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Well Logging

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Well Logging University of Kirkuk / Faculty of Science / Applied geology department rd 3 year 2020 WELL LOGGING Dr. Radhwan Khaleel Hayder INTRODUCTION: -What is a “Log” and ‘’Well Logging’’. LOG OR WELL LOGGING (THE BOREHOLE IMAGE): - Data are organized and interpreted by depth and represented on a graph called a log (a record of information about the formations through which a well has been drilled). - Visual inspection of samples brought to the surface (geological logs). Example cuttings and corres. - Study of the physical properties of rocks and the fluids through which the bore hole is drilled. - Traditionally logs are display on girded papers. Now a days the log may be taken as films, images and in digital format. - Some types of well logs can be done during any phase of a well's history: drilling, completing, producing or abandoning. -Well logging is performed in boreholes drilled for the oil and gas, groundwater, mineral, environmental and geotechnical studies. -The first electrical resistivity well log was taken in France, in 1927. THE IMPORTANCE OF WELL LOGGING: 1-Determination of lithology. 2-Determination of reservoir characteristics (e.g. porosity, saturation, permeability). 3-Determination formation dip and hole size. 4-Identification of productive zones, to determine depth and thickness of zones. 5-Distinguish between oil, gas, or water in a reservoir, and to estimate hydrocarbon reserves. 6-Geologic maps developed from log interpretation help with determining facies relationships and drilling locations. 1 ADVANTAGES AND LIMITATIONS OF WELL LOGGING: Advantages: 1- Continuous measurements. 2- Easy and quick to work with. 3- Short time acquisition. 4- Economical. Limitations: 1- Indirect measurements. 2- Limited by tool specification. 3- Affected by environment. WELL LOG “LOGGING OPERATION”: The process of logging involves a number of elements (Figure. 1) 1- Truck and the rig. 2- Mechanical Winches and ordinary driving machine. 3- Cables and Sondes. 4- Drill Bits and Drilling Mud PETROLEUM DRILL BITS (DIAMOND): Core drilling bit shape and number of diamonds depends on the dominant rock type being penetrated. Some types of them:(Figure. 2) - Large teeth for shale. - Short teeth for hard rocks. - Button bit for very hard rocks. 2 Figure. 1. Basic Well Logging Technique Figure. 2. Petroleum Drill Bits types 3 TYPES OF BOREHOLES: According to Casing operation: - The name open hole is applied because these logs are recorded in the uncased portion of the well bore. - The closed hole logs. The name closed hole is applied because these logs are recorded in the cased portion of the well bore. - The various types of logs, the ones used most frequently in hydrocarbon exploration are called open hole logs. According to conductivity of the borehole: Conductive (water base drilling mud) and Non-conductive boreholes (oil base mud, air drilled or cased holes). Figure. 3.Cased holes Open holes PRINCIPAL OF WELL LOGGING: - A well log is a record of certain formation data versus depth. - The appropriate down hole logging tools instrument called ‘sonde’, about 3.5 inches in diameter is lowered into mud filled hole on logging cable. - This tools will measure the electrical, acoustic and radioactive properties of the formation. 4 - The result will be analyzed to determine which of the layers are porous and permeable and likely to contain hydrocarbon. - Survey is normally done from the bottom up. As the sonde is pulled up the hole, a continuous measurement signal is sent to the surface where the data is processed and recorded as a curve. FORMATION EVALUATION: Geological evaluation of the lithology, stratigraphy, fluids and reservoir properties are essential for exploration and field development. -Critical information from a well comes from: 1- The mud log. 2- Cores. 3- Cutting. 4- Geophysical logs. WHAT IS FORMATION EVALUATION: - Formation Evaluation (FE) is the process of interpreting a combination of measurements taken inside a wellbore. - FE data can be gathered by logging while drilling tools . WHY FORMATION EVALUATION: - Provide continuous record of penetration rate, lithology, hydrocarbon shows and determine the ability of a borehole to produce petroleum. - To evaluate hydrocarbons reservoir (oil , gaz) and in petroleum exploration and development. BASIC INFORMATION NEEDED IN LOG INTERPRETATION: - Formation Water: (connate water in the formation) held by capillary pressure in the pores of a rock serves to inhibit the transmission of hydrocarbons. Table. 