logging

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 models are concerned with characterizing and quantifying the

structure prior to any production from the field. Hence, key parameters

that the models aim to determine are:

• 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 bulk volume; 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 water

• Sh = 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

• HWC = hydrocarbon/water contact; the depth 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 rock

subject 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 between low, 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:

• 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 that the 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 P50 value.

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:

• Reserves = the part of the STOIIP or 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-

2 Well Logging and Formation Evaluation

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

reserves, probable reserves, and possible reserves.

• Remaining reserves = that part of the reserves that has not yet been

produced

• Cumulative production = that part of the reserves that has already been

produced

• UR = ultimate recovery; the total volume of reserves that will be produced

prior to abandonment of the field

• NPV = net present value; the future economic value of the field, taking

into account all future present value costs and revenues

• RF = recovery factor; the reserves as a proportion of the STOIIP

(or GIIP)

• Bo = oil volume factor; the factor used to convert reservoir volumes of

oil to surface (stock tank) conditions. Likewise Bg for gas.

In order to produce the hydrocarbons, wells are needed and a development

strategy needs to be constructed. This strategy will typically be presented

in a document called the field development plan (FDP), which

contains a summary of current knowledge about the field and the plans

for future development.

Once an FDP has been approved, the drilling campaign will consist of

well proposals, in which the costs, well trajectory, geological prognosis,

and data-gathering requirements are specified. The petrophysicist plays a

part in the preparation of the well proposal in specifying which logs need

to be acquired in the various hole sections.