Common logging tools and techniques used in the oil and gas industry are as follows: Resistivity logs: Water is a better conductor of electricity than oil and gas. Petroleum fluids are much more resistive to electricity than formation water having measurable salinity.

Defining the depths of the fluid contacts, gas/water contact (GWC), oil/water contact (OWC), and gas/oil contact (GOC), or defining both of the latter in some reservoir situations, is essential for volumetrics calculations and important for detailed petrophysical calculations.

The density log usually labelled ‘RHOB’ measures the density of the borehole and the rocks penetrated by the drill bit. The unit for density is gram per cubic centimetre. The logs (sonic and density) measurements are usually made every 6 in. down hole, but must be aligned to be ‘on-depth’ with each other.

Electrical resistivity is a measure of a material’s property to oppose the flow of electric current. This is expressed in Ohm-meters (Ω⋅m). A high resistivity means that a material does not conduct electric charge well.

Google it and you will find out about 100k ohm-m for oil, and a ridiculous 10 billion ohm-m for gas. Enormously resistive value. Archie knew this, that is why there is no hydrocarbon resistivity in his equation.

Words in this image: GWC: Gas/water contact, GOC: Gas/oil contact, GDT: Gas down to, GUT: Gas up to, GLC: Gas/liquid contact, OWC: Oil/water contact, ODT: Oil down to, OUT: Oil up to, OLC: Oil/liquid contact, WDT: Water down to, WUT: Water up to, OOWC: Original oil/water contact, FWL: Free water level, HDT: …

The methods used for determining the fluid contacts include fluid sampling, water and hydrocarbons saturation estimation from geophysical well logs, analyses of conventional or sidewall cores, and formation pressure measurements.

By their definition, petrophysics is the study of the physical and chemical properties of rocks and their contained fluids. The Society of Petrophysicists and Well Log Analysts (SPWLA) mission is to increase the awareness of petrophysics, formation evaluation and well logging best practices in the oil and gas industry.

The resistivity of a material is dependent upon the material’s electronic structure and its temperature. For most (but not all) materials, resistivity increases with increasing temperature.

Identifying Fluid Contacts. Gravity segregation of fluids puts gas on top of oil (or water) and oil on top of water in most reservoirs. The gas/oil or gas/water contact is fairly sharp. The oil/water contact is usually gradational, covering a few to many feet. The top of the transition zone is the base of clean oil production.

Log analysts look for formations with low background radiation because they have the potential to contain moveable hydrocarbons. Resistivity tools measure the formation at different depths of investigation. The resulting log presents shallow-, medium- and deep-reading curves.

Running and analyzing TDT or pulsed neutron logs to find water contact changes over time is called reservoir monitoring. By combining dated water contacts from both cased hole and open hole logs, a clearer picture of reservoir performance can be obtained.

Well log analysis revealed that the range of formation water resistivity (R w ) vary from 0.32 Ω-m in the deeper subsurface to 0.89 Ω-m in the shallow surface indicating the presence of fresh water bearing sands at shallow depth.