Nowadays a series of laboratory experiments on carbonate cores and logs were conducted in order to understand and quantify the observed changes. It is estimated that more than 60 % of the world's oil and 40 % of the world's gas reserves are held in carbonate reservoirs. The Middle East, for example, is dominated by carbonate fields, with around 70 % of oil and 90 % of gas reserves held within these reservoirs, that’s why we need to understand the nature of carbonate rocks.
Magnetic susceptibility is a widely used property that, in its most basic of magnetic inferences, gives some indication of the amount of magnetic minerals, mainly the mineral magnetite. Magnetic susceptibility is a common measurement employed in paleoclimate reconstruction of terrestrial, marine, and lacustrine environments. Another important petrophysical parameter is permeability, which is an indication of the ability for fluids (gas or liquid) to flow through rocks. It is affected by pore connectivity in a rock.
Magnetic susceptibility is probably the most easily measurable petrophysical parameter, because it can be measured not only in the special laboratories, but also in the field on rock outcrops. Magnetic susceptibility of rocks is in principle controlled by the type and amount of magnetic minerals contained in a rock. Sometimes, it is dominantly controlled by paramagnetic minerals (mafic silicates such as olivine, pyroxenes, amphiboles, micas, tourmaline, garnets), often by ferromagnetic minerals (iron oxides or sulphides, represented for instance by magnetite and/or pyrrhotite, respectively) and much less frequently by diamagnetic minerals (calcite, quartz). As the ferromagnetic minerals mostly belong to accessory minerals that are often sensitive indicators of geological processes, the magnetic susceptibility is a useful parameter in solving some petrologic problems. If it is positive, the material can be paramagnetic. In this case, the magnetic field in the material is strengthened by the induced magnetization. Alternatively, if it is negative, the material is diamagnetic. As a result, the magnetic field in the material is weakened by the induced magnetization. Generally, non-magnetic materials are said to be para- or diamagnetic because they do not possess permanent magnetization without external magnetic field. Ferromagnetic or antiferromagnetic materials, which have positive susceptibility, have permanent magnetization even without external magnetic field.
Permeability is typically determined in the laboratories by application of Darcy's law under steady state conditions or, more generally, by application of various solutions to the diffusion equation for unsteady flow conditions. In the field it can be also measured by using special devices and tools.
Permeability and magnetic susceptibility measurements are widely used in geological and soil studies, paleomagnetic and environmental studies, core logging & correlation.
These measurements on rock outcrops are very fast, one measurement taking from few seconds to few minutes, and one can execute numerous detailed measurements in reasonable time.
During the experiments magnetic susceptibility a number different carbonate samples (DR) were measured by using MS2E Surface Scanning Sensor, Bartington (Figure 1). The MS2E sensor is designed to perform high resolution measurements of magnetic susceptibility along flat surfaces which have a roughness less than 1 mm, the sensing surface is at the end of a ceramic tube. The tube is mounted on a metal enclosure which houses the electronic circuitry. The sensor is connected directly to the MS2 meter via a TNC-TNC coaxial cable which may be up to 30 meters in length:
In turn permeability values of the same rock samples were measured by using NER’s TinyPerm tool (New England Research, Inc.), which is a portable hand-held air permeameter used for measurement of rock matrix permeability or effective fracture apertures on outcrops and at the core scale.This instrument is designed to be used either in the field or indoors to take measurements on rocks or other porous materials. For rock matrix, the permeability measurement range varies from approximately 10 millidarcys to 10 darcys.
Fig. 1. TinyPerm for permeability measurements, New England Research, http://www.ner.com/
These carbonate rock samples were collected at petroleum reservoir analogues that are located in the Dominican Republic. After getting lateral magnetic susceptibility measurements and permeability value of the different types and textures of the carbonates from each point the average values were loaded to Surfer 8 Software and the following contour maps were created for each rock sample using x and y axes. Magnetic susceptibility values represent respectively values taken at the z position of the measuring probe scanner at the first map, and permeability value for the second contour map.
It is difficult to present the use of magnetic susceptibility and permeability in general geological terms, because this is a very complex problem. In our opinion, instead of attempting for generalization, it is more convenient to present a set of case histories of the use of magnetic susceptibility with permeability in solving various geological and petrophysical problems, in particular for carbonate petroleum reservoirs.
These carbonate rock samples were collected at petroleum reservoir analogues that are located in the Dominican Republic. After getting lateral magnetic susceptibility measurements of the different types and textures of the carbonates from each point the average values were loaded to Surfer 8 Software and the following contour maps were created for each rock sample using x and y axes. Magnetic susceptibility and permeability values represent respectively values taken at the z position of the measuring probe scanner.
Fig. 2. Magnetic susceptibility distribution in the centre map, permeability in the right map of the coral reservoir properties (DR 1) shown in the left picture
Fig. 3. Magnetic susceptibility distribution in the centre map, permeability in the right map of the organic carbonate reservoir properties (DR 2) shown in the left picture
The data shown in Figure 2 indicate that heterogeneous distribution of the petrophysical properties, in particular porosity, can be evaluated and systemized using magnetic susceptibility mapping. The lower zones of the magnetic susceptibility (-7 and lower) are generally correlated with higher porosity and permeability distribution. The high zones of the permeability (10,7 units and higher) are generally correlated with higher porosity distribution. The same regularity is observed in Figure 3, where more lower values of the magnetic susceptibility (-12) correspond to higher pore content, when the high zone of permeability (11 units and higher) are generally correlated with higher porosity distribution.
Fig. 4. Magnetic susceptibility distribution in the centre map, permeability in the right map of the coral in the coral (DR 4) shown on the left picture
Fig. 5. Magnetic susceptibility distribution of travertine rock in the centre map, permeability in the right map of the coral in the travertinecarbonate reservoir (DR 7) shown in the left picture
The coral internal structure (Figure 4) correlates with the higher magnetic susceptibility in the center of the plate and with lower permeability. The travertine reservoir section type indicates heterogeneous distribution of the magnetic susceptibility and permeability (Figure 5) together with often positive values of the magnetic susceptibility. The values of magnetic susceptibility and permeability in this sample depends on heterogeneity of the rock, primary by vugs existence and higher permeability in some parts of the rock. Positive values of magnetic susceptibility is a result of sand contents in this travertine's sample.
In general, results show that magnetic susceptibility is one of the most easily measurable petrophysical parameters. Magnetic susceptibility of rocks is in principle controlled by the type and amount of diamagnetic, paramagnetic and magnetic minerals in a rock. Magnetic susceptibility values indicated the amount of minerals presented in the carbonates, and also provided graphically important information for distribution of porosity/permeability, coral types and diagenetic changes, which were determined by using contour mapping. Importantly, the results show that carbonate reservoir rocks have a lower magnetic susceptibility and high permeability correlation. Also this zones contains more pores and have higher porosity. Magnetic susceptibility appears to be a parameters that can indicate reservoir quality index of carbonate petroleum reservoirs.
References:
1. Frantisek Hrouda, Marta Chlupakova and Martin Chadima. Use of Magnetic Susceptibility of Rocks in Geological Exploration. — Brno, 2009.
2. Wayne M. Ahr. Geology of Carbonate Reservoirs. — Texas A&M University, Wiley, 2009.
3. SEG-2009–2149 Characterization of Heterogeneities in Carbonates. — Colorado School of Mines, 2009.
4. SPE-58995-MSIntegrated Reservoir Characterisation of a Fractured Carbonate Reservoir, 2000.
