Week#8 & #9-1 Compressibilities & Drive Mechanisms

Compressibility Notes

1. Compressibility Overview

  • Compressibility is the capacity of a substance to change in volume under pressure.

2. Isothermal Compressibility

  • Definition: Change in volume with pressure per volume at constant temperature.

3. Effects of Compressibility in Oil Material Balance

  • Simple oil material balance ignores compressibility effects.

  • Below bubble point: Ignoring rock compressibility is acceptable.

  • Above bubble point: Must account for oil, water, and rock compressibility due to fluid expansion being the primary driving force.

4. Gas Compressibility Calculation

  • Formula: Cg = 1/kPaa or 1/psia = -dV/dP.

  • At constant temperature and volume, use the z-factor to determine compressibility.

5. Objectives in Compressibility Calculations

  • Goal: Calculate gas compressibility at various pressures and temperatures using graphical and analytical methods.

6. Graphical Method

  • Plot compressibility vs. pressure at specified temperatures (e.g., 74, 94, and 124 °C).

7. Regions of z-factor Behavior

  • Region 1: dz/dp < 0 (Pressure: 0-15000 kPaa, T= 94°C).

  • Region 2: z ≈ const. (Pressure: 15000-20000 kPaa, T= 94°C).

  • Region 3: dz/dp > 0 (Pressure: > 20000 kPaa, T= 94°C).

8. Numerical Method to Calculate Cg

  • Approximate the derivative dz/dP using points from z-factor tables selected around reservoir pressure.

9. Example of Gas Compressibility Calculation

  • Determine gas compressibility using z-factor graph and table at a reservoir pressure of 10,000 kPaa and 94°C.

10. Oil Compressibility

  • Objective: Calculate oil compressibility (Co) for samples above bubble point pressure; will require adding the effect of free gas below the bubble point.

  • Co = - (dV/dP) at constant temperature.

  • If m is the slope of B° vs. pressure, Co is derived from changes in B vs. pressure.

11. General Oil Recovery Mechanisms

  • Primary Recovery (RF up to 15%): Natural energy sources.

  • Secondary Recovery (RF up to 40-50%): Water and/or gas injection (e.g., water flooding).

  • Tertiary Recovery (EOR) (up to 15-20% more): Thermal recovery, steam flooding, chemical flooding.

12. Major Drive Mechanisms

  • Solution Gas Drive: Pressure stabilization through gas expansion.

  • Gas Cap Drive: Expansion of gas cap displacing oil.

  • Fluid Expansion Drive: Expansion of oil and rock above bubble point.

  • Water Drive: Recovery through active aquifer support.

  • Gravity Drainage: Density differences driving oil, water, and gas movements.