Page 102 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

90 Reservoir Engineering

Cores taken with a pressure core barrel are often frozen at the well-site for
transportation to the laboratory. (Cores are left in the inner core barrel.)
Normally, the inner barrel containing the cores is cut into lengths convenient
for transport. Because of the complexity of the operation, the pressure core
barrel is not used as extensively as the conventional core barrel. An alternate
procedure involves bleeding off the pressure in the core and core barrel while
the produced liquids are collected and measured. Analysis of the depressured
core is done by conventional techniques. Fluids collected from the barrel during
depressuring are proportionately added to the volumes of liquid determined
from core analysis. In this manner a reconstructed reservoir core saturation
is provided.

Core Preparation
Depending on the type of core testing to be done, core samples may be tested
as received in the laboratory or they may be cleaned to remove resident fluids
prior to analysis. Details for cutting, cleaning, and preparing core plugs can be
found in API RP-40: Recommended Practice for Core-Analysis Procedure [ 1921,
available from API Production Department, 211 North Ervay, Suite 1700, Dallas,
TX 75201.

Core Analysis

Conventional core analysis procedures are described in detail in API RP-40
and elsewhere [191]. A good discussion on core analysis procedures is in the
textbook written by Amyx, Bass, and Whiting [19].

Porosity. A number of methods [192] are suitable for measuring porosity of
core samples. In almost all the methods, the sample is cleaned by solvent
extraction and dried to remove liquid. Porosity can be determined by saturating
the dry core with brine and measuring the weight increase after saturation.
Another common method includes compressing a known volume of gas (usually
helium) at a known pressure into a core that was originally at atmospheric
pressure. Several other techniques have been used; one of the more common
methods is the mercury porosimeter in which pressure on the core plug is
reduced and the volume of the expanded air or gas is measured accurately. A
summation of fluids technique, which measures and sums the oil, gas and water
volumes in a freshly recovered reservoir core sample, is often used for plugs or
sidewall samples of non-vuggy consolidated rocks that contain minimum amounts
of clay [188].
Equations commonly used for calculation of porosity by gas expansion or
compression include:

(5-80)

(5-81)