Material Balance

 Introduction

Globally, there are different techniques applied in the oil and gas industry to estimate

hydrocarbon reserves. These techniques include the analogy, volumetric, decline

curve, material balance and reservoir simulation. The application of these techniques

is dependent on the volume and quality of data available with some level of

uncertainties. In Chap. 2, we have established that the analogy method is applied

by comparing factors for the current field or wells while the volumetric or geologic

method combined the extent of the reservoir (area), the pore volume of the reservoir

rock, the content of fluid within the reservoir pore volume and PVT properties.

When production and pressure data from the field become available, decline

curve analysis and material balance calculations become the predominant methods

of calculating reserves since the hydrocarbon reserve estimation is a continuous

process for a field that is producing. These methods greatly reduce the uncertainties

in reserves estimation; however, during early depletion, caution should be exercised

in using them.

Material balance equation (MBE) makes use of the basic concept of conservation

of mass which states that the cumulative observed production, expressed as an

underground withdrawal, must be equal to the expansion of the fluids in the reservoir

resulting from a finite pressure drop or expressed as the mass of fluids originally in

place equal to mass of fluid remaining plus the mass of fluid produced. MBE is seen

by the Reservoir Engineers as the basic tool for interpreting and predicting the

performance of oil and gas reservoirs. It helps engineers to get a feel of the

reservoir. To better understand this subject, several textbooks and materials were

consultated. these are: Craft & Hawkins (1991), Dake (1978, 1994), Mattar &

Aderson (2005), Numbere (1998), Pletcher (2002), Steffensen (1992), Matter &

McNeil (1998), Tracy (1955) & Tarek (2010).

5.1.1 Assumptions of Material Balance Equation

To apply the material balance equation, there are several assumptions made by the

engineers to successfully carry out an evaluation on oil and gas reservoirs. These are:

• The reservoir is considered to be a tank

• Pressure, temperature, and rock and fluid properties are not space dependent

• Uniform hydrocarbon saturation and pressure distribution (homogenous

reservoir)

• Thermodynamic equilibrium always attained.

• Isothermal condition apply

• Production data is reliable

Limitations of Material Balance Equation

The implication/limitation of the above stated assumptions in evaluating reservoir

performance is that, material balance uses a model that is existing as an imagination

of the reservoir to actually tell or forecast the behaviour of the reservoir. This is

established as a result of the production of hydrocarbon from the reservoir with

natural energy or by gas or water injection. These implications are given below:

• It is considered to be a tank model with a zero dimension which implies that it

does not reflect the area drained

• the shape or geometry of the reservoir

• the manner in which the wells drilled into the reservoirs are positioned and

orientation are not considered

• the dynamic effects of fluid are not considered

• the heterogeneous nature of the reservoir and no time parameters

These implications lead to the statement made by Warner et al. (1979) that the

material balance method has some limitations, though it can be used as a

pre-processing tool to infer fluid in place, drive mechanisms and identify aquifer

for a more sophisticated tool “reservoir simulation”. This sophisticated tool gives an

insight into dynamic rock and fluid properties for evaluation of past reservoir

performance, prediction of future reservoir performance, and reserves estimation.

5.2 Data Requirement in Performing Material Balance

Equation

5.2.1 Production Data

• Cumulative oil, gas and water volume produced

• cumulative gas-oil ratio

5.2.2 PVT Properties

• Oil, gas and water formation volume factor

• Compressibility of water

• Solution Gas-Oil Ratio

5.2.3 Reservoir Properties

• Rock Compressibility

• Connate water saturation

Other Terms

• Initial volume of oil in reservoir

• Initial gas cap

• Water and gas injection if any

5.3 Sources of Data Use for the MBE

Uses of Material Balance Equation

However, despite the assumptions and limitations of the material balance approach,

there some basic uses which could guide reservoir engineers prior to full field

reservoir study. These are:

• Determination of the hydrocarbon in place, gas cap size etc.

• Reservoir pressure estimation from historical production and/or injection

schedule.

• Predict the future performance of the reservoir and the average production of the

wells sunk into the reservoir for a given pressure schedule

• Determine the presence, type and size of an aquifer.

• Estimation of fluid contacts (Gas/Oil, Water/Oil, Gas/Water).

• Material balance equation can be used to calculate fluid saturation as production

increases

5.5 PVT Input Calculation

The PVT properties can either be obtained from the laboratory analysis or generated

from existing correlations. Some of these developed correlations are given below.

Standing Correlations

Glaso Correlations

 Al-Marhouns