Characterization of carbonate rocks by combination of scattering, porosimetry and permeability techniques

Abstract

A mineralogical and microstructural study was conducted on rocks from formations in Cavala Gulf basin (PRINOS), Greece. The samples were obtained from six different depths of the petroleum drill and were characterized by ultra small-angle neutron scattering (USANS), nitrogen porosimetry and mercury porosimetry. The microstructure of all the samples is fractal ($D_{\text{s}}\sim 2.5$) within the experimental Q region. Mercury porosimetry measurements showed that all samples present rather similar pore structure with rather low porosity and exhibit polydisperse pore size distributions, in agreement with the USANS data. Furthermore, three powder segments with different grain size in the range between 90 and 200 μm, possessing narrow size distributions, were selected with the aim to elucidate the possible existence of large macropores. As a next step, the pore structure and liquid flow properties of standard core samples (Berea sandstone) were examined by combining nitrogen and mercury porosimetry as well as permeability measurements. A good correlation of the experimental findings with the predictions of capillary permeability models was established. These models were also involved to predict the permeability properties of the rocks under investigation with main inputs the experimentally determined porosity and mean pore diameter. The aforementioned procedure was followed because the rocks exhibit extremely low permeability factors and, thus, the accuracy of the measurements with the existing experimental core holder setup would be uncertain.

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 a_m (N₂) = 0.162 nm², where, a_m, 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/D_p = 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.