Well Logging Engineering Lecture 3 PDF

University of Mosul College of Petroleum & Mining Engineering Department of Petroleum & Refining Engineering Well Logging Engineering Third Year Lecture 3 Assist. Prof. Dr. Muneef Mahjoob Moh...

University of Mosul College of Petroleum & Mining Engineering Department of Petroleum & Refining Engineering Well Logging Engineering Third Year Lecture 3 Assist. Prof. Dr. Muneef Mahjoob Mohammed 2024 – 2025 Well logging 2024 – 2025 Dr. Muneef Mahjoob Mohammed Fundamentals of quantitative log interpretation: 4- Resistivity: Resistivity is the measurement of resistance. The reciprocal of resistivity is conductivity. The measured units are ohm-meter2/meter, and are called ohm-meter. 𝑟𝑥𝐴 R= ------------------ (8) 𝐿 Where: R: resistivity (ohm-meter). r: resistance (ohm). A: cross sectional area of substance being measured (meter2). L: length of substance being measured (meter). Resistivity is a basic measurement of a reservoir’s fluid saturation and is a function of porosity, type of fluid (i.e. hydrocarbons, salt or fresh water), and type of rock. Because both the rock and hydrocarbons act as insulators but saltwater is conductive, resistivity measurements made by logging tools can be used to detect hydrocarbons and estimate the porosity of a reservoir. 4.1 Resistivity of Rocks: Reservoir rocks contain the following constituents: Matrix material High Resistivity Formation water Low resistivity Oil High resistivity Gas High resistivity Water-based mud filtrate Low resistivity Oil-based mud filtrate High resistivity All have a high electrical resistivity (electrical insultors) except the formation water and water- based mud filtrate, which are good electrical conductors and have a low electrical resistivity. 1 Well logging 2024 – 2025 Dr. Muneef Mahjoob Mohammed Therefore, the resistivity of the reservoir rocks depends only upon the water or water based mud filtrate occupying its pore space. 4.1.1 Uninvaded Formations: For uninvaded formations, the measured bulk resistivity of the rock depends only upon the amount of the aqueous formation fluids present in the rock, and the resistivity of those aqueous fluids. Since the amount of formation fluids depends both on porosity ( Ø ) and water saturation (Sw), we can say that the resistivity of the formation ( Rt) depends upon porosity, water saturation, and the resistivity of the formation water. This resistivity is called the true resisitivity of the formation. It is the resistivity of the formation in the uninvaded zone, where the rock contains some saturation of oil (So), gas (Sg), and water (Sw) and where So + Sg + Sw = 1. The aim is to use knowledge of the resistivity of the formation, together with independent knowledge of the porosity and resistivity of the formation water, to calculate water saturation (Sw), and hence enable ourselves to calculate the STOOIP. 4.1.2 Invaded Zones: In most cases, there is an invaded zone, where the formation fluids have been disturbed by the drilling fluid. The resistivity of the formation in this zone depends upon the resistivity of the mud filtrate (Rmf), the resistivity of any remaining formation water Rw, the saturation of the mud filtrate (Sxo), the saturation of the remaining formation water (Sw) (if any), and the porosity of the rock (Ø). If these values and the depth of invasion are known, the resistivity measured in the invaded zone can be corrected to account for the presence of the mud filtrate. 4.2 The Effect of Temperature: The resistivity of formation fluids and water-based drilling muds varies greatly with temperature. Archie’s experiments showed that the resistivity of the formation 100% water saturated (Ro), filled with water having a resistivity of Rw can be related by means of a formation resistivity factor (F): Ro = F x R w ------------------ (9) Where the formation resistivity factor (F) is equal to the resistivity of the formation 100% water saturated (Ro) divided by the resistivity of the formation water (Rw). 2 Well logging 2024 – 2025 Dr. Muneef Mahjoob Mohammed Archie’s experiments also revealed that formation factors can be related to porosity by the following formula: 1.0 F = ∅𝑚 --------------------- (10) Where m is a cementation exponent whose value varies with grain size, grain size distribution, and the complexity of the paths between pores (tortuosity). The higher the value for tortuosity the higher the m value. Water saturation (Sw) is determined from the water filled resistivity (Ro) and the formation resistivity (Rt) by the following relationship: 𝑅 Sw = ( 𝑅𝑜 )1/n ------------------ (11) 𝑡 Where n is the saturation exponent whose value varies from 1.8 to 2.5 but is most commonly 2. By combining the formulas (9) & (11), the water saturation formula can be rewritten in the following form: 𝐹 𝑥 𝑅𝑤 Sw = ( )1/n ------------------ (12) 𝑅𝑡 This formula is the Archie equation for water saturation (Sw). 4.3 The effect of salt composition: The resistivity of the formation fluids depends upon the concentration and type of salts dissolved in it. The dissolved salts in a solution are expressed as an NaCl equivalent. Figure 1 shows a typical chart for correction the variation of the resistivity of aqueous fluids with temperature. 3 Well logging 2024 – 2025 Dr. Muneef Mahjoob Mohammed Figure 1: Resistivity of NaCl fluids as a function of temperature. 4