Page 50 - Handbook Of Multiphase Flow Assurance

Quality of fluid samples 45

Laboratories for the sample collection should be validated, and personnel involved in ac-
tual sampling should be aware of the proper well flow duration requirements before the
sampling takes place, to ensure that the sample is not contaminated with drilling mud and
that sample itself is uniform and representative of the reservoir fluid.

Sampling hydrocarbon fluid
API Recommended Practice 44 provides detailed recommendations on how to conduct the
sampling for typical hydrocarbon fluids. Sampling specialists and laboratories would have
additional procedures on how to properly collect and transport samples of fluids which may
contain small amounts of hydrogen sulfide or mercury.
Information whether a sampling program should have provisions for H 2 S or mercury may
be obtained from regional analog fluids, from the analysis of samples collected earlier in the
same region about which published information is available or from geologic analysis which
provides indications whether similar rock structures may contain certain minerals which get
dissolved by the reservoir fluid.

Quality of fluid samples

Quality of the samples is of the most critical importance to the preconcept and concept
evaluation phases of a project when the technical feasibility of developing an asset is evalu-
ated. A flow assurance specialist should be able to use a few simple quality checks to find the
most representative samples or to compensate for sample contamination.

Oil sample quality checks
Collecting a surface sample from a separator onshore is much simpler than collecting a
downhole sample from an openhole wellbore in deepwater. Thus a close attention must be
paid to quality checks of the samples collected downhole.
Several initial checks are described below. These checks omit the sampling process and
conditions, container preparation and sample transfer and instead focus on the measured
properties control which allows the flow assurance specialist to review data in the laboratory
reports for the factors which can affect flow assurance work.

Hydrocarbon fluid sample quality checks

1. Check for GOR range
GOR of the samples collected from similar depths of the same reservoir should be fairly
constant, within a few hundred scf/stb. If GOR of some of the samples differs significantly
from the rest, particularly if GOR is lower than the average, it may indicate that some of the
gas was lost during downhole sample collection or transfer.
In this case the reservoir fluid sample composition should be checked for methane
content.