Abnormal Pressure from Anticline Gas Cap

I got a question about how an anticline gas cap can create the abnormal pressure. The anticline with gas cap can be the potential high pressurized zone. Because of reservoir connectivity between fluid underneath gas and gas reservoir, it can generate abnormal pressure. This example below demonstrates you how this situation could happen.

Well#1 was already drilled into oil reservoir at 6500’MD/6000’ TVD and its formation pressure is 0.52 psi/ft.

The second well, well#2, is planned to drill into the gas cap, which has reservoir connectivity to the well#1, at 5400’MD/4800’ TVD and a geologist estimates gas cap thickness of 950 ft and pressure gradient is 0.1 psi/ft.

What is minimum mud weight to be able to successfully drill well#2?

Firstly, I would like to share which basic concepts I will use for this case .

Hydrostatic Pressure
Pressure gradient

Convert pressure to equivalent mud weight
Equivalent Mud Weight

You need to determine formation pressure of well#1.

Well#1: Well depth: 6,500’MD/6,000’TVD and Pressure gradient: 0.52 psi/ft

Formation pressure of well#1 = 0.52 x 6000 = 3120 psi

Convert pressure to equivalent mud weight = 3120 ÷ (0.052 x 6000) = 10.0 ppg

You may think that 10.0 ppg should be good mud weight to drill well#2. –> Wrong answer.

Let’s take a look at the well#2.

Abnormal Pressure - Anticline Gas Cap 2

Pressure at the bottom of gas cap = formation pressure of well 1 @ 6000’TVD – hydrostatic pressure of reservoir fluid

Pressure at the bottom of gas cap = 3120 psi – 0.52 x (6000 – 5750) = 2990 psi

Determine the reservoir pressure at the top of gas cap:

Pressure at the top of gas cap = Pressure at the bottom of gas cap – hydrostatic of gas gradient

Pressure at the top of gas cap = 2990 – (0.1×950) = 2895 psi

Since we know the formation pressure, we can calculate to equivalent mud weight (EMW).

EMW of well#2 = 2895 ÷ (0.052 x 4800) = 11.6 ppg

You need mud weight at least 11.6 ppg to drill well#2 successfully.

Conclusion: This case clearly shows you that the anticline gas cap can be high pressurized zone and you may not be able to drill the well with the same mud weight as the adjacent drilled into the same reservoir.

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

 Well Control Books

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7 Responses to Abnormal Pressure from Anticline Gas Cap

  1. Agha Ghulam Nabi says:

    Since, I am member of this blog and have been receiving mails intermittently but all these mails have really added alot in my knowledge.

    This blog shall be continued and for knowledge sharing purpose.

  2. Adriano says:

    why can’t we say directly that

    the pressure at the top of the gas cap= 0.52*4800=2496 psi ? please explain the reason

  3. DR. MOHAN DOSHI says:

    This is a great site and all drilling engineers and mud engineers must see it. I like the formulas and the calculations, and I have found them very useful in my day to day work.

    Thanks so much – continue the good show.

  4. Fatin Asheelo says:

    Pressure at the bottom of gas cap = 3120 psi – 0.52 x (6000 – 5750) = 2990 psi

    I have problem in understanding where/how does the value 5750 come from ?

    thank you in advance.

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