Page 190 - Introduction to Petroleum Engineering

RESERVOIR CHARACTERIZATION ISSUES 177
9.8 RESERVOIR CHaRaCTERIZaTION ISSuES

Well logs are a major source of data about the reservoir. Table 9.2 is a summary of
the principal applications of some widely used well logs. The columns in Table 9.2
show that well logs provide information about rock type, fluid content, porosity,
pressure prediction, and structural and sedimentary dip. This information is essential
for characterizing the reservoir. Some common reservoir characterization issues are
introduced below.

9.8.1 Well Log Legacy

Well logging technology has been changing for approximately a century. Well logs
acquired during that period of time represent a data legacy that has a value as intel-
lectual property because old well logs can still provide useful information about a
reservoir at the time the well log was acquired. In many cases, however, changes in
well logging tools required changes in analysis techniques.

9.8.2 Cutoffs

Well log measurements can provide information about reservoir rock over a
range of properties that includes economically producible pay zones and zones
that contribute very little oil and gas to the production stream. Reserves calcula-
tions are more accurate when the analysis of well logs includes only pay zones.
A well log cutoff specifies the minimum value of a measured property so that
unproductive rock volume does not get included in fluid in place and reserves
calculations.
We illustrate the use of cutoffs in the calculation of reserves by recalling that the
volume of reserves V is the product of original hydrocarbon in place and recovery
R
factor R . Therefore the volume of reserves V is
F R
V = φ AhSR F (9.14)
R
h
TabLE 9.2 Principal applications of Common Well Logs
Log Type Lithology Hydrocarbons Porosity Pressure Dip
Electric
SP X
Resistivity X X X
Radioactive
Gamma ray X
Neutron X X
Density X X
Sonic X X X X
Dipmeter X

Source: After Selley and Sonnenberg (2015), page 86.