Andy Heath

Representation and scaling of faults in fluid flow models

Fault representation and scaling in flow models are examined with respect to fault zone properties, the accuracy with which they can be determined, and how these variables and fault geometries can be incorporated realistically in to flow models. Outcrop data show that fault displacement/thickness ratios and permeability vary widely. For simple single fault models, results for numerical models are compared with analytical and statistical methods. Representation of a fault as a transmissibility surface conflates the effects of four variables--fault zone thickness and permeability, grid-block size and matrix (host-rock) cell permeability. Random spatial variation of transmissibility factor values is well represented by a uniform transmissibility factor which is the arithmetic mean of the values representing log-normally distributed permeability and thicknesses. Realistic ranges of fault zone thicknesses can be represented without grid-block refinement by an upscaling method based on simple transformation of transmissibility factor curves derived from a range of coarse grid-block models. Sub-seismic faults have significant effects on effective permeability of model volumes at kilometre scales only when the faults are assigned a permeability less than c. 0.001 of the matrix permeability.

Faults in conventional flow simulation models: a consideration of representational assumptions and geological uncertainties

Even when geologically based methods are used to determine fault rock permeabilities and thicknesses for input into flow simulators, a wide range of simplifying assumptions regarding fault structure and content are still present. Many of these assumptions are addressed by defining quantitative and flexible methods for realistic parameterization of fault-related uncertainties, and by defining automated methods for including these effects routinely in full-field flow simulation modelling. The fault effects considered include: the two-phase properties of fault rocks; the spatial distributions of naturally variable or uncertain single-phase fault rock properties and fault throws; and the frequencies and properties of sub-resolution fault system or fault zone complexities, including sub-seismic faults, normal drag and damage zones, paired slip surfaces and fault relay zones. Innovative two-phase or geometrical upscaling approaches implemented in a reservoir simulator pre-processor provide transmissibility solutions incorporating the effect, but represented within the geometrical framework of the full-field flow simulation model. The solutions and flexible workflows are applied and discussed within the context of a sensitivity study carried out on two faulted versions of the same full-field flow simulation model. Significant influence of some of these generally neglected fault-related assumptions and uncertainties is revealed.

X-ray diffraction measurements on potassium nitrate under high pressure using synchrotron radiation

Potassium nitrate (KNO3) has seven polymorphs in the pressure range 0-4.0 GPa. The authors have studied it at room temperature as a function of pressure up to 9.3 GPa and have confirmed the structure of phase IV previously refined by neutron diffraction at 0.36 GPa. The compressibility measured in the range 0.3-9.3 GPa is found to be anisotropic with the axial compression ratios a:b:c=1.0:0.64:0.50. The relative merits of X-rays and neutrons for high-pressure studies are discussed.

Assessment of the effects of sub-seismic faults on bulk permeabilities of reservoir sequences

Abstract Single-phase horizontal bulk permeabilities for 3 km × 3 km volumes of varying thickness of a typical Brent permeability sequence have been calculated, both before and after faulting by a range of sub-seismic fault arrays, with maximum fault displacements of 20m – 2m. The models incorporate realistic juxtaposition geometries across fault surfaces. Results are expressed in terms of fractional bulk permeabilities (Kf), i.e. ratio of bulk permeability of faulted model and of pre-faulting model. Fault and fault array variables modelled and tested were fault density, spatial distribution, orientation distribution and fault zone permeability relative to the host rocks, expressed as transmissibility factor (Tf). Realistic fault zone thicknesses were incorporated by use of a scaling factor. Low, moderate and high fault densities have significant and markedly different effects on Kf whereas the effects of spatial and orientation distribution variations are slight except at very low Tf values (Tf < 0.001). The relative insignificance of fault spatial distributions is due to closer fault spacing resulting in locally high hydraulic gradients which increase flow through fault surfaces unless these surfaces have very low Tf values. Prediction of fault zone hydraulic properties remains the most important factor contributing to modelling uncertainties.

