Gas Material Balance & Decline Curves

Estimate original gas in place using p/z plot analysis and forecast future production using Arps decline curve methods.

Production Material Balance Reserves

Material Balance Analysis

Pressure-Production Data

Time (months) Pressure (psia) Cumulative Prod (Bcf)

Gas Properties

Air = 1.0
°F

Decline Analysis

MMscf/d
1/year
0 = exponential, 1 = harmonic
years

Results

Theory & References

Material Balance Equation

For volumetric gas reservoir:

p/z = pi/zi × (1 - Gp/G)

Where:

  • p = pressure (psia)
  • z = gas compressibility factor
  • Gp = cumulative production (scf)
  • G = original gas in place (OGIP)

Plot p/z vs Gp → straight line → intercept at Gp axis = OGIP

Decline Curve Analysis

Exponential: q = qi × e^(-Di×t)

Hyperbolic: q = qi / (1 + b×Di×t)^(1/b)

Harmonic (b=1): q = qi / (1 + Di×t)

Where:

  • q = rate at time t
  • qi = initial rate
  • Di = initial decline rate
  • b = hyperbolic exponent (0-1)

Drive Mechanisms

  • Depletion: p/z straight line, recovery 70-85%
  • Water Drive: p/z curves upward, recovery can exceed 90%
  • Compaction: p/z curves downward

Straight line indicates volumetric depletion (no water influx)