Well Logging and Formation EvaluationPAB2084 & PCB 2044

SP Log Rw calculation

Dr. Ahmed Salim

Learning OutcomeAt the end of the practice, students should be able to

Determine Rw from SP curve by: Classical methods or Using Silva-Bassiouni method

Rw from SP (1) Rw from the SP-Classical Method1) Determine formation temperature Tf.2) Find Rmf at formation temperature.3) Correct SP to SSP using bed thickness and Ri/Rm.4) Compute the Rmf/ Rwe ratio from the SSP.5) Compute Rwe from the above ratio6) Convert Rwe at formation temperature to an Rw value.

Example Determine water resistivity (Rw) value for the formation at 7446 ft using the SP log shown below

From the log trackSP = -40 at formation depth 7446Bed Thickness = 8 ft (7442 to 7450 ft)Resistivity short normal (Ri)= 28 .m

Formation Temperature TfFormation depth 7446

Arps formula and a chart for determining resistivity change due to temperature.

Rm and Rmf at Formation TemperatureArps formula

SP Chart for Bed Thickness CorrectionInput: Ri/RmRm value (from Heading) at formation temperature.Ri from resistivity log.Output: SP to SSP correction factor (CF)SSP= SP*CF

SSP = -40*1.3=-52 mv

The graphic Solution of the SP EquationInput: SSP and TemperatureOutput: Rmf / Rwe ratio

Rmf/RweInput: SSP= -52 and Temperature = 130Output: Rmf / Rwe = 5Rwe = Rmf / 5

Rw from RweInput: Rwe and TemperatureOutput: Rw

Rw from RweInput: Rwe = 0.106 and Temperature = 130Output: Rw = 0.11

Second Method in step3 use Rmfe 3) Convert Rmf at formation temperature to an Rmfe valueFollowing figure makes the conversion of Rmf to Rmfe. It is also used in step (6), where Rwe is converted to Rw. The figure is constructed for "average" formation waters containing some magnesium and calcium cations in addition to sodium chloride. Mud filtrates may not contain the same ionic mixture and when the mud is fresh (Rmf > O.1 m at 75F). It is normal to convert Rmf to Rmfe by using: Rmfe=0.85 Rmf

Rw from SP Classical Method 3) Convert Rmf at formation temperature to an Rmfe value

Rw from SP Classical Method 4) Compute the Rmfe/ Rwe ratio from the SPThe SP equation may be used to compute the Rmfe/ Rwe ratio

Kc = 61 + 0.133 ToFKc = 65 + 0.24 ToC5) Compute RweRwe is found by dividing the known value of Rmfe by the Rmfe/ Rwe ratio found from the SP.6) Convert Rwe at formation temperature to an Rw valueThe conversion of Rwe to Rw is a simple matter of reversing the process used to convert Rmf to Rmfe in step (3), using Figure in step (3).

(3) Rw from the SP-Silva-Bassiouni MethodA recent study has found that a simple method is available and theoretically justified. The entire process is reduced to a single chart, shown in the right Figure.

Steps: SP-Silva-Bassiouni Method

Example 2Determine Rw for Zone A of the attached figure which is a water bearing sand.Given: Ts= 180 o FRmf= 0.64 at 78 o F

Ans. 0.028 .m at 180 o F

Figure for Example 2

Thank You