Page 100 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

88 Reservoir Engineering

Table 5-20
Routine Core Analysis Tests
Type of analysis Use of results
Porosity A factor in volume and storage determinations.
Permeability-horizontal Defines flow capacity, crossflow, gas and water coning
and vertical and relative profile capacity of different zones, pay
and nonpay zones.
Saturations Defines presence of hydrocarbons, probable fluid
recovery by test, type of recovery, fluid contacts,
completion interval.
Lithology Rock type, fractures, vugs, laminations, shale content
used in log interpretation, recovery forecasts, capacity
estimates.
Core-gamma ray log Relates core and log depth.
Grain density Used in log interpretation and lithology.
From Reference 180.

recommended by the American Petroleum Institute [192]. Some of the infor-
mation available in these sources will be highlighted.

Coring

Well coring refers to the process of obtaining representative samples of the
productive formation in order to conduct a variety of laboratory testing. Various
techniques are used to obtain core samples: conventional diamond-bit coring,
rubbersleeve coring, pressure coring, sidewall coring, and recovery of cuttings
generated from the drilling operation. Conventional coring is normally done
in competent formations to obtain full-diameter cores. Rubber sleeve-coring
improves core recovery in softer formations. Pressure coring, although relatively
expensive, is used to obtain cores that have not lost any fluids during lifting of
the core to the surface.
A common problem with all of these techniques is to decide when to core.
In many instances, cores from the interval of interest are not obtained because
of abrupt stratigraphic changes. A second problem is that, typically, non-
productive intervals of the desired strata are obtained. These intervals did not
initially contain a significant amount of hydrocarbon.
Core Preservation
The importance of not altering wettability with drilling mud filtrate has been
discussed in this chapter in the section entitled “Wettability and Contact Angle.”
Preventing wettability changes in core material, after it has been recovered at
the surface, can be equally important so that subsequent laboratory measure-
ments are representative of formation conditions.
Cores obtained with drilling muds that minimize wettability alteration, and
that are protected at the well-site to prevent evaporation or oxidation, are called
preserved cores. They are also referred to as fresh cores or nativestate cores.