Back to Basics and Rule of Thumb Methods

In Petrophysical Evaluations

Ko Ko Kyi

Retired Principal Petrophysicist

The use of powerful computers and sophisticated software nowadays has made

petrophysical evaluations look easy; something which can be quickly performed at a push

of a button. Without knowing the basic theories and principles behind petrophysics, blindly

relying on software to produce these evaluations can lead to misinterpretation which can

have serious consequences such as missed opportunities or wrong economic assessment,

either over or under estimation, of the hydrocarbon potential. There are times where digital

data is not available and manual interpretation of the petrophysical data has to be carried

out by hand. At such a time, it becomes very important to know the basics and “the rule of

thumb” methods to carry out a meaningful interpretation. There are also times, where

conventional interpretation with standard methods cannot provide a satisfactory evaluation

and some “tricks of the trade” need to be used to provide the best possible solution to the

problem. In this presentation, some case histories are presented on the importance of

knowing the “rules of thumb” and “tricks of the trade” to help petrophysicists in their work.

Gas Correction

Worried about how gas correction is done? There are complex methods and there are also rules of

thumb “easy-to-do” methods, to derive gas-corrected porosity using neutron and density logs.

Gas correction methods for porosity in a gas bearing sand

1. Using iteration process with

petrophysical software

2. Using approximate equations:

PHI_23 = (2*DPHI+NPHI)/3

PHI_GM = SQRT[(DPHI^2+NPHI^2)/2]

PHI_SLB = (7*DPHI+2*NPHI)/9

PHI_85 = 0.85*DPHI

Notes:

Both DPHI and NPHI must be computed using

Sandstone Matrix.

Gas corrected porosity PHIT is obtained from the

triple lithological model.

All the approximate methods are overestimating

the porosity compared to the triple lithology model

result in gas bearing zone.

All the results tend to agree in liquid hydrocarbon

and water bearing intervals

Gas correction methods for porosity in a gas bearing limestone reservoir

1. Using iteration process with software

2. Using approximate equations:

PHI_23 = (2*DPHI+NPHI)/3

PHI_GM = SQRT[(DPHI^2+NPHI^2)/2]

PHI_SLB = (7*DPHI+2*NPHI)/9

PHI_85 = 0.85*DPHI

Notes:

Both DPHI and NPHI must be computed using

Limestone Matrix.

Gas corrected porosity PHIT is obtained from the

iterative process using neutron-density crossplot.

PHI_GM (Gaymard’s method) seems to be the

closest to the gas corrected porosity PHIT.

Water Saturation Computation Without Porosity Log

Ever been stumped when asked to

compute water saturation based on

one resistivity log only?

You can make it happen with a certain

assumption and a pinch of salt.

Sw = Sqrt(R0/Rt)

Where,

R0 = Resisitivty in the water leg

Rt = Resistivity in the hydrocarbon

bearing interval.

Assumption: The reservoir quality does not

change and porosity remains constant over

the zone of interest.

Variable Rock Textural Properties,

The Dreaded Variable “m” in Carbonates

Rxo and Rt almost overlaying each

other at values between 1-2 ohm-m

Improvement of hydrocarbon

saturation using ratio method

Pickett Plot and Hingle Plot

Pickett Plot is usually used to determine Rw

(formation water resistivity).

It can also be used to graphically display zones

with different water saturation values.

Hingle Plot is also used to determine

Rw, without having to compute

porosity first. It can also display zones

with different water saturation values.

Credit to original owner of the plots

Buckles Number and Buckles Plot

Uses of Buckles Plot

• Identify Swirr Zones for Analysis (i.e. Capillary Pressure)

• Identify One Type of Low Resistivity Pay

• Identify Stratigraphic Flow Units (Depositional

Environment)

Ref: ConocoPhillips

Buckles Number can give an indication if a certain

zone is at irreducible water saturation and therefore

will not produce any water.