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Qwestrum Engineering360 · Petroleum & Energy · Reservoir Engineering

Material Balance Equation

Material balance estimates hydrocarbons in place and drive behavior by equating cumulative withdrawals with expansion and support terms.

Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

Drive mechanisms: solution gas, water, gas cap
OOIP original oil in place from MBAL
Cumulative plot identifies drive type

Topic details

Introduction

Craft & Hawkins and Dake treat MBE as a field-scale accounting framework rather than a single formula. B.Tech solutions usually gain marks by writing each term meaning before substitution.

Key relations & formulas

Formulas (Indian textbook notation)

$oil tank: N = N_{p} + (N-N_{p})\frac{(Bo-Boi)}{Boi} + \frac{(W_{e} - W_{p})}{Boi}$

Formulas (Indian textbook notation)

$gas \frac{p}{z} method: \frac{p}{z} vs G_{p} linear depletion$

Formulas (Indian textbook notation)

$aquifer influx W_{e} Schilthuis or Fetkovich$

Notation and sign conventions

Relation 1 —

oil tank: N = N_{p} +

Formulas (Indian textbook notation)

$oil tank: N = N_{p} + (N-N_{p})\frac{(Bo-Boi)}{Boi} + \frac{(W_{e} - W_{p})}{Boi}$

Write this relation with symbols exactly as in Dake Reservoir Engineering — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

gas \frac{p}{z} method: \frac{p}{z} vs G_{p} linear depletion

Formulas (Indian textbook notation)

$gas \frac{p}{z} method: \frac{p}{z} vs G_{p} linear depletion$

Write this relation with symbols exactly as in Dake Reservoir Engineering — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

aquifer influx W_{e} Schilthuis or Fetkovich

Formulas (Indian textbook notation)

$aquifer influx W_{e} Schilthuis or Fetkovich$

Write this relation with symbols exactly as in Dake Reservoir Engineering — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Concept in depth

When production occurs, reservoir fluids expand, dissolved gas evolves, and water influx may provide pressure support. MBE captures these competing effects without full simulation and is particularly useful for early reserve estimation and drive-mechanism diagnosis using Havlena-Odeh straight-line plots.

Assumptions and validity limits

State assumptions explicitly before using any relation for material balance equation — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Reservoir Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Reservoir Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to material balance equation.
4. Use equation 1:

oil tank: N = N_{p} +

.
5. Use equation 2:

gas \frac{p}{z} method: \frac{p}{z} vs G_{p} linear depletion

.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Material Balance Equation appears in field development plans. In Indian petroleum curricula this topic is tested because it connects theory to reservoir behaviour and recovery.

GATE and semester exams often combine material balance equation with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use material balance equation?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

Students often apply depletion assumptions to water-drive reservoirs, ignore cumulative water production in the equation, and mismatch reservoir barrels with stock tank barrels.

Quick revision checklist

Before attempting material balance equation problems, confirm you can:

1. Drive mechanisms: solution gas, water, gas cap
2. OOIP original oil in place from MBAL
3. Cumulative plot identifies drive type

Revise the solved examples in Dake Reservoir Engineering — Standard reference and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Gas p/z Screening

Problem

If p/z declines approximately linearly from 3200 to 2400 psi while Gp rises from 0 to 0.8 Tscf, what does it indicate?

Solution

Near-linearity of p/z versus Gp suggests volumetric gas depletion with limited water influx; OGIP can be estimated from the x-intercept method.

Conceptual check — Material Balance Equation

Problem

In a Reservoir Engineering semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of material balance equation." What should a complete answer include?

Exams & GATE

Havlena-Odeh material balance — GATE PE favourite.

📖 Standard books (India)

Dake Reservoir Engineering — Standard reference
Read: Syllabus unit
Referenced in Indian B.Tech syllabus

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