Magnetic properties of rocks show a wide range of geoscience applications and they are rapid and non-destructive to measure. Magnetic susceptibility is one of the most common magnetic parameters that allows to distinguish between different rock types and to understand geological processes that are related to Fe-mineralogy. The physical basis for the discrimination is the nature of magnetic minerals, their distribution, and grain size. Thus, magnetic methods show great potential for reservoir rock characterization, fabric determination, ore deposits, environmental issues, and even pore fabric studies. We particularly encourage contributions dealing with reservoir characterization applying magnetic methods from all areas of geology including method developments and applications of magnetic methods in all kinds of geological reservoir characterization.
9:00am - 9:30amSession Keynote
Magnetic pore fabrics and how they predict preferred fluid migration paths in porous rocks
University of Bern, Switzerland, Switzerland
The shape preferred orientation and connectivity of pores in reservoir rocks largely controls fluid migration properties, for example, by defining preferred flow directions. An accurate determination of preferred flow directions, observed as permeability anisotropy, is an integral part of reservoir characterization, due to profound effects on fluid migration. Numerous research fields, including groundwater studies, hydrocarbon exploitation, contamination mitigation, and CO2 sequestration, therefore seek methods to reliably characterize pore fabrics and permeability anisotropy. Many traditional methods face trade-offs between sample size and resolution, and measurements of permeability anisotropy require several oriented cores, where anisotropy may be masked by core-scale heterogeneity, and assumptions on the fabric orientation need to be made when less than six cores are measured. The magnetic pore fabric method has the potential to overcome these difficulties, and has shown promising empirical relationships to both the preferred orientation of pores, and permeability anisotropy. Magnetic pore fabrics are determined by impregnating rock with ferrofluid, and then measuring the anisotropy of magnetic susceptibility. These measurements provide a full 3D average fabric measure from a single core. So far, interpretation was compromised by large variability in the empirical relationships published in different studies. Here, experimental developments, and a conceptual and numerical model are presented that enable more thorough and quantitative interpretation of magnetic pore fabrics.
9:30am - 9:45am
Characterization of pore space in sandstone using the anisotropy of magnetic susceptibility
1Karlsruher Institute of Technology, Germany; 2Gubkin University, Russia
The pore space in siliciclastic rocks is one of the most important petrophysical properties in reservoir rock characterization. Of particular interest is the 3D distribution of pore space and permeability for the purpose of reservoir model development. We used a magnetic technique to determine the preferred orientation of the pore space. The approach is based on the injection of a magnetic ferrofluid, which is a stable colloidal suspension of approx. 10 nm-sized magnetite particles, into a rock specimen. The anisotropy of magnetic ferrofluid susceptibility (AMFFS) is then measured for its AMS and the orientation was compared with the rock’s AMS.
We used red Permo-Triassic sandstones of different Buntsandstein and Rotliegend facies, which represent Europe’s highest geothermal water and hydrocarbon reservoir potential. The used porosity methods were helium pycnometry, mercury injection porosimetry, computer tomography and ferrofluid injection. Although the used methods show a strong deviation for single samples, which is related to the different pore size ranges for which each method provide reliable information, the trends are comparable. The computer tomography showed pore space network to be parallel with the bedding of the sandstone. The AMFFS is also mostly similar to the rock’s AMS with principal minimum susceptibility axis normal to the bedding. However, small deviations of AMS and AMFFS axis orientation occur. This deviation could indicate that the ferrofluid fabric is related to a special size of pores. Further investigations are needed to verify this hypothesis.
9:45am - 10:00am
Identification of magnetic enhancement at hydrocarbon/water contacts.
1Imperial College London, United Kingdom; 2CGG, United Kingdom
Pyrolysis experiments and calculated thermostability diagrams show that iron bearing minerals (< 60nm) can be produced inorganically during oil formation in the ‘oil-kitchen’ or be precipitated in the reservoir via alteration or replacement of existing minerals. Here we use this observation to find a magnetic proxy that can be used to identify hydrocarbon fluid contacts by determining the morphology, abundance, mineralogy and size of the magnetic minerals present in reservoirs. We address this by examining core samples from the Tay Sandstone Member in the Western Central Graben in the North Sea.
The magnetic properties of core samples from the study area were determined using room-temperature measurements on a Vibrating Sample Magnetometer (VSM), low-temperature (0-300K) measurements on aMagnetic Property Measurement System (MPMS) and high-temperature (300-973K) measurements on a Kappabridge susceptibility meter while hydrocarbon fluid contacts were determined using wireline logs.
We observed magnetic enhancements at both gas-oil and oil-water contacts that are detectable both through magnetic susceptibility measurements and magnetic hysteresis measurements. This magnetic enhancement is due to the precipitation of new nanometric iron oxide (magnetite) and iron sulphide (greigite) phases. The magnetic enhancement may be caused by diagenetic changes or preferential biodegradation at the top of the oil column during early filling and at the oil water contact.
10:00am - 10:15am
Using mineral magnetics to track migration in the Bittern and Pict Fields, Central North Sea
Imperial College, United Kingdom
Minerals magnetics has been proposed as a means of improving our understanding of petroleum systems. We have carried out extensive rock magnetic experiments on core samples from the Tertiary reservoir sands of the Bittern and Pict Field, UK Central North Sea. This together with Petroleum Systems Modelling/Analysis has revealed the potential to characterise hydrocarbon migration pathway and fill-pattern through siderite identification. We have also suggested a plausible mechanism responsible for these observations. Furthermore, the presence of different flow zones or ‘compartments’ in petroleum reservoirs may also be identified through the distribution of magnetic minerals. In terms of ease of application in oil exploration and reservoir characterization, there is a potential to define this parameter very efficiently through the measurement of magnetic susceptibility.
10:15am - 10:30am
Effect of cyclic loading at elevated temperatures on the magnetic susceptibility of a magnetite-bearing ore
1Karlsruhe Institute of Technology, Institute of Applied Geosciences, Germany; 2Institute of Geophysics, Polish Academy of Sciences, Poland; 3Karlsruhe Institute of Technology, Institute for Applied Materials, Germany
Rocks are often subjected to dynamic stress that occurs during earthquakes, volcanic activity as well as human-induced activities. The aim of this study is to test if mechanical fatigue in rocks can be monitored by magnetic methods. For this purpose, the effect of cyclic-mechanical loading (150 + 30 MPa) on the magnetic susceptibility and its anisotropy of a magnetite-bearing ore with varying temperatures and environment was investigated. Our study shows that magnetic susceptibility decreases significantly (up to 23%) under air conditions and even in vacuum (up to 4 %) within the first ca. 1000 cycles. Further loading does not significantly affect the magnetic susceptibility which then remains more or less constant. However, a stronger decrease of susceptibility parameters is observed at higher temperatures. As magnetic susceptibility was measured after decompression of the loaded sample at room temperature, magnetostriction cannot be the reason for these changes. After cyclic loadings in air, the transformation of magnetite to hematite is the major mechanism affecting bulk magnetic susceptibility. The weak changes in magnetic susceptibility after vacuum loadings are related to the formation of damage and deformation structures observed on the surface of magnetite grains. We have shown that cyclic loading can change significantly the induced magnetization of a rock due to mineral transformation below < 1000 cycles and that mechanical fatigue, which is a precursor of the failure of a rock, is closely associated with these transformations. Therefore time-dependent magnetic susceptibility measurements can be used as a proxy parameter of mechanical fatigue.