Well Log Interpretation

Basic Relationships

Drilling and Completion

Cable-Tool1800’s

Drilling and Completion

Rotary Drilling1900’s - present

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

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.

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.

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.

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.

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.

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.

Producing Methods

Water drilling platforms may be jacked up from seabed or be floating platforms held in place by anchors and/or propellors

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.

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.

Formation Evaluation

Typical Mud Log

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.

Geophysical Logs

Factors affecting log response include:

Nature of drilling mud;

Porosity & Permeability;

Nature of pore fluids;

Lithology

Some Definitions

Porosity: Pore Volume/Total VolumeEffective Porosity: Interconnected pore spaceFracture Porosity

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 : Viscosity of fluid

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

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)

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

Some Definitions

Resistivity (R) measured in ohm-metersConductivity = 1/R

r = resistance (Ohms)A = area across which electrons flowL = length along which electrons flow

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.

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

Estimating Water Saturation:Some 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

Estimating Water Saturation

Based on experiments, Archie found that F could also be related to a tortuosity factor, a, the porosity, , and a “cementation exponent”, m, by

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.

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.

Estimating Water

Saturation

Experimentally determined formation factors for various lithologies

Borehole Environmen

t

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.

Borehole Environmen

t

Dh = Hole Diameter

R = ResistivityRm = mud

Rmc = mud cake

Rmf = mud fluids (filtrate)Rxo = rock and filtrate

Rt = rock and formation fluids

Resistivity profiles around

Borehole

Well logs that measure resistivity can detect hydrocarbons by measuring water saturation Sw using the Archie Equation. The petroleum saturation isSp = 1 – SwKnowing Sp is critical to calculating reserves and determining the economic viability of a well.

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.

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

Resistivity profiles around

Borehole

The graph shows resistivity profiles in rock filled with oil (or fresh water).

Log Headers

The log header usually has all the necessary information needed to calculate Sw.

Typical Resistivity

Logs

Mud uses fresh water. What’s in the rock pores?

Typical Resistivity

Logs

Mud uses saline water. What’s in the rock pores?

Typical Resistivity

Logs

Mud uses fresh water. What’s in the rock pores?

Typical Resistivity

Logs

Mud uses saline water. What’s in the rock pores?

Resistivity = fcn (Temp, Salinity)