Page 176 - Introduction to Petroleum Engineering

LOGGING ENVIRONMENT 163

Adjacent bed Borehole

Uninvaded zone Uninvaded zone

Mud Flushed Zone of
Adjacent bed cake zone transition

FIGuRE 9.2 Schematic of invasion zones.

An idealized representation of the wellbore and formation is illustrated in
Figure 9.2. It is used to make analysis of well log response more tractable. The figure
shows four zones near the wellbore: the mud cake, the flushed zone, the zone of
transition, and the uninvaded zone. If pressure in the borehole during drilling is
greater than pressure in the formation, the pressure difference will drive drilling mud
into the permeable formation. Larger particles in the drilling mud will be filtered out
at the rock face of the borehole and create a mud cake adjacent to the borehole. The
liquid with any small particulates that pass through the mud cake is called the mud
filtrate. The mud cake can reduce the flow of fluids between the formation and the
well. The formation damage caused by mud cake and filtrate can be quantified by
well testing that yields a parameter called “skin.”
During drilling, portions of fluids originally in the flushed zone are displaced by
the invading mud filtrate and pushed into the transition zone. During logging, fluids
in the flushed zone include mud filtrate with any suspended solids, some native brine,
and any remaining oil and gas. Reservoir rock and fluid properties in the uninvaded
zone have not been altered from their original state by fluids from the drilling
operation.

9.1.2 Open or Cased?
Open‐hole logging refers to logging before setting casing in the well. Sidewall
coring, or the removal of a core of rock from the borehole wall, requires an uncased
hole. Most logging tools are designed for uncased holes. Open‐hole logging, which
is the most widely used logging method, makes it possible to assess the commercial
viability of a formation before spending the money to complete a well. Open‐hole
logs can be used to determine fluid contacts in a formation, obtain geological prop-
erties such as porosity and rock type, make pore pressure measurements, and obtain