Characterization of carbonate rocks by combination of scattering, porosimetry and permeability techniques
Introduction
Hydrocarbons are produced from granitic and/or volcanic reservoirs in more than twenty basins all over the world. Such basic basins are found in Algeria, China, Russia, Ukraine, Romania, Indonesia, Argentina, Great Britain, Yemen and many others [1], [2]. Several studies determined the relation between pore structure (pore size distribution, pore volume, fractality) and permeability properties at various petroleum reservoirs [3]. The area that forms the scope of this study lies in the Gulf of Cavala at Greece. The reservoir pressure varies from 140 to 400 bar and the exploitation takes place using gas lift. The petroleum reservoir expands at an area of four square kilometers and its depth varies from 2500 to 2900 m. In general, the oil productivity is considered to be low. This fact can be attributed to both the low porosity and the low permeability of the reservoir formation, which is verified by the outcome of the measurements carried out on this study. The determination of the complete pore size distribution in carbonate rocks is even more challenging. In clastic rocks, like sandstones, pores are observed between compacted grains, but also in the form of micro and mesoporosity often attributed to the presence of clays [4]. The solid–void interface in sandstones is known to be a surface fractal [5], [6], [7].
Small-angle neutron scattering (SANS) is a non-destructive and non-intrusive technique for providing structural details about inhomogeneities of scattering objects, such as pores, in the size range from 1 nm to about 200 nm. In addition, during last decade there is a gradually increasing interest for Ultra SANS (USANS) facilities. The USANS technique is a useful tool for the investigation of larger inhomogeneities having sizes extending up to few micrometers in real space [8], [9], [10]. In the present study, USANS measurements have been performed for the characterization of the carbonate samples.
The nitrogen adsorption technique is able to detect pores in the size range from 0.35 to about 500 nm. The specific surface area (SSA) was determined by the Brunauer–Emmet–Teller (BET) method [11] using am (N2) = 0.162 nm2, where, am, is the molecular area of nitrogen at 77 K, i.e. the area occupied by a nitrogen molecule in the complete monolayer.
Mercury porosimetry is a routine method for determining the pore structure of solids with pore sizes varying from large mesopores to macropores [12]. By applying the high and the low-pressure mercury intrusion techniques pore size distributions in the size ranges of 16 μm to 3.6 nm and 180 μm to about 16 μm, respectively, can be precisely determined.
The relation between porosity and permeability was also studied through the application of capillary (Dullien) and phenomenological (Rumpf and Gupte) permeability models [13], [14], [15]. Phenomenological models are based on the definition of dimensionless quantities or groups of dimensionless quantities (Reynolds number, Porosity, L/Dp = length to pore diameter ratio and other dimensionless parameters that characterise the properties of the porous media) that are interrelated with a stable functional correlation independently of the conditions of flow and the structure of the pore medium. On the other hand, capillary flow models are developed to describe the flow in porous media for which it is assumed that the pores are parallel cylinders aligned in one direction. These models are based on the Hagen–Poiseuille equation that is valid for the one-dimensional laminar flow. In addition, the experimental permeability results for the standard Berea sandstone samples were obtained via the pressure drop technique, which is extensively described in the following section.
Section snippets
Experimental
The six carbonate rocks were exploited by Gulf of Cavala, Greece during a research drilling performed by Kavala Oil Company. All samples were studied with EDX analyzer. Table 1 presents the lithology of the samples. It is worth mentioning, that apart from the EDX measurements, the nitrogen porosimetry measurements also confirm the clay existence in the samples.
The USANS measurements were carried out using the V12a DCD instrument at BENSC, HMI, Berlin. The wavelength was λ = 0.476 nm. The samples
Results and discussion
The USANS data were evaluated by using the PRINSAS software [19]. The slit desmearing, for transforming the USANS data to the point geometry, was also performed by the same software, based on the method of Lake [20]. The surface fractal dimension, Ds, was determined from the desmeared I(Q) vs. Q plots (Fig. 1). According to Bale and Schmidt [21], at Porod’s region ; where Ds is the surface fractal dimension. For a smooth surface Ds = 2 (Porod’s law). When however, the surface texture is
Conclusions
A series of carbonate rocks were characterized by combining a variety of techniques. The USANS measurements revealed that the microstructure of the sample is fractal with surface fractal dimension, . The evaluated probability density of the pore size distribution is characteristic for a polydisperse system of randomly oriented pores, following a power law which is also ubiquitous for fractal systems. The nitrogen adsorption isotherms suggested the presence of clay content in the samples.
Acknowledgments
The project is co-funded by the European Union–European Social Fund and National Resources–(EPEAEK-II), Project “Archimedes 04-3-001/7”. We would like also to thank the Kavala Oil for kindly offering of the carbonate rocks.
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