## Logging tools environmental corrections

### Logging tools environmental corrections

In real world  logging ,its not easy to determine true gamma ray, neutron porosity,bulk density and true resistivity of the uninvaded zone precisely for  variety of reasons. There are various Factors affecting these responses which includes  hole size, mud density, bed thickness, depth of invasion, bore hole mud salinity, formation pressure , formation salinity, formation temperature, and tool stand off.

Uncorrected values of these curves could  have strong impact on formation evaluation and must be corrected before starting evaluation.

There are some pre-calculations before starting environmental correction,these pre-calculations are explained below:-

1.Determination of Geothermal Gradient:-Geothermal gradient is needed to calculate temperature at any particular depth in well bore. Temperature increases with depth, and the temperature gradient of a particular region depends upon its geologic or tectonic activity . As a rule of thumb higher tectonic activity leads to higher geothermal gradient. Geothermal gradients are commonly expressed in degrees Fahrenheit per 100 feet (0F/100′). In case if  geothermal gradient of an area is not known, then there are standard charts and  formula are available to get geothermal gradient.

Information available in log header may help in calculating geothermal gradient using equation below:

Where:

BHT = bottom hole temperature (from header)

TD = total depth (Depth-Logger from header)

Tms = mean surface temperature

Note that both the chart method and the formula method require a value for mean surface temperature (Tms). This refers to the average annual temperature of a region, and not the temperature at which resistivity measurements were made during the logging job

Determination  of Formation Temperature (Tf):

Once the geothermal gradient (gG) has been established, it is possible to determine the temperature for any particular depth. This is often referred to as formation temperature (Tf). Formation temperature can be calculated using the following equation.

Where:

Tms = mean surface temperature

D = depth at which temperature is desired

Determining Rmf from Rm

Correcting Resistivity for Temperature:

Resistivity decreases with increasing temperature, and hence  value of Rmf and  Rw determined at one depth needs to  be corrected for the changing formation temperature (Tf) through the wellbore. It is vital that formation water resistivity (Rw) be corrected for temperature,Failing to correct Rw to a changing temperature will result in erroneous values of water saturation (Sw). Therefore, it is possible to calculate a hydrocarbon-bearing zone as a wet zone or wet zone as hydrocarbon bearing zone  if the temperature correction is not applied.

Correction can be applied using the charts available with vendors or by applying Arps equation . Charts provided by vendors may also be used to determine the resistivity of a solution (such as Rm, Rmf, Rw, etc.) at a given temperature when the NaCl concentration of that solution is known, and vice versa. An example chart for conversion is given below:-

In the above chart temperature is given on top in Degree Celsius,Enter the chart from top at provided temperature and  on Y-axis enter  provided water resistivity.Point of intersection of both the entries is the  equivalent NACL content of water in parts per million.Go along the salinity line to required temperature at which you need to determine resistivity,resistivity on Y-axis in that zone is water resistivity at that required temperature.

Resistivity conversion can be done mathematically also using  Arps equation.Arps equation  for converting resistivity at given temperature to resistivity at another temperature is given below:-

R2 = R1 * (T1+k)/(T2+k)

where:

R2 = resistivity value corrected for temperature

R1 = resistivity value at known reference temperature (T1)

T1 = known reference temperature

T2 = temperature to which resistivity is to be corrected

k = temperature constant

k = 6.77 when temperature expressed in 0F