Estimated mud weight required to safely drill the well

I have an interesting question to share with you about how to estimate minimum mud weight required to safely TD the well.

The question is shown below.

7” casing shoe was set at 6,500’MD/5,000’ TVD. The geologist team in town expects 2 hydrocarbon reservoirs and information is listed below;

Formation sand A: Expected depth 5,500’ TVD, pressure gradient is 0.48 psi/ft.

Formation sand B: Expected depth 8,800’ TVD, pressure gradient is 0.49 psi/ft.

The planned TD is 9200’MD/9000’TVD and the drilling team requires 250 psi overbalance while drilling.

What is the mud weight required to drill the well with 250 psi overbalance?

First of all, let’s draw a simple diagram like this.

Knowledge required for this example:

Calculate formation pressure from pressure gradient

Convert pressure in to equivalent mud weight

 

Let’s take a look at each point.

 Formation sand A: 

Formation pressure of sand A = 0.48 x 5500 = 2,640 psi

With over balance of 250 psi, the hydrostatic pressure required is 2,890 (2640 + 250) psi.

Convert pressure into mud weight = 2890 ÷ (0.052 x 5500) = 10.2 ppg.

 Formation sand B: 

Formation pressure of sand B = 0.49 x 8800 = 4,312 psi

With over balance of 250 psi, the hydrostatic pressure required is 4,562 (4312 + 250) psi.

Convert pressure into mud weight = 4562 ÷ (0.052 x 8800) = 10.0 ppg

 

Please always use round up number for the mud weight.

For this case, you must use 10.2 ppg mud weight in order to drill to TD safely.

The point that I would like to emphasize on this topic is that you need to calculate how much pressure required for all sands. Please do not use only the sand that has the maximum pressure gradient.

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition

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10 Responses to Estimated mud weight required to safely drill the well

  1. eduardo says:

    hola soy eduardo de argentina trabajo en un perforador deceo aprender mas sobre mi trabajo y siemper me matan con las formulas en especiar cuando estan con herramientas en el pozo por ej: cuanto tarda un tapon en llegar a la punta del trepano y salir tengo que calcular los diametros interiores y los exteriores pero no tengo las formulas me las podrian enviar por correo si las tienen? atte gracias

  2. Obi Anselm says:

    How can you determine pore pressure at the depth about 10 to 15ft from the last casing shoe using d-exponent in a known blowout recorded zone assuming if the mud weight was just increased from 10.5 to 13.5 ppg. The last caing shoe was 9183, the casing was 9 5/8 inches.
    Mr. A.C. Obi

    • From the information that you give me, I can tell only that the formation pressure must be more than 13.5 ppg.

      If you give more information, I may have better ideas on this issue.

      Regards,
      Shyne

  3. Xavier says:

    Can someone tell me how to calculate Pore pressure and Fracture Pressure using only D’exponent realtime during drilling.

  4. Leonardo Santana says:

    Hi Obi,
    you can determine the pore pressure with you corrected d-exponent using Eaton approach. You need the normal pressure gradient of the area. Please take note that you will miscalculate the pore pressure using the d-exponent without correction.

    As Shyne said, the pressure in the kick zone has to more than 13.5 ppg because you already got a kick.

  5. sakai laut solero says:

    Hi,

    Is it normal if mudweight that I calculate keep decreasing?

    For example,

    Conductor = 9.8 ppg
    Surface = 9.8 ppg
    Intermediate = 8.9 ppg
    Production = 8.6 ppg

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