Research Paper on Relationship between Conventional Core and Special Core Analyses

Core analyses provide information required for reservoir characterization, description, as well as simulation, recovery methods, log calibration and all other purposes directly or indirectly involved throughout the life cycle of a field. Special Core analysis, (SCAL) is a procedure common in the petroleum industry; it's the process of performing flow experiments on the core plugs which are taken from a petroleum reservoir. It is distinguished from the Conventional core analysis (CCAL), mainly by conducting additional experiments, particularly determining relative permeability, capillary pressure, and measurements of two-phase flow properties. Conventional Core Analysis, also known as routine core analysis, uses a small sample to represent a large interval of the core. When the pore system is homogenous, it produces acceptable results. Plugs for conventional core analysis are collected usually once per foot or three to four times per meter. (Routine core analysis - Schlumberger Oilfield Glossary, 2018)

CCAL and SCAL are closely related-they both depict the porosity-permeability relation in the reservoir, but SCAL does this in more details. CCAL includes the measurements of basic petro physical properties including permeability (vertical and horizontal), porosity, and grain and rock density. SCAL includes both the measurements and the detailed study of relative permeability, capillary pressure, surface and interfacial tension as well as the wettability. It gives us detailed ideas concerning the quality of the rock, their wetting nature-whether water wet or oil wet.

Since The properties of these two core analyses methods are related they complement each other. CCAL alone is not enough in instances when more exquisite details about the core are required(Zheng & Ju, 2015). Routine Core analysis answers three fundamental questions about a reservoir; does the rock being analyzed contain the fluid-like space, also known as porosity; do the areas include hydrocarbons, and lastly does those hydrocarbon fluids (if any) be produced permeability? Core Gama logging is vital as it provides a link of the core depth, and compares it to that of the wellbore depth, as well as core computed tomography (CT) Scans that help to indicate the heterogeneity of the center.

The results from the routine core analysis are critical when a detailed understanding of the reservoir is required. This is done through the Special Core Analysis. Archie exponents, for example, are obtained from the electrical measurements for calibrating electrical logging measurements of saturation, porosity as well as the Nuclear Magnetic Resonance (NMR). ("Core Analysis Services for Conventional Reservoirs | Schlumberger," 2018). The capillary pressure measurements can indicate saturation Distribution of pore throats by centrifuge, mercury injection, or acceptable plate methods. These are used for evaluation of saturation distribution. Relative permeability, also determined from the SCAL, determines the multiphase flow character of the formation. This can be performed at elevated conditions of temperature and pressure. USBM or Amort-Harvey Methods determine Wettability, another component determined by this analysis.

Typically, petroleum business is among the most lucrative companies on the globe. However, various factors within the rocks make it easier to tap into the oil reservoirs like permeability, porosity, and solubility of the surrounding rocks. A keen analysis of the adjacent soils and rocks can highlight the best way or technique to use to reach the reservoirs. The reservoirs can be tapped using the drilling method whereby pipes are drilled horizontally or parallel to the oil reservoirs and then pumped to the surface. Additionally, research on the adjacent rocks also ensures that the impurities that can be present in the oil easily identified to fasten the purification process. Moreover, other fractures like rock texture, mineral composition, nature and type of rock also affect a rocks wettability and capillary pressure.

Factor affecting rock wettability and capillary pressure

Rock mineralogy

Rock mineralogy refers merely to the composition of minerals within petroleum products in the reservoirs. Rocks with a high concentration of mineral ions have increased wettability because the regions are having minerals act as areas of high density, therefore, attracting fluids from regions of low frequency and this increases wettability in the ricks (Munoz, 2007). Through adhesive forces, the, unlike molecules, can attract ensuring that rock particles and petroleum molecules can easily imbibe increasing wettability. Additionally, a high concentration of mineral ions also increases the capillary pressure in the rocks. More so because the strong adhesive forces present to ensure that water can move through the lines of weaknesses on the rocks easily as a result of the forces of attraction between the walls of the rocks and the petroleum particles. These mineral ions can be magnesium, calcium or sulfate ions. Therefore, increased mineral composition of concentration increases capillary pressure and wettability (Imokawa and Kawai, 1989). On the other hand, petroleum products with minerals can be very volatile id the minerals are not stabilized therefore a study of the adjacent rocks acts as a safety precaution.

