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Oil Reservoir Engineering
Transcript of Oil Reservoir Engineering
FORCES By Suez University
Faculty of Pet. & Min. Eng.
Petroleum Engineering Dept. Oil is produced from wells drilled into underground porous rock formations. The ensemble of wells draining a common oil accumulation or source or surface area defined by the well distribution termed an "Oil Field" or "Oil Pool". The part of the rock that is oil productive is termed an "Oil Reservoir" by variety of the subsurface location of the reservoir rock; its entrained fluids are subject to elevated temperature and pressure – the reservoir temperature and reservoir pressure.
Reservoir rocks are mostly sedimentary in origin. They are either mechanical or chemical deposition of solid–materials or simply the remains of animals or plant life. Oil Reservoirs (Definitions) Considered on hand–specimen scale reservoir rocks have defined ranges of physical properties which are of paramount interest to the reservoir engineer. The three engineering characteristics of the reservoir rock are “porosity”, “oil, gas and water saturation” and “permeability, Specific, effective and relative”. Physical properties of reservoir rocks * Definitions * of a material is defined as that fraction of the bulk volume of this material that isn't occupied by the solid framework of the material. POROSITY 2 types The percentage of total void space with respect to the bulk volume REGARDLESS OF INTERCONNECTED of the pore voids. A rock may have considerable absolute porosity and yet have no conductivity to fluid for lack of pore interconnection. ABSOLUTE POROSITY EFFECTIVE POROSITY The percentage of INTERCONNECTED void space with respect to the bulk volume. It is an indication of conductivity to fluid but not necessarily a measure of it. * Geological Factors * affecting porosity Degree of sorting Compaction Cementation Clay content Mode of packing Rock compressibility 1 2 3 4 5 7 6 Well–sorted, moderately rounded sand grains settle in water giving a packing of 30 to 40% porosity. In poorly sorted sediments, the smaller grains fit into the space between the larger ones, and porosity is considerable decreased. It's a geological factor which reduces porosity due to overburden pressure of the overlying sediments. Sandstones, whoever, exhibit very little compressibility 3x10–6 whereas shales may be reduced to a small fraction of there original sedimentation volume. It's the agent which has the greatest effect on the original porosity and which affects the size, shape and continuity of pore channels. Clay may often act as cementing material. Clay is deposits of the same time as sand grains and generally it adheres to them so that after deposition considerable porosity still exists and the over–all porosity of sandstone may not be lowered greatly by a small amount of clay. Their effect on porosity at great depth under overburden pressure is of interest. With increasing overburden pressure, quartz grains in sandstone show a progressive change from random packing to closer packing. Some crushing and plastic deformation of the sand grain occurs. Granulation and crushing
of sand grains One may get qualitative picture of the geometrical structure of sands by consideration of packing of spheres of uniform size. This, too, is of infinite variety. However, it will suffice to note here two basic and extremely types, namely: the cubic and rhombohedra packing. Unit cells of such packing are shown For CUBIC PACKING RHOMBOHEDRA System For Of particular interest is the fact that the radii cancel and the porosity of packing of uniform spheres is a function of packing only. Pore volume compressibility Solid volume compressibility Bulk volume compressibility = + * Experimental porosity measurements * Preparation of samples for measuring it is porosities: They are selected to be preferably 10 to 20 cm3 in bulk volume and are obtained from the center of the core .their surfaces are cleaned to remove traces of drilling mud. The samples are extracted in a soxhlit using oil solvents such as benzene, toluene alight hydrocarbon fraction. During the extraction, the sample should be kept in a paper thimble, covered with plug of cotton in order to avoid erosion of loosely cemented grains. After extraction, the samples are dried in an over a 100 to 105 co and cooled in a desiccators. This operation removes the solvent and moisture from the samples. Russel volumeter Glass pycnometer Ruska porosimeter " " devices A glass pycnometer with a cap which rests on a ground taper joint and with a sample hole through the cap is filled with mercury, the cap is pressed into its seat and the excess mercury which overflows through a hole in the cap is collected and removed. The pycnometer is opened and the sample is placed in the surface of the mercury and submerged by a set of pointed rods which project from the lower side of the cap. The cap is again pressed into its seat, which causes a certain amount of mercury equivalent to the bulk volume of the sample to overflow. The rods which submerge the sample should be adjusted so that the sample does not touch the sides of the pycnometer; this avoids trapping air bubbles. Either the volume of mercury which overflows or the loss of weight of the mercury in the pycnometer may be measured and the core bulk volume calculated. (1) (3) (2) As the determination of the bulk volume by glass pycnometer can not be applied to loosely cemented samples which have a tendency to disintegrate when immersed in mercury, and a serious source of error of the trapping of air bubbles at the surface of the sample, Russle volumeter provides for direct reading of