading) and is available to start in the Department of Geology in October 2009

Project Summary:
The petrophysical interpretation of heterogeneous sandstone or ‘shaly sandstone’ reservoirs is complicated because the presence of clay minerals and clay-sized particles has unusual and often unpredictable effects on the geophysical properties, such as electrical resistivity and acoustic velocity or travel time, which can lead to significant underestimation of hydrocarbon volumes. Well established relationships between geophysical properties and reservoir parameters (density, porosity, saturation, permeability) for ‘clean’ sandstone reservoirs often yield unsatisfactory results in ‘shaly sandstone’ reservoirs.
This project will investigate existing petroacoustic models for heterogeneous sands using existing data (core and downhole logs) and propose alternative models and strategies for predicting acoustic properties. The hypothesis behind this project is that accurate prediction of Vp, Vs and density from acoustic models depends on the detailed distribution and nature of clay/silt particles and clay minerals in heterogeneous shaley sand formations.
Key Objectives: (1) review the range of petroacoustic models in the literature and document the evolution of these models, identifying the relative importance of the several degrees of freedom in each model, together with the limitations of each model. (2) develop a petroacoustic classification scheme using appropriate data sets demonstrating variability in the nature of the shaly sands, based on mineralogy, physical grain size properties/sedimentology, burial history (physical and chemical compaction) and petrophysical parameters (3) determine the optimal strategy for modelling acoustic properties of each member of the classification scheme (4) propose novel modelling approaches based on the limitations and constraints of existing models (5) Produce Rock Physics Templates for predicting the elastic properties of each member of the classification scheme.
Methodology and Approach: A major difficulty arising from recent work on the acoustic modelling of reservoir formations is that models have evolved to contain a large number of variables. Many of these are easily defined for clean formations, but in shaly sandstones their estimation is problematical and they are poorly constrained. We propose to revisit the prediction of acoustic properties, will clearly document the evolution of petroacoustic models, and investigate their use in different formations. This requires careful assessment of sedimentology of the reservoirs combined with detailed evaluation of the models. The student will spend a considerable part of the project investigating the mathematical basis of existing models and developing novel approaches based on the underlying sedimentology.
Objectives:
Petroacoustic model evolution. Gassmann’s equations 4,5 underpin a considerable part of acoustic modelling yet apply only to monominerallic rocks in which the fluid is free to move. A review of petroacoustic models in the literature 2,3,4,5 will document the evolution of these models and identify their importance and limitations.
Petroacoustic model classification of shaly sands. The petrophysical parameters (especially clay volume, effective porosity and saturation) are intricately linked to the sedimentology6, and together these must form the basis of any characterisation or classification scheme. This links to objective (1) in defining which approach to modelling is most likely to work and also in identifying the constraints when failure is more likely. Data availability scenarios should be considered in the petroacoustic classification schemes of shaly sands, such as the exploration phase of prospect analysis where limited data may be available.
Modelling strategies. Based on the results of (2) a modelling strategy will be developed to predict Vp and Vs in different ‘shaly sandstone’ formations. This will include work to identify the different members of the classification scheme from other downhole logs, thus enabling a predictive capability from downhole data alone. This will include strategies to deal with the upscaling effects of modelling the effective moduli at an appropriate scale such that the seismic response can be predicted for different 'shaly sandstone' formations.
New models. The development of novel approaches and alternative models will be based on the limitations and constraints of existing models.
Proof of concept. A calibrated Rock Physics Template7 for the prediction of both downhole log data and the modelled seismic response for each member of the shaly sands classification scheme (as defined in objective 2). This will include how the elastic properties are directly affected by sedimentology, physical and chemical compaction, and fluid saturation. These templates will be validated by data sets that include those classes of heterogeneous sandstone reservoirs, and include core, log and 3D pre-stack seismic data.
Training Elements and Project Management:
Training will include: sedimentological analysis at core-scale and sub-core scales (<10-2 m) using optical and electron microscopic imaging; clay mineralogy; petrophysics; petroacoustic modelling, data analysis; mathematical modelling. Transferable skills will be developed in project planning, data management, and report and paper writing
Project Management: The student will be based at the University of Leicester. Regular joint meetings will be held with the supervisors and students based at BG group. During the course of the PhD project the student will spend between 3 and 18 months working within the company (BG group, Reading), in addition to formal meetings with BG group supervisors.
References: 1Baines, V., Bootle, R, Pritchard, T, Macintyre, H, & Lovell, M.A. 2008. Predicting shear and compressional velocities in thin beds. Proceedings of the 49th SPWLA Annual Logging Symposium, Edinburgh. 2Dvorkin, J. and Gutierrez, M.A. 2001a. Grain Sorting, Porosity, and Elasticity. Society of Exploration Geophysicist Annual Meeting. 3Dvorkin, J. and Gutierrez, M.A. 2001b. Textural sorting effect on elastic velocities, part II: elasticity of a bimodal grain mixture. Society of Exploration Geophysicist Annual Meeting, Expanded Abstracts. 4Gassmann, F. [1951] Elastic waves through a packing of spheres. Geophysics, 16, 673-685. 5Simm, R.W., 2007. Practical Gassmann Fluid Substitution in Sand/Shale Sequences, First Break, 25, 61-68. 6Thomas, E.C., and Stieber, S.J. 1975. The distribution of shale in sandstones and its effect on porosity. SPWLA 16th Annual Logging Symposium. Paper T. 13pp. 7Chi, X., Han, D., 2009. Lithology and fluid differentiation using a rock physics template, The Leading Edge, 28, 60-65.
Eligibility and how to apply:
Funding is available for UK citizens and EU citizens.
Potential applicants:
* must hold a good degree in geosciences with an aptitude for numerical geology, geophysics or petrophysics
* must have excellent written and spoken English and be highly motivated to work within a team; and
* must not already hold a PhD degree.
Candidates should apply to the Postgraduate Admissions Office using the Postgraduate Application Form at http://www.le.ac.uk/graduateoffice/application_form.pdf
In addition to the application form, please include in your application: examples of your technical work that illustrate good writing, two letters of reference, curriculum vitae, a letter explaining your motivation to do this particular project and any other information that you may find important.
For informal enquiries please contact Mike Lovell mtl@le.ac.uk Department of Geology University of Leicester, University Road Leicester LE1 7RH.  Tel:+44 (0)116 252 3647. Fax 0116 252 3918.
Closing date is 31st July 2009.