What is “Logging” and “Log Analysis”? What is “Petrophysics”?

I- “Logging” is the measurement versus depth/time, of one or more physical quantities in or around a wellbore 

1- Timing and tool conveyance 

• During drilling – MWD/LWD – Coring 
• After drilling – Wireline (or E-line) 

2- Data is displayed vs. depth or time 

3- Hardcopy printout of data as curves is referred to as a “log”

II- “Log Analysis” is understanding and interpreting what these log measurements represent, and how they can be used to define or refine geological, petrophysical, and engineering models. 

III-“ Petrophysics” is suggested as the term pertaining to the physics of particular rock types. This subject is a study of the physical properties of rock which are related to the pore and fluid distribution." --G.E. Archie (1950), a pioneer in the application and quantification of rock physical relations to geosciences and petroleum engineering 

1- Sources of Data for Petrophysical Analysis 

• Cuttings 
• Core (Conventional and Sidewall) 
• Fluid Samples (Mud / DST / Formation Tester) 
• Well Logs and Images (LWD & Wireline) 
• Seismic 
• Experience 

2- Formation Properties 

• Rock type 
• Fluid volume and type 
• Saturations 
• Porosity 
• Permeability

IV- What does the Petrophysicist do? 

Basic tools: 

• Borehole Logging Tools (LWD or Wireline) :  Caliper, Gamma Ray, Resistivity, Density and Neutron, Acoustics, NMR, Formation Test
• Core Data 

Plan and QC logging programs

Interpret well log data to provide: Matrix type, Fluid Types & Pressure Gradients, Fluid Saturations & Volumes, Porosity & Permeability. 

 The Archie ‘Clean Sand’ Equation “foundation stone” of most log interpretation 

Φ = porosity 

Rw = resistivity of brine (water)

Rt = resistivity of the formation 

Sw = water saturation 

a, m, and n constants are best determined by core analysis 

a: a constant (often taken to be 1) 

m: cementation factor (varies around 2) 

n: saturation exponent (generally 2) 

V- Why does the Petrophysicist need Sw?

Recoverable Oil & Gas 

Recovery factor F = estimate of fraction of oil or gas in place likely to get to surface 

Factors affecting recovery factor : Permeability, Fluid type, Fluid Viscosity, Reservoir Pressure 

Typical recovery Factors : 

 Oil: Low 0.3 High 0.6 

Gas: Low 0.7 High 0.95 

if  Shc = (1 - Sw) 

Oil in place (bbls) = 7,758 x F x (1 - Sw) x h (ft) x A(acres)