What do resistivity logs measure?

Resistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current.

What is mud resistivity?

In a permeable formation, mud resistivity is commonly 1 to 3 orders of magnitude lower than the formation resistivity, or, in the case of oil-based mud (OBM), it can be much higher.

What is resistivity in drilling?

1. n. [Formation Evaluation] The ability of a material to resist electrical conduction. It is the inverse of conductivity and is measured in ohm-m. The resistivity is a property of the material, whereas the resistance also depends on the volume measured.

What is flushed zone?

1. n. [Formation Evaluation] The volume close to the borehole wall in which all of the moveable fluids have been displaced by mud filtrate. The flushed zone contains filtrate and the remaining hydrocarbons, the percentage of the former being the flushed-zone water saturation, Sxo.

How is resistivity measured in the uninvaded zone?

The resistivity in the uninvaded zone is the true resistivity of the original fluid and formation. The uninvaded zone is logged as deep resistivity Resistivity can be interpreted as a measurement of a formation’s fluid saturation as it is a function of the formation’s rock type, porosity, fluid type, and fluid volume.

Where does the resistivity of a formation depend?

Invaded Zones. In most cases there is an invaded zone, where the formation fluids have been disturbed by the drilling fluid. The resistivity of the formation in this zone depends upon the resistivity of the mud filtrateR mf, the resistivity of any remaining formation waterR w, the saturation of the mud filtrate S

How is the resistivity of the flushed zone measured?

The resistivity of the flushed zone can be measured with reasonable precision if the depth of invasion is greater than the depth of investigation of the Rxo logging tool, but the depth of invasion and the resistivity profile and geometry of the zone of transition are difficult to estimate.

How is inversion used in formation resistivity determination?

Inversion means first building a parametric model of the formation, then estimating from the log values a “first guess” of the parameter values. Then, a modeling code is used to compute the log response to the model. This is compared with the actual logs.