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February 18, 2017

Gamma Ray and Spontaneous potential corrections

1.Gamma ray correction:-

Gamma ray log is subjected to number of effects like: –

Sonde position in the hole.

Hole size.

Mud weight.

Casing size and weight.

Cement thickness

Since there are innumerable combinations of hole size, mud weight and tool position, Gamma ray charts are designed to correct their gamma ray logs back to standard set of conditions.

While processing the data on computer,Interpreter  needs to input required  parameters and gamma ray corrected values are output however its worth to manually do environmental correction using chartbook to understand the concept.

For GR borehole correction is done to correct  for: –

1.Distance between tool and bore hole wall.

2.Density of mud in the borehole

Gamma ray borehole correction chart from various vendors  can be  used for this purpose if it needs to be done manually.The environmental correction charts provided in the Gamma Ray section of the chart book correct for two separate effects:

1.The distance between the tool and the borehole wall; and,

2.The density of the mud filling the borehole.

Both the above conditions affect the amount of natural radiation that is measured at the gamma ray detector.

 

Figure:- Gamma ray borehole correction for different hole size ,tool diameter,mud weight and tool position. Note more gamma ray correction is needed if tool is centered.

Tool diameter and tool position are both important factors to consider when applying GR environmental corrections . In a hole of given diameter, the amount of gamma radiation received by a large diameter tool will be greater than that received by a smaller diameter tool because the amount of mud surrounding the larger tool is less. Standard gamma tools have an outer diameter of 3-5/8 inches. Furthermore, if the tool is eccentered, then some gamma rays will be attenuated by the thicker shield of mud on one side of the tool. With a centered tool there is still attenuation of the received gamma rays, but it is much more regular in nature. These effects are accounted for while doing environmental corrections.

Figure:- Effect of tool position on incoming gamma ray.As we can see in above plot,eccentralized tool gets more gamma ray ,without any interference of mud on one side of tool whereas centralized tool gets same amount of gamma ray from all around the borehole.

  1. Spontaneous PotentialCorrections

In relatively thin beds, the spontaneous potential (SP) curve is suppressed due to the influence of overlying and under lying beds. This is particularly true in sand-shale sequences where sands are overlain and underlain by shales.  In cases such as this, the SP response in the thin formation of interest may not attain its full deflection because of the adjacent shales.    The purpose of SP correction is to correct the SP response in these type of  situations. The correction is called SP bed thickness correction.

Figure:- SP Bed thickness correction chart.

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