1. Isothermal oil compressibility Prepared by: Rawshan Rashid Supervisor : Mr. Alaa University Of Zakho Petroleum Engineering Department reservoir engineering 3 rd Stage 2019-2020 26/12/2020

2. Contents Oil compressibility Oil compressibility above the bubble point pressure Oil compressibility below the bubble point pressure Conclusion 2

3. Oil Compressibility Oil compressibility is defined as the measure of change in volume as theresult of change in prevailing pressure. under conditions of constant temperature. C o is the oil compressibility also called coefficient of isothermal compressibility. C o is usually determined from the pressure-volume measurements of reservoir fluids. These data are usually obtained from reservoir fluid analysis. 3

4. A convenient method of estimating the coefficient of isothermal compressibility for reservoir fluids for a finite change in pressure and volume is to use this simple equation: Where: C o = isothermal compressibility, psi −1 V = oil volume, ft 3 P =the pressure, psi 4 Oil Compressibility

5. Normally, an increase in fluid pressure (p2 > p1) causes the fluid volume to decrease (V1 > V2). Hence the negative sign, to make the value of the isothermal compressibility positive. Values of oil compressibility can typically range from 5 × 10 −6 psi −1 to 12 × 10 −6 psi −1 or more. 5 Oil Compressibility

6. Oil compressibility, when plotted versus pressure, shows a significant discontinuity at the bubble point pressure. 6 Oil Compressibility Oil compressibility curve

7. above the bubble-point pressure (in undersaturated conditions) the oil is a single phase liquid containing large amounts of dissolved solution gas (R so ). 7 Isothermal oil compressibility above the bubble point pressure oil compressibility above the bubble point is a weak function of pressure (Oil is slightly compressible fluid). Co psi -1

8. Isothermal oil compressibility above the bubble point pressure For a crude oil system, the isothermal compressibility coefficient of the oil phase C o is defined for pressures above the bubble-point can be measured in the laboratory. Mathematically, it can be expressed by one of the following equivalent expressions: Where: C o = isothermal compressibility, psi −1 ρ o = oil density lb/ft 3 B o = oil formation volume factor, bbl/STB 8

9. Below the bubble point pressure (saturated condition), gas comes out of solution causing a sharp increase in compressibility, which causes the discontinuity shown in the plot. Once below the bubble point, oil compressibility becomes a much stronger function of pressure. This behavior is clearly noticed in the differential liberation test. This is due to shrinkage associated with the liberation of the solution gas with decreasing reservoir pressure. 9 Isothermal oil compressibility below the bubble point pressure

10. At pressures below the bubble-point pressure, the oil compressibility is defined as: Where : B g = dry gas formation volume factor, B o = oil formation volume factor, bbl/STB R s = solution gas ratio, R v =vaporized oil ratio. At higher R s values, the crude oil has higher compressibility values due to the solution gas. 10 Isothermal oil compressibility below the bubble point pressure

11. Conclusion  isothermal compressibility coefficients are required in solving many reservoir engineering problems and plays an important role in reservoir simulation, material balance calculations, design of high-pressure surface-equipment, the determination of skin, dimensionless time, and the interpretation of well test analysis, specifically for systems below the bubble point pressure.  Accurate information on the oil fluid compressibility above and below bubble point pressure is very important for reservoir evaluation. 11