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
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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.
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