Towards seasonal forecasting of malaria in India

BackgroundMalaria presents public health challenge despite extensive intervention campaigns. A 30-year hindcast of the climatic suitability for malaria transmission in India is presented, using meteorological variables from a state of the art seasonal forecast model to drive a process-based, dynamic disease model.MethodsThe spatial distribution and seasonal cycles of temperature and precipitation from the forecast model are compared to three observationally-based meteorological datasets. These time series are then used to drive the disease model, producing a simulated forecast of malaria and three synthetic malaria time series that are qualitatively compared to contemporary and pre-intervention malaria estimates. The area under the Relative Operator Characteristic (ROC) curve is calculated as a quantitative metric of forecast skill, comparing the forecast to the meteorologically-driven synthetic malaria time series.Results and discussionThe forecast shows probabilistic skill in predicting the spatial distribution of Plasmodium falciparum incidence when compared to the simulated meteorologically-driven malaria time series, particularly where modelled incidence shows high seasonal and interannual variability such as in Orissa, West Bengal, and Jharkhand (North-east India), and Gujarat, Rajastan, Madhya Pradesh and Maharashtra (North-west India). Focusing on these two regions, the malaria forecast is able to distinguish between years of “high”, “above average” and “low” malaria incidence in the peak malaria transmission seasons, with more than 70% sensitivity and a statistically significant area under the ROC curve. These results are encouraging given that the three month forecast lead time used is well in excess of the target for early warning systems adopted by the World Health Organization. This approach could form the basis of an operational system to identify the probability of regional malaria epidemics, allowing advanced and targeted allocation of resources for combatting malaria in India.

Geological implications of a large pressure difference across a small fault in the Viking Graben

Abstract Two discrete pressure cells with a 128 bar pressure difference within a 100 m thick Tarbert reservoir interval are separated by a fault with a throw of 50 m. Given that high permeability reservoir rocks are juxtaposed across the fault, the observed across-fault pressure difference cannot be maintained for reasonable fault rock permeabilities over geologically significant periods (> 10,000 years). A high resolution flow model of a 60 km 2 area straddling the pressure compartments is used to investigate the main parameters controlling pressure compartmentalisation. Single-phase hydrodynamic flow modelling demonstrates that the observed pressure distribution requires low across-fault transmissibilities and relatively high hydrodynamic flow rates (10 m 3 /day). The most significant contributor to the high flow rates is gas generation and migration into the high pressure cell. Low fault transmissibilities are attributed either to shale smearing or, less likely, to extensive quartz cementation of the fault rock. Our study shows that pressure compartmentalisation in the North Sea can be controlled by relatively small displacement faults and highlights the importance of high resolution 3-D geological models in understanding overpressure distribution.

Vibrational spectroscopy at high pressures. LVI, A Raman study of some alkali metal halates, and the X-ray compressibility of KClO3

Abstract Raman spectra at high pressure have been obtained for KBrO3, NaClO3 and NaBrO3 in a diamond anvil cell. A phase change has been found for KBrO3 at 19 kbar; at 24 kbar for NaBrO3; and at 36 kbar for NaClO3. The bulk compressibility of KClO3 has been determined by X-ray diffraction using synchrotron radiation and found to be 2.36 × 10−2 GPa−1, in good agreement with Bridgmans value.

3-D Flow Connectivity Associated with Relays

Simple streamline simulation models are used to illustrate some of the properties of relays in a production flow context. The prevailing view of an unbreached relay is essentially of a hole in a fault providing an uninterrupted lateral flow path across it.

A software package for data acquisition and analysis: application to a ruby R-line spectroscopic system

A spectroscopic system used for the calibration of pressure using the ruby R-line method has been designed and interfaced to a BBC model B microcomputer. The methodology of the real-time software design is detailed including flow diagrams for the three main programs employed in the system. The software includes both ambient and high-pressure data acquisition and analysis programs. Suggestions are made for the application of the software system to other experimental tasks and also to other compatible microcomputers.