Rock samples obtained from the reservoir are cut to thin sections with a thickness of 5 microns by thin sectioning machine. Then these thin sections are studied under a polarized microscope through the light refraction. In this way, the type of minerals are determined based on the angle of extinction.
To perform the routine and special core analysis, it is required to prepare the core plugs form the whole core with high precision. Therefore, core plugs are provided horizontally, vertically, and under different angles by Core Plugging Machine. Then, its surfaces flattened by the core cutting machine.
Core plugs are washed using Soxhlet Extraction Apparatus. Washing operations are performed by organic solvents such as toluene. After completing this step, the samples are ready for core analysis experiments.
Core plugs obtained from reservoir after washing and oven drying are ready for measuring core analysis parameters. In the following, the most important routine core analysis tests are represented.
Reservoir porosity is measured by helium or nitrogen gas using Boyle’s method. Due to the high precision pressure sensors used in Petro Ahoura systems, the porosity of all types of reservoir rocks, including sandstone and carbonate, is calculated with the accuracy of 0.1%.
Density Measurement of Rock Matrix
We first weigh the plug sample in a dry state without any fluids inside its empty space. Then calculate the volume of rock solids by using a vacuum flask. By knowing rock mass and dry weight, the density of rock grains can be calculated.
Gas Permeability Measurement Using Steady State and Unsteady State Methods
One of the methods for obtaining absolute rock permeability is to allow gas flow inside the rock. For high permeability rocks, after a short period of time the gas flow becomes stable and steady state condition will be stablished. Therefore, for these types of rocks the absolute permeability and equivalent liquid permeability can be calculated based on the Darcy law and the Klinkenberg method. For low and ultra-low permeability rocks sometimes it becomes impossible to implement steady state analysis. Therefore, in these cases unsteady state method is used to calculate permeability. In this method gas is charged inside the rock within the core holder and then it is allowed to evacuate from core holder. The length of time that gas completely discharged from the core sample is related to the permeability and the absolute permeability can be calculated through this principle. By using Petro Ahoura BIPERM system rock permeability can be calculated through both steady and unsteady state method. therefore, the permeability measurement can be achieved for a wide range of permeability.
Rock Wettability Analysis
Wettability is an important reservoir rock and fluid property which is the result of the surface tensions between rock, oil, and water. Wettability is the tendency of the rock surface to spread a fluid in the presence of another immiscible fluid. In fact, when two immiscible fluid are in touch with each other, one of them spread across the surface and the other ball up and avoid to contact with the surface. In general rocks can divided into two main categories: water wet and oil wet rocks. The rock wettability has a significant effect on the distribution of phases inside the reservoir.
There are several methods for measuring wettability:
Sessile Drop Analysis
In this method a drop of the fluid is placed on the rock surface while there is a second fluid surrounding the drop. The pictures are taken from the drop by using camera and contact angle is measured.
Wilhelmy Plate Method
In this method a rock plate is oriented perpendicular to the interface of the two immiscible fluid. The plate is connected to a balance which measure interfacial tension and measured contact angle indirectly, and consequently wettability.
The method combines two spontaneous imbibition measurements and two forced displacement measurements. The sample is submerged in the fluid to be imbibed and the displaced volume of nonimbibed fluid is measured.
It is similar to the Amott test with the difference that the capillary pressure displacement is done by centrifuging, this method is one of the most common methods for measuring the rock wettability under reservoir pressure and temperature conditions.
Important properties such as formation resistivity factor, formation resistivity index, Archie’s parameters, and saturation index is determined during electrical properties test.
Capillary Pressure Measurement
Capillary pressure is measured by different methods which have their own advantages and disadvantages.
In this method, also known as porous diaphragm, the core plug is saturated with a fluid (for example, water) which its end is in contact with the diaphragm. This diaphragm is impermeable to the injected fluid (for example, oil), therefore the injected fluid penetrate to the core plug and removes water from the rock. At each stage, with the equilibrium of the pressure reached, the total volume of the outflow fluid can be obtained from the rock and converted to saturation. Therefore, the capillary pressure graph can be plotted.