1. Illustrates the distribution of discovered oil and gas fields based on geologic age 5 -Water Saturation (Sw): is the percentage of pore volume in a rock which is occupied by formation water. (Sw) = Formation water occupying pores / Total pore space in the rock * 100 % -Subsurface Temperature: Formation temperature (Tf) is important in log analysis because the resistivity of the drilling mud, mud filtrate, mud cake and formation water change with temperature. The following are needed to determine formation temperature: • Formation depth • Bottom hole temperature (BHT) • Total depth of the well • Surface temperature Formation temperature can be obtained by graphical means (assuming a linear geothermal gradient) or by calculation using the linear regression equation. Y=mx+c where: x = depth Y = temperature m = Geothermal gradient c = constant (surface temperature) G.G = Bottom hole temperature (BHT) = surface temperature / Total depth (TD) For example: Temperature Gradient Calculation. Assume: y = bottom hole temperature (BHT) = 250ᴼF x = total depth (TD) = 15,000 ft c = surface temperature = 70°F Solve form (i.e. slope or temperature gradient) m = y – c / x Therefore: m = 250ᴼ – 70ᴼ / 1500 ft m = 0.012ᴼ/ ft or 1.2ᴼ/100 ft 6 Formation Tmperature Calculation: Assume: m = temperature gradient = 0.012ᴼ/ ft x = formation depth = 8,000 ft c = surface temperature = 70ᴼ Remember: y = mx + c Therefore: y = (0.012) X (8,000)+ 70ᴼ = 166ᴼ formation temperature at 8,000 ft After a formations' temperature is determined either by chart or calculation, the resistivity of the different fluids (Rm, Rmf or Rw) can be corrected to formation temperature. Resistivity values of the drilling mud (Rm), mud filtrate (Rmf) and mudcake (Rmc), and the temperature at which they are measured, are recorded on the log's header. Formation water resistivity (Rw) is obtained by analysis of water samples from a drill stem test, or from a catalog of water resistivity value , and as follows: RTF = Rtemp (Temp+6.77) / (Tf+6.77) Where : RTF: Resistivity at formation temperature. Rtemp: Resistivity at a temperature other than formation temperature. Temp: temperature at which resistivity was measured. RTF: formation temperature. Using a formation temperature of 166° and assuming an Rw of 0.04 measured at 70°. The Rw at 166° will be: Rw166 = 0.04 X (70+6.77)/(166 + 6.77) Rw166 = 0.018 7 Notes: 1- The four most fundamental rock properties used in petrophysical logging are (1) porosity; (2) permeability; (3) water saturation and (4) resistivity. 1/n 2- The Archie equation for water saturation is: Sw= (F x Rw/Rt) Sw = water saturation of uninvaded zone. F = formation factor. Rw = formation water resistivity Rt = formation resistivity (uninvaded zone) 3- Where a porous and permeable formation is penetrated by the drill bit. the drilling mud invades the formation as mud filtrate (Rmf). 4. The invasion of the porous and permeable formation by mud filtrate creates invasion zones (Rxo and Ri) and an uninvaded zone (Rt). Reading resistivity logging tools provide information aboul the invaded and uninvaded zones. 5. -Lithology: The lithology of a formation must be known because: (1) porosity logs require a matrix value sandstone, limestone, or dolomite in order to determine porosity ; (2) the formation factor varies with lithology; (3) the variation in formation factor causes changes in water saturaiion values. 6. The four fluids that affect logging measurements are: (I) drilling mud. Rm; (2) mud filtrate. Rmf; (3) formation water, Rw; and (4) hydrocarbons. 7. The resistivities of the drilling mud (Rm). mudcake (Rmc), mud filtrate (Rmf) and formation water (Rw) all vary with changes in temperature. Consequently, a formations temperature (Tf) must be determined and all resistivities corrected to Tf. 8 .Clay content: (Rock may be clean or it may contain clay) Caly content affects the permeability and log readings 9. Consolidation: A mechanical property in which reduction in volume takes place by expulsion of water under static loads. BOREHOLE ENVIRONMENT: Where a hole is drilled into a formation, the rock plus the fluids in it (the rock-fluid system) are altered in the vicinity of the borehole. The borehole rock and the rock surrounding it are contaminated by the drilling mud, which affects logging measurements. 8 BOREHOLE DIAMETER (DH): -A well’s borehole size is described by the outside diameter of the drill bit . - But, the diameter of the borehole: may be larger or smaller than the bit diameter because of: 1- Wash out and/or collapse of shale and poorly cemented porous rocks. 2- Build-up of mud cake on porous and permeable formations (Fig4 ). - Borehole sizes normally vary from 7 inches to 12 inches. The size of the borehole is measured by a caliper log. DRILLING MUD (RM): - The mud helps remove cuttings from the well bore, -The lubricate and cool the drill rotary bits - The maintain an excess of borehole pressure over formation pressure and prevents blowouts. Note: The Resistivity values for drilling mud, mud cake and mud filtrate are recorded on a log’s header (Figure 2) and are used in log interpretation. 9 Figure. 4. is a schematic illustration of a porous and permeable formation that is penetrated by a borehole filled with drilling mud. This schematic diagram illustrates an idealized version of what happens when fluids from the borehole invade the surrounding rock. Dotted lines indicate the cylindrical nature of the invasion. - Invaded Zone: The zone which is invaded by mud filtrate is called the invaded zone. It consists o f a flushed zone (Rw) and a transition or annulus (Ri) zone.
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