Understanding Resistivity Invasion Profiles

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Petrophysical analysis plays an important role in the success of a company. To keep a broad topic simple I will speak in regards to the Invasion Profile caused by a resistivity tool. The petrophysicist’s knowledge, in terms of the reservoir and the migration of hydrocarbons, not only helps determine, and possibly increase, that well’s overall production, but supports future reserves in any adjacent wells that maybe in the plan for future. The invasion of drilling fluid into the rock plays an important role in this analysis.

What is Invasion?

  • Due to the differential pressure in the borehole, drilling fluid is forced into the rock, causing invasion of this fluid into the reservoir fluid

  • The distance the rock is “invaded” by the drilling fluid helps determine the invasion diameter, which makes up the invased zone of the rock

  • “Invasion profiles” can be found using conductivity measurements, that provide different depth of investigations, to determine how far the drilling fluid has penetrated the rock

  • To ensure the rocks true resistivity (Rt) is being measured the deep induction reading is typically used as the depth of investigation penetrates the furthest.

Why is Rt important?

Looking back at Archies Equation, to determine the water saturation of a formation, in addition to knowing porosity and formation water resistivity (typically found in catalogues) we also need to know the formation resistivity. Depending on how these curves react to one another helps determine what to use as your formation resistivity in this calculation.

In addition to resistivity, measurements such as porosity, natural gamma ray and spectral gamma ray are available using Cordax's unique, safe memory logging method.