Water salinity

Water salinity refers to the concentration of sodium chloride ions in the petroleum products within the rocks. The greater the concentration of sodium chloride ions (Na+) in the petroleum, the higher the wettability of the rock or soil as stated by (Kalia and Balakotaiah, 2007). This is because water molecules move from a region of high concentration to a region of low concentration through a concentration gradient. Therefore, the osmotic pressure of the soil will be increased making it easy for water particles to move within the rocks through attraction to the sides with high sodium concentration and this increases the rocks wettability. Salinity also increases capillary pressure especially around lines of weaknesses with thin films because the adhesive forces travel for shorter distances to the walls of the rocks. Therefore, salinity increased a rocks wettability and capillary pressure.

Consequently, petroleum products with a high sodium chloride concentration can easily draw oil products from other places which have a lower concentration via a concentration gradient. However, the viscosity of oil products is also very essential at this point because it enables easy movement of petrol from one point to another. But the movement is also dependent on the size of the openings or the lines of weakness. Thin lines of weakness have a high capillary pressure which increases the movement of petroleum up to the surface while bigger one wider lines of weaknesses have reduced capillary pressure and therefore movement of oil is not as fast as around the thin films.

Pressure

Pressure is the movement of water, air and gaseous molecules from a region of high concentration to regions of low concentrations. Rocks within areas with high pressure attract petroleum molecules from areas with lower pressure and this increases wettability (Garing & Guadagnini, 2015). Additionally, rocks with a reduced pressure have reduced water content and therefore wettability in reduced. On the other hand, rocks in high-pressure points increase capillary pressure which ensures that fluids can move to the surface easily and fast as compared to areas with low pressure. Therefore, high pressure increases wettability and capillary pressure in the rocks. Furthermore, pressure ensures that fluids can be pushed to the surface at an increased pace making the process of petroleum extraction easy and fast.

Temperature

Temperature can be defined as the hotness or coldness of a particular place or point. Increase in temperature increases the inter-molecular movements of particles within the rocks as a result of increased kinetic energy. As result fluids and petroleum molecules and particles easily move in the soil and imbibe to increase wettability. On the other hand, low temperatures reduce kinetic motion within the soil. Therefore particle movement is prolonged, and this causes reduced wettability (Zheng & Ju, 2015). Increased temperatures also ensure that fluids can move from one point to the other as vapor and this increases wettability in the soil. Additionally, increased temperature also increases capillary pressure within the rocks by increasing the kinetic energy of particles within the soil. Temperature also affects the weathering process of rocks, and therefore the hard and impermeable layers are removed through exfoliation making it easier for petroleum products to be extracted.

Oil composition

The amount of oil composition within the rocks highly depends on the gases present within the micro pores of the rock particles. Methane and other hydrocarbons normally compete in the soil under extremely high temperatures and pressure to determine the type of gas present in the reservoir (Mergelov et al, 2016). However, most of these hydrocarbons cannot exist both at the same place. Therefore, the competition between the gases ensures that only pure gases remain in the reservoirs while the rest is taken up to the ground.

Thickness of the swi layer

The thickness of the swi layer is very significant in the permeability of rocks. Rocks with a thicker swi layer have reduced wettability because the layer acts as a barrier blocking the movement of fluids in the rock (Zheng & Ju, 2015). Additionally, thin swi layers increase wettability of the soil or rock because fluids will easily move and imbibe with the rock. On the other hand, a thick swi layer reduces capillary pressure by preventing movement of fluids within the rocks. However, fluids can still penetrate through lines of weaknesses in the rocks. A thick swi layer also can prevent outward movement of petroleum products in the rocks, and this affects the overall petroleum production.

Diagenesis Diagenesis refers to the chemical and physical changes that occur in sediments to form sedimentary rocks. Sediments are organic compounds, and at times they also produce fossil fuels and petroleum. The faster the rate of sedimentation the greater the amount of petrol in the rocks. However, in some cases, the petroleum has a lot of impurities from the organic products, and therefore they have to be distilled of filtered after being siphoned to the surface. During the formation of sedimentary rocks, the sediments decompose under high pressure, and this increases wettability especially during imbibition. On the other hand, diagenesis increases capillary pressure in the rocks due to increased adhesive forces in the soil (Figueiredo and Niemi, 2015).

 Factors affecting relative permeability

Wettability

The ability of a rock to imbibe with fluids makes the rock easily relatively permeable. This is because the rock can easily allow the movement of fluids in the rocks. Additionally, wettability favors the process of imbibition within the rocks, and this promotes the movement of water through capillary forces.

Capillary forces

Strong capillary forces, for instance, cohesive forces which attract the same molecules in the rocks increase relative permeability in the rocks. Additionally, adhesive forces which allow different molecules to attract one another also increase the relative permeability of the rocks. Additionally, there are various factors like Rock porosity; permeability and viscosity which are essential features in rocks relative permeability properties, because they determine the movement of water molecules from one place to the other in the soil...