Book Chapter 1 Wellloggingandformationevaluation Toby Darling

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CHAPTER

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BASICS

1.1 TERMINOLOGY
Like most professions, petroleum engineering is beset with jargon.
Therefore, it will make things simpler if I first go through some of the
basic terms that will be used throughout this book. Petroleum engineering is principally concerned with building static and dynamic models of
oil and gas reservoirs.
Static modelsare concerned with characterizing and quantifying the
structure prior to any production from the field. Hence, key parameters
that the models aim to determine are:

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STOIIP = stock tank oil initially in place; usually measured in stock
tank barrels (stb)
GIIP = gas initially in place; usually measured in billion standard cubic
feet (Bcf)
GBV = gross bulkvolume; the total rock volume of the reservoir
containing hydrocarbon
NPV = net pore volume; the porespace of the reservoir
HCPV = hydrocarbon pore volume; the porespace actually containing
hydrocarbon
f = porosity; the proportion of the formation that contains fluids
k = permeability; usually expressed in millidarcies (md)
Sw = water saturation; the proportion of the porosity that contains waterSh = hydrocarbon saturation; the proportion of the porosity that contains
hydrocarbon
FWL = free water level; the depth at which the capillary pressure in the
reservoir is zero; effectively the depth below which no producible
hydrocarbons will be found

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Well Logging and Formation Evaluation

HWC = hydrocarbon/water contact; thedepth below which the formation is water bearing as encountered in a particular well. Likewise,
OWC for oil and GWC for gas
GOC = gas oil contact; the depth below which any gas in the reservoir
will be dissolved in the oil
Gross thickness = the total thickness of the formation as encountered
in a particular well
Net thickness = the part of the gross thickness that contains porous rocksubject to given cutoff criteria
Pay thickness = the part of the net thickness that is considered to be
capable of producing hydrocarbons in a particular well

Because of inherent uncertainties in all the parameters used to determine STOIIP or GIIP, geologists will usually develop probabilistic
models, in which all the parameters are allowed to vary according to distribution functions betweenlow, expected, and high values. The resulting
static models may then be analyzed statistically to generate the following
values, which are used for subsequent economic analyses:

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P50 STOIIP: the value of the STOIIP for which there is a 50% chance
that the true value lies either above or below the value
P15 STOIIP: the value of the STOIIP for which there is only a 15%
chance thatthe true value exceeds the value. Often called the high case.
P85 STOIIP: the value of the STOIIP for which there is an 85% chance
that the true value exceeds the value. Often called the low case.
Expected STOIIP: the value of the STOIIP derived by taking the
integral of the probability density function for the STOIIP times the
STOIIP. For a symmetric distribution, this will equal the P50value.

Similar terminology applies to GIIP.
In order to predict the hydrocarbons that may be actually produced from
a field (the reserves), it is necessary to construct a dynamic model of the
field. This will generate production profiles for individual wells, subject
to various production scenarios. Additional terminology that comes into
play includes:

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Reserves = the part of the STOIIPor GIIP that may be actually produced for a given development scenario. Oil companies have their own
rules for how reserves are categorized depending on the extent to which
they are regarded as proven and accessible through wells. Terms fre-

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Basics

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quently used are proven reserves, developed reserves, scope for recovery reserves,...