Page 161 - Hydrocarbon Exploration and Production Second Edition

148 Data Gathering

A complicating factor when acquiring downhole data is the contamination of
the measured formation by mud filtrate, which is discussed in detail at the end of
Section 6.4. During the drilling process, mud filtrate will enter the newly penetrated
formation to varying degrees. In a highly permeable formation, a large quantity of
fluid will initially enter the pores. As a result the clay platelets suspended in
the mud will quickly accumulate around the borehole wall. The formation
effectively filters the penetrating fluid forming a mudcake around the borehole wall
which in turn will prevent further invasion. In a less permeable formation, this
process will take more time and invasion will therefore penetrate deeper into the
formation.
In recent years, there has been a large increase in the volume of data that can be
acquired and transmitted by wireline logging. Historically, a logging tool would
generate one or two curves and two or more tools could be run linked together in a
‘toolstring’ to generate a series of curves from one run. Recent advances in
technology and IT power have led to the development of sophisticated tools
which record a data array at any one point rather than a single value. Downhole and
surface mathematical processing is conducted to transform the array data into a
product set of curves to be used by all subsurface disciplines. The following table
provides a summary of the mainstream wireline tool types that are routinely run
today.

Generic Device Tool Examples MeasurementType Application

Gamma GR, NGT, Natural gamma Lithology,
Spectralog radiation correlation
SP SP Spontaneous Lithology,
potential permeability
(indicator)
Density LDL, ZDL, SDL Bulk density Porosity, lithology
Neutron CN, CNL, DSN Hydrogen index Lithology, porosity,
gas indicator
Acoustic BHC, XMAC, DSI Travel time, acoustic Porosity, seismic
waveform calibration
Resistivity DLL, HRLA, HDLL Electrical resistance Saturation,
of formation permeability
indicator
Induction ILD, AIT, HILT, Induced electrical Saturation (OBMs)
HDIL, HRAI current
Image FMI, STAR, CBIL, Resistivity or Sedimentology,
EI, OBMI, CAST acoustic pixellated fracture/fault
image analysis
NMR MRIL, MREX, Nuclear magnetic Porosity,
CMR resonance permeability,
saturation
Formation RFT, MDT, RCI Pore pressure Fluid types, pressures
tester pressure and contacts