Chapter 3 reservoir fluid properties and pvt analysis

Vietnam National University - Ho Chi Minh City University of Technology Faculty of Geology & Petroleum Engineering Department of Drilling - Production Engineering Course Reservoir Engineering Trần Nguyễn Thiện Tâm Email: trantam2512@hcmut.edu.vn 12/11/2017 Reservoir Engineering Chapter Reservoir fluid properties and PVT analysis 12/11/2017 Reservoir Engineering References Tarek Ahmed, Reservoir Engineering Handbook, 4th edition Gulf Professional Publishing, 2010 12/11/2017 Reservoir Engineering Contents  Reservoir fluid properties  Classification of reservoir fluids 12/11/2017 Reservoir Engineering Reservoir fluid properties To understand and predict the volumetric behavior of oil and gas reservoirs as a function of pressure, knowledge of the physical properties of reservoir fluids must be gained These fluid properties are usually determined by laboratory experiments performed on samples of actual reservoir fluids In the absence of experimentally measured properties, it is necessary for the petroleum engineer to determine the properties from empirically derived correlations 12/11/2017 Reservoir Engineering Properties of natural gases 12/11/2017 Reservoir Engineering Properties of natural gases A gas is defined as a homogeneous fluid of low viscosity and density that has no definite volume but expands to completely fill the vessel in which it is placed Generally, the natural gas is a mixture of hydrocarbon and nonhydrocarbon gases The hydrocarbon gases that are normally found in a natural gas are methanes, ethanes, propanes, butanes, pentanes, and small amounts of hexanes and heavier The nonhydrocarbon gases (i.e., impurities) include carbon dioxide, hydrogen sulfide, and nitrogen 12/11/2017 Reservoir Engineering Behavior of ideal gases The kinetic theory of gases postulates that gases are composed of a very large number of particles called molecules For an ideal gas, the volume of these molecules is insignificant compared with the total volume occupied by the gas It is also assumed that these molecules have no attractive or repulsive forces between them, and that all collisions of molecules are perfectly elastic 12/11/2017 Reservoir Engineering Behavior of ideal gases Based on the above kinetic theory of gases, a mathematical equation called equation-of-state can be derived to express the relationship existing between pressure p, volume V, and temperature T for a given quantity of moles of gas n This relationship for perfect gases is called the ideal gas law and is expressed mathematically by the following equation: pV = nRT where p = absolute pressure, psia V = volume, ft3 T = absolute temperature, °R n = number of moles of gas, lb-mole R = the universal gas constant, which, for the above units, has the value 10.730 psia ft3/lb-mole °R 12/11/2017 Reservoir Engineering Behavior of ideal gases The number of pound-moles of gas, n, is defined as the weight of the gas m divided by the molecular weight M, or: m n M m pV    RT M  where m = weight of gas, lb M = molecular weight, lb/lb-mol Since the density is defined as the mass per unit volume of the substance, m pM g   V RT where ρg = density of the gas, lb/ft3 12/11/2017 Reservoir Engineering 10 Constant-composition expansion tests Section B: The pressure is reduced in steps at constant temperature by removing mercury from the cell, and the change in the total hydrocarbon volume Vt is measured for each pressure increment 12/11/2017 Reservoir Engineering 100 Constant-composition expansion tests Section C: The saturation pressure (bubble-point or dewpoint pressure) and the corresponding volume are observed and recorded and used as a reference volume Vsat 12/11/2017 Reservoir Engineering 101 Constant-composition expansion tests Section D, E: The volume of the hydrocarbon system as a function of the cell pressure is reported as the ratio of the reference volume 12/11/2017 Reservoir Engineering 102 Constant-composition expansion tests 12/11/2017 Reservoir Engineering 103 Differential liberation (vaporization) tests In the differential liberation process, the solution gas that is liberated from an oil sample during a decline in pressure is continuously removed from contact with the oil, and before establishing equilibrium with the liquid phase This type of liberation is characterized by a varying composition of the total hydrocarbon system 12/11/2017 Reservoir Engineering 104 Differential liberation (vaporization) tests The differential liberation test is considered to better describe the separation process taking place in the reservoir and is also considered to simulate the flowing behavior of hydrocarbon systems at conditions above the critical gas saturation As the saturation of the liberated gas reaches the critical gas saturation, the liberated gas begins to flow, leaving behind the oil that originally contained it This is attributed to the fact that gases have, in general, higher mobility than oils 12/11/2017 Reservoir Engineering 105 Differential liberation (vaporization) tests 12/11/2017 Reservoir Engineering 106 Differential liberation (vaporization) tests The test is carried out on reservoir oil samples and involves charging a visual PVT cell with a liquid sample at the bubble-point pressure and at reservoir temperature The pressure is reduced in steps, usually 10 to 15 pressure levels, and all the liberated gas is removed and its volume is measured at standard conditions The volume of oil remaining VL is also measured at each pressure level It should be noted that the remaining oil is subjected to continual compositional changes as it becomes progressively richer in the heavier components 12/11/2017 Reservoir Engineering 107 Differential liberation (vaporization) tests The above procedure is continued to atmospheric pressure where the volume of the residual (remaining) oil is measured and converted to a volume at 60°F, Vsc The differential oil formation volume factors Bod (commonly called the relative oil volume factors) at all the various pressure levels are calculated by dividing the recorded oil volumes VL by the volume of residual oil Vsc, or: VL Bod  Vsc 12/11/2017 Reservoir Engineering 108 Differential liberation (vaporization) tests 12/11/2017 Reservoir Engineering 109 Separator tests Separator tests are conducted to determine the changes in the volumetric behavior of the reservoir fluid as the fluid passes through the separator (or separators) and then into the stock tank 12/11/2017 Reservoir Engineering 110 Separator tests 12/11/2017 Reservoir Engineering 111 PVT Experiments for Gas Condensate  Constant composition expansion – CCE  Constant volume depletion - CVD 12/11/2017 Reservoir Engineering 112 Constant-Composition Test 12/11/2017 Reservoir Engineering 113 Constant-Volume Depletion (CVD) Test 12/11/2017 Reservoir Engineering 114