Well Log Interpretation Basic Relationships

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Well Log InterpretationBasic Relationships
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Drilling and Completion
Cable-Tool 1800s
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Drilling and Completion
Rotary Drilling 1900s - present
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Drilling and Completion
Rotary Drill Bits attached to hollow pipe
(drill string) in 33 lengths. Mud is circulated
through drill string, out the bit and back up the
hole outside the drill string
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Drilling and Completion
The drill string is attached to the kelly a
square rod that slides through the Kelly Bushing.
The kelly bushing rotates the Kelly, drill string
and bit.
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Drilling and Completion
The mud circulating through the drill string
cools the bit and carries the rock chips back up
the hole outside the drill string.
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Drilling and Completion
The mud carrying the rock chips goes to the
shaker screen which separates the rock chips
which are examined by the Geologist to determine
lithology and stratigraphic position.
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Drilling and Completion
Casing is set to prevent cave-ins and produce
fluids. Cement holds it in place and helps
prevent corrosion and leaks. Well logs are
normally run before casing is set.
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Role of Geologist
A Mudlogger records lithology from well cuttings,
tests for petroleum with UV light. Bottomhole T
P, rate of penetration, composition of gasses
are collected automatically.
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Role of Geologist
A well site geologist may be present. Duties
include advising driller on anticipated
lithologies and pressures, Picking casing and
coring points, and supervising logging. Routine
wells are monitored remotely from office.
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Producing Methods
Water drilling platforms may be jacked up from
seabed or be floating platforms held in place by
anchors and/or propellors
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Producing Methods
At sea, a single producing platform may have a
dozen or more deviated wells. These platforms may
be fixed to bottom or floating. Oil, gas and
water are separated and the petroleum piped to
shore. Wells must be cased and perforated.
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Formation Evaluation

• Geological evaluation of the lithology,
  stratigraphy, fluids and reservioir properties
  are essential for exploration and field
  development. Critical information from a well
  comes from
• the mudlog,
• cores (regular and sidewall)
• geophysical logs.

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Formation Evaluation
Typical Mud Log
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Geophysical Logs
Routine geophysical logs typically measure
natural electrical currents, electrical
resistivity, sonic velocity, and a variety of
radioactive parameters, to derive information
like density, porosity, permeability, composition
of fluids and lithology.
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Geophysical Logs

• Factors affecting log response include
• Nature of drilling mud
• Porosity Permeability
• Nature of pore fluids
• Lithology

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Some Definitions
Porosity Pore Volume/Total Volume Effective
Porosity Interconnected pore space Fracture
Porosity
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Some Definitions

• Permeability Ability to transmit fluids unit
  called a Darcy
• Q Rate of Flow
• K Permeability
• A Cross section area
• P1 P2 Pressure drop across L
• L Length
• m Viscosity of fluid

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Some Definitions
If there is more than one fluid, then things get
complicated. Wetting One fluid wets the rock
(usually water), and the other(s) are blobs
surrounded by the wetting fluid Water Saturation
(Sw) volume of water in pores/Volume of
pores If there are only oil and water, then So
1 - Sw
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Some Definitions
The wetting fluid adheres to the rock and will
not move. It is usually water and is called the
irreducible water saturation (Swirr)
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Some Definitions

• If more than one fluid then Permeability is more
  complicated
• Ka Absolute permeability K of fluid at 100
  saturation
• Ke Effective permeability Permeability of one
  fluid in presence of another
• Kr relative permeability Permeability of
  fluid at some saturation divided by Ka,
  permeability at 100 saturation Ke/Ka

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Some Definitions
Resistivity (R) measured in ohm-meters Conductivit
y 1/R r resistance (Ohms) A area across
which electrons flow L length along which
electrons flow
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Some Definitions

• Resistivity (R) is measured by electrical logs
• Rock is high R (low conductivity)
• Oil is high R
• Pure Water is high R
• Salt water is low R
• Drilling mud can be either high or low R
  depending on whether saline or fresh water is
  used.

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Estimating Water Saturation
Water saturation, Sw, is the bottom line. If it
is less than one, then there is probably
petroleum. Sw can be estimated from a
resistivity measurement using the Archie
Equation, an empirical relationship derived from
experiments done by G.E. Archie in 1942
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Estimating Water SaturationSome Definitions
Rw Resistivity of water in the rock pores
(measured) R0 Resistivity of rock saturated by
water of Rw. Formation Resistivity Factor, F
R0/Rw, and R0 F Rw
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Estimating Water Saturation
Based on experiments, Archie found that F could
also be related to a tortuosity factor, a, the
porosity, F, and a cementation exponent, m, by
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Estimating Water Saturation
Archie also found experimentally that

• n is the saturation exponent with a value of
  about 2 and
• R0 resistivity of rock 100 saturated by water
  of Rw (unknown).
• Rt is the resistivity of the combined rock and
  fluid measured by the log.

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Estimating Water Saturation
Combining these relationships produces the Archie
Equation

• n and F are experimentally determined, but there
  are tables for typical rock types.
• Rw resistivity of fluids in the rock and must
  be measured at the well site.
• Rt is the resistivity of the combined rock and
  fluid measured by the logging tool.

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Estimating Water Saturation
Experimentally determined formation factors for
various lithologies
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Borehole Environment
During drilling, the mud is kept at a higher
pressure than the formation fluids to avoid
blowouts. As a result, mud fluids invade the rock
leaving a mudcake along the bore hole. Mud fluid
invasion produces artificial resistivities.
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Borehole Environment
Dh Hole Diameter R Resistivity Rm mud Rmc
mud cake Rmf mud fluids (filtrate) Rxo rock
and filtrate Rt rock and formation fluids
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Resistivity profiles around Borehole
Well logs that measure resistivity can detect
hydrocarbons by measuring water saturation Sw
using the Archie Equation. The petroleum
saturation is Sp 1 Sw Knowing Sp is critical
to calculating reserves and determining the
economic viability of a well.
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Resistivity profiles around Borehole
Formation water is typically saline and normally
has a low Rmf. Water used in drilling muds is
usually saline but may be fresh with high
Rmf. The resistivity profile around a borehole
depends on whether the mud uses fresh or saline
water.
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Resistivity profiles around Borehole
Logs typically measure three resistivities
shallow, intermediate and deep corresponding to
the three zones around the borehole. The graph
shows resistivity profiles in rock filled with
saline water
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Resistivity profiles around Borehole
The graph shows resistivity profiles in rock
filled with oil (or fresh water).
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Log Headers
The log header usually has all the necessary
information needed to calculate Sw.
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Typical Resistivity Logs
Mud uses fresh water. Whats in the rock pores?
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Typical Resistivity Logs
Mud uses saline water. Whats in the rock pores?
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Typical Resistivity Logs
Mud uses fresh water. Whats in the rock pores?
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Typical Resistivity Logs
Mud uses saline water. Whats in the rock pores?
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Resistivity fcn (Temp, Salinity)