Oilfield Terminology & Definitions.
Back to "M"
Reservoir Management & Improved Recovery Oilfield Terminology

Material Balance Definition / Meaning

9 views

Material balance is a fundamental principle in petroleum reservoir engineering that applies the law of conservation of mass to a reservoir system. It accounts for all fluids entering, leaving, and accumulating within the reservoir volume over time. The material balance equation (MBE) is a zero-dimensional model that does not consider spatial variations; instead, it treats the entire reservoir as a single tank with average properties. This simple yet powerful tool is used to estimate original hydrocarbons in place, predict reservoir performance, and evaluate drive mechanisms.

Fundamental Principle

The basis of material balance is that the mass of fluids originally in the reservoir must equal the sum of mass remaining plus mass produced. In equation form, for an oil reservoir: Initial oil + initial gas + initial water = remaining oil + remaining gas + remaining water + produced oil + produced gas + produced water + water influx (if any). For practical applications, the equation is expressed in terms of reservoir volumes and rock/fluid expansion.

The Material Balance Equation

The general MBE for an oil reservoir (Schilthuis form) is written as:

N = (Np [Bo + (Rp - Rs) Bg] + We - Wp Bw - Gi Bg) / ( (Bo - Boi) + (Rsi - Rs) Bg + m Boi (Bg/Bgi - 1) + (1+m) Boi (cf + cw Swi)/(1 - Swi) Δp )

Where:

  • N = initial oil in place (STB)
  • Np = cumulative oil produced (STB)
  • Bo, Bg, Bw = formation volume factors for oil, gas, water (RB/STB or RB/scf)
  • Rp = cumulative gas-oil ratio (scf/STB)
  • Rs = solution gas-oil ratio (scf/STB)
  • We = cumulative water influx (RB)
  • Wp = cumulative water produced (STB)
  • Gi = cumulative gas injected (scf)
  • m = initial gas cap size (ratio of gas cap volume to oil zone volume)
  • cf = formation compressibility (1/psi)
  • cw = water compressibility (1/psi)
  • Swi = initial water saturation (fraction)
  • Δp = pressure drop from initial to current (psi)

The equation can be simplified for specific reservoir types (e.g., under-saturated oil reservoir, gas reservoir, or water-drive reservoir).

Applications in Reservoir Management

Material balance is used extensively in reservoir management for:

  • Estimating original fluids in place: Matching historical production and pressure data to solve for N or G.
  • Evaluating drive mechanisms: Determining the dominant energy source (solution gas drive, gas cap drive, water drive, compaction drive). A diagnostic plot (e.g., Havlena-Odeh) helps identify the drive index.
  • Predicting future performance: Forecasting reservoir pressure decline and recovery under various production scenarios.
  • Validating reservoir simulation models: The MBE provides a check on the overall mass balance in a numerical simulation.
  • Optimizing recovery strategies: Assessing the potential for waterflooding or gas injection by comparing with a material balance prediction.

Typically, material balance is applied at the field or reservoir level, not for individual wells, unless they drain separate compartments.

Assumptions and Limitations

Assumption Implication
Homogeneous reservoir properties Average properties may not capture heterogeneity; significant error in layered or fractured reservoirs.
No spatial gradients Pressure is uniform throughout the reservoir; not valid for highly permeable compartments or during rapid depletion.
Representative fluid samples PVT properties must accurately reflect reservoir fluid; poor samples lead to unreliable N estimates.
Accurate production and pressure data Measurement errors in rates, cumulative production, and static pressures can distort the match.
Single tank model Cannot model areal sweep or vertical coning; use reservoir simulation for detailed studies.

Despite these limitations, material balance remains a cornerstone of reservoir analysis due to its simplicity and low data requirements compared to simulation.

Usage Example

A reservoir engineer collects five years of production and pressure data from a solution gas-drive oil reservoir. Using the material balance equation and a Havlena-Odeh plot, she determines the original oil in place is 50 million STB. This estimate helps the firm decide whether to implement a waterflood to improve recovery, as the current primary recovery factor is only 15%.

Key Takeaway: Material balance is an essential, inexpensive tool for reservoir evaluation and management. When combined with other techniques like volumetric estimation and decline curve analysis, it provides a robust foundation for reservoir development decisions.