Overburden pressure in carbonate reservoirs is of great importance in understanding and improving productivity and recovery of hydrocarbons because it affects porosity, permeability, capillary pressure, particularly micro-inter-crystalline-framework and micro-fracture structures. As a result of this complexity, a Ruska Gas Permeameter was used at different pressures to measure the rock properties such as porosity and permeability of reservoir cores. The results were compared to CT-scan and NMR data.
In this research over 50 core samples were prepared from AB-Teymur carbonate oil reservoir (Iran). The samples were placed in the core holder and overburden pressures of 500, 1000, 1500, 2000 and 3000psi were applied. Then Helium porosity and permeability were measured.
This paper describes a systematic experimental methodology to analyze the different permeability and porosity values for reservoir characterization. The new study has shown the role of particularly micro-intercrystalline pore structure in carbonate rocks through digenesis in providing various ranges of porosity and permeability. The different hydrostatic levels in the reservoir rocks achieve a range of micro-fracture scenarios. Hence carbonate formations generally have complex porosity and permeability based on variation of overburden pressure in comparison to siliciclastic sandstones and there are not enough works published in the literature.
In carbonate reservoirs, overburden pressure has significant effects on pore architecture, geometry, primary and secondary porosity and absolute permeability. As a result of complicated carbonate lithofacies and diagenesis processes, pore structure and pore sensitivity are more problematic than in siliciclastic sandstones. Overburden pressure may change the reservoir quality, capacity, producibility and recovery factors.
Routine petroleum engineering reservoir parameter calculations are generally performed in the absence of the overburden pressure effect. This leads to serious errors in further development of a hydrocarbon-bearing reservoir. Thus, measurements under stress conditions become vitally important1. In the estimations of reservoir properties, the effect of overburden pressure is usually disregarded and this results in severe errors during the data processing and reservoir characterization measurements.
The objective of this study is to investigate the effect of overburden pressure in vuggy carbonate oil reservoir by using real core samples. In this research the range of permeability and porosity reduction vs. increasing overburden pressure at a carbonate reservoir characterized by various facies is investigated. The results are compared to mass balance, nuclear magnetic resonance (NMR) and computerized tomography (CT scanning) data.
In this investigation about forty two core samples from 3100m to 3450m depth of real carbonate reservoir in south west Iran were tested. The plugs were cored and cut to proper size. Then they were cleaned and dried. These processes were very time consuming and difficult because of the tight rock matrix.
A Ruska Gas Permeameter was used for a gas expansion experiment to measure the porosity and permeability variation at different overburden pressures. In the laboratory tests, indicative different core plugs from different depths were used for each application. The bulk density and grain density of all samples were measured. Then the pore volume of each plug was established by helium gas expansion and the sample porosity and permeability were determined.
