Page 189 - Introduction to Petroleum Engineering

176 WELL LOGGING
means that coring is a time‐consuming and expensive process. Consequently, only a
few wells are cored. Cored wells are usually cored over a limited section of the well
where core samples can provide the most useful information.
Cores are especially useful for characterizing productive formations. Each core
from the formation is a small sample of reservoir rock. The core can be used to
determine important reservoir properties such as lithology, porosity, and perme-
ability. Core properties can be detrimentally modified during the coring process
because reduction in temperature and pressure as the core is lifted to the surface
alters core fluid content. Core samples may be obtained using a process known
as native‐state coring that is designed to keep core samples at original in situ
conditions. The more complicated native‐state coring process increases cost and
tends to limit native‐state coring to special situations.
Cores and any results of core analysis are routinely compared to open‐hole logs as
a way to improve interpretation of the logs.

9.6.3 Sidewall Core
Samples of formations can also be collected by devices lowered on a wireline. Some
of these devices use percussive means to drive a sampling cup into the formation.
Other devices use rotary saws for cutting samples. While percussive sampling is
faster, rotary cutting provides better samples.

9.7 MEaSuREMENT WHILE DRILLING aND LOGGING
WHILE DRILLING

Measurement while drilling (MWD) refers to real‐time measurements of drilling
progress with sensors in the BHA. MWD allows better control of the drilling oper-
ation for reaching targets efficiently. Originally, MWD sensors measured drilling
direction only, but currently they can also measure weight on bit, torque, RPM, and
other drilling‐related parameters. Natural gamma‐ray logging sensors are included
in some MWD assemblies. All of this information is communicated to the surface
with pressure pulses in the drilling mud. Opening and closing a valve in the BHA
generates the pulses. Rates of 1–3 bits per second are common. A battery pack or
a small turbine driven by drilling mud provides electrical power for the MWD
equipment.
Over the years, more and more logging sensors have been included in BHAs,
which led to the coinage of the acronym LWD for logging while drilling. Many of the
logging tools available for open‐hole logging are available for LWD. Logging
information is communicated to the surface with pressure pulse systems in the MWD
assemblies. In some cases, logging information is stored in memory devices for later
retrieval. In place of mud pulse telemetry, some MWD and LWD operations use
electromagnetic systems that communicate at about 10 bits per second. MWD and
LWD depths are tied to driller depths. Drillers determine depths by summing the
lengths of drill pipe joints in the hole relative to the kelly bushing.