bulk volume. A saturated sample is placed in a sample bottle after a zero reading is established with fluid in the volumeter. The resulting increase in volume is the bulk volume. Only saturated or coated samples may be used in the device. This device has the advantage of applicability to loosely cemented sample with irregular surfaces. Since the liquid used is transparent, trapped air bubbles may be seen and steps taken to remove them. A micrometer piston is used to pressure the sample cup, so that the mercury reaches a given reference on the manometer. Let the reading be Rb in the absence of a core sample in the cup. When the core floats on the mercury within the cup, the displacement of the micrometer piston gives a reading Rc to reach the same reference mark. Break of the well core, clean the surface of the sample to remove the drilling mud, measure the bulk volume by any one of the procedure described above, crush the sample to its grains, wash the grains with suitable solvent to remove oil mud and water, and determine the volume of the grains. It is of course necessary to dry the rock grains before their volume is determined. The volume of the dry grains may be determined in a pycnometer containing a suitable liquid as kerosene. 3 asim A. Hennawi MADE y: B * Definitions * Cores or underground rock samples, which brought to the surface, are universally found to have entrained in their pores varying amounts of liquid. In a typical oil field, water called interstitial or connate–water and frequently free gas pressured in addition to the oil. SATURATION types is the pore volume filled by water divided by the total pore volume. The water saturation (Sw): Sw + So = 1 * Fluid Saturation * FACTORS affecting Mud Filtration Pressure
Gradiesnt 1 2 In the case of rotary drilling, the differential pressure across the well face causes mud and mud filtrate to invade to formation immediately adjacent to the well surface, this flushing the formation with mud and this filtrate displacing some of the oil and perhaps some of the original interstitial water. The displacement process changes the original fluid contents to fluid saturation. Pressure gradient between the surface and the formation permits the expansion of the entrapped water, oil and gas. Thus the contents of the core at the surface have been changed from those which existed in the formation. * Uses of core determined fluid saturations * The saturation values obtained directly from rock samples are used to: In summary, it is seen that although fluid–saturation determinations made on core samples at the surface may not give a direct indication of the saturation within the reservoir, they are of value and do yield very useful and necessary information. •Determine the original oil–gas contact, original oil water contact and weather sand is productive of oil or gas.
•Establish a correlation of the water content of cores and permeability from which it can be determined whether a formation will be productive of hydrocarbon. *Fluid saturation measurements* Liquid is the pore volume filled by oil divided by the total pore volume. The oil saturation (So): If oil and water are the only fluids present, the volume filled by water plus the volume filled by oil must equal the total pore volume, therefore: In many pools, in addition to oil and connate water, free gas is also present. The free gas saturation is defined by: is the pore volume filled by gas divided by the total pore volume. The gas saturation (Sg): Sw + So + Sg = 1 And then: - The saturation will vary from place to place; the water saturation tending to be higher at the lower part due to gravity.
- Water tending to be higher in the less porous section.
- The saturation will vary with cumulative with drawal. Three factors should be remembered concerning fluid saturation: DISTILLATION METHOD
METHOD Methods for the determination of reservoir fluid saturation in place consist in analyzing reservoir core samples for water and oil, the saturation in gas being obtained by difference since the sum of the three fluids is equal to unity. •Take a sample ranging in volume from 50 to 60 cc from the central part of the larger core.
•Place the core in an extraction thimble and weighed.
•Put the thimble in the flask containing a liquid solvent such as toluene or a gasoline fraction boiling at about 150 ˚C.
•A reflux condenser is fitted to the flask to return the condensate to a calibrated glass trap.
•The liquid hydrocarbon is boiled and the water present in the sample vaporized, carried into the flask condenser, and caught in the trap. When the volume in the trap remains constant under continued extraction, the volume of the water collected is read and the sample containing the sample is then transferred to a soxhlet for the final extraction.
•The thimble and the sample are then dried and weighted. •The total fluid saturation is obtained by weight difference and includes both oil and water.
•By weight difference again, the weight of oil is obtained, and, by use of an appropriate oil density, its volume is calculated.
•The saturations on a percentage of pore–volume base are readily calculated for both water and oil. TWO
Methods A solvent is injected into the centrifuge just of center.
Owing to centrifugal force, it is thrown to the outer radii, being forced to pass through the core sample. The solvent removes the water and oil from the core. The outlet fluid is trapped and the quantity of water in the core is measured.
This method provides a very rapid method because of the high forces which can be applied; at the same time that the water content is determined, the core is cleaned in preparation for the other measurements.