Page 186 - Hydrocarbon Exploration and Production Second Edition

CHA P T E R 7

Volumetric Estimation

Introduction and Commercial Application: Volumetric estimation is concerned with
quantifying how much oil and gas exists in an accumulation. The estimate will vary
throughout the field lifetime as more information becomes available and as the
technology for gathering and interpreting the data improves. A volumetric estimate
is therefore a current estimate, and should be expected to change over time. Two
main methods of estimating volumetrics are used: deterministic and probabilistic.
Deterministic methods average the data gathered at various points in the reservoir,
from well logs, cores and seismic to estimate the field-wide properties. Probabilistic
methods use predictive tools, statistics, analogue field data and input regarding the
geological model to predict trends in reservoir properties away from the sample
points. This section will concentrate on the deterministic methods and the
techniques used for expressing uncertainty in these volumetric estimates.
The volumetrics of a field, along with the anticipated recovery factors (RFs),
control the reserves in the field – those hydrocarbons which will be produced in the
future. The value of an oil or gas company lies predominantly in its hydrocarbon
reserves which are used by shareholders and investors as one indication of
the strength of the company, both at present and in the future. A reliable estimate of
the reserves of a company is therefore important to the current value as well as the
longer term prospects of an oil or gas company.

7.1. Deterministic Methods

Volumetric estimates are required at all stages of the field life cycle. In many
instances, a first estimate of ‘how big’ an accumulation could be is requested. If only
a ‘back of the envelope’ estimate is needed or if the data available is very sparse, a
quick look estimation can be made using field-wide averages.
The formulae to calculate volumes of oil or gas are

N 1
STOIIP ¼ GRV fS o ðstbÞ
G B o
N 1
GIIP ¼ GRV fS g ðscfÞ
G B g
UR ¼ HCIIP Recovery factor ðstbÞ or ðscfÞ
Reserves ¼ UR Cumulative production ðstbÞ or ðscf Þ
STOIIP is a term which normalises volumes of oil contained under high
pressure and temperature in the subsurface to surface conditions (e.g. 1 bar, 151C).
In the early days of the industry this surface volume was referred to as stock tank oil

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