3D Pore Scale Characterisation of Carbonate Core: Relating pore types and interconnectivity to petrophysical and multiphase flow properties.
Authors: Knackstedt Mark Alexander , Sok Rob , Adrian Sheppard , Arns Christoph Hermann 
Published: February 2016

Abstract
Carbonate rocks are diverse and their pore space complex. Large scatter in petrophysical and multiphase properties of carbonates is caused by the variation in pore type, pore interconnectivity and the microporosity. To date the classification of the interconnectivity of the different pore types and pore shapes and their relative contribution to flow, electrical and displacement properties has been made on the basis of visual inspection and 2D thin section analysis. In this paper we present a 3D image and analysis study of a range of carbonate core material at a 3 micron scale. The 3D interconnectivity of the vugs, macropores, and meso/micropores is directly characterized.
We undertake simulations of resistivity (m and n) and drainage capillary pressure directly on the image data under various wettability conditions. Prediction of drainage capillary pressure, m and n from image data are compared to laboratory measurements where available on the same core material and found to be in good agreement. The role of different pores and their interconnectivity are correlated to the behaviour of m and mercury injection capillary pressure for different cores. The distribution of fluids during drainage are directly probed within the 3D pore space via experimental and numerical methods. A strong complexity and variability in the RI curves for different carbonates is noted under different wettability conditions. The implications of structure and topology to relative permeability and ultimate recovery are also discussed.

Introduction
Carbonate reservoirs contain more than 50% of the world's hydrocarbon reserves. In carbonate rocks, the processes of sedimentation and diagenesis produce microporous regions and a wide range of pore sizes, resulting in a complex spatial distribution of pores and pore connectivity. Developing a reliable petrophysical interpretation for predicting the single and multiphase transport properties and producibility of carbonates remains difficult. Much of the poor reliability in estimating carbonate properties is due to the complex pore structure exhibited by carbonates. Unlike sandstones, many carbonate sediments have a multi-modal pore size distribution with organisms playing an important role in forming the reservoirs [1,2]. Carbonate rocks are further complicated by significant diagenesis resulting from chemical dissolution, reprecipitation, dolomitization, fracturing, etc. For these reasons the pore structure is expected to be very heterogeneous and is known to exhibit pore sizes ranging from sub-micron to centimetres. This feature distinguishes the petrophysical properties and productivities of carbonate fields from other sedimentary rocks [3].
In previous work we have described the development of a capacity to characterize and predict a range of multiphase flow and petrophysical properties from experimental 3D microtomographic images of rock microstructures [4]. Rock properties derived from fragments of a range of cores including homogeneous and reservoir sands have been compared with conventional laboratory measurements and shown to be in good agreement [5–7]. In this paper we consider a number of carbonate samples; samples are imaged and analysed over a range of length scales using high resolution X-ray microtomography (µ-CT). The samples imaged include sucrosic dolomite samples with significant intercrystalline porosity, oomoldic core, vuggy limestone and bioclastic grain/packstone. The variation in pore morphology across these samples in 3D is quantified. Many samples exhibit a substantial presence of sub-micron porosity. Although pores at the submicron scale cannot be imaged with the current facility we use x-ray attenuation to describe the distribution of microporosity in the sample. The resultant porosity map agrees with experimental mercury injection capillary pressure (MICP) measurements performed on the same core material. Mapping microporosity allows the visualisation of the spatial distribution of the macro / meso / micro porosity contributions. Pore networks allow one to characterize the pore topology.