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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.**

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: ** **Formulas and Calculations for Drilling, Production and Workover, Second Edition**

** ****Well Control Books**

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Thanks for your kind comment.

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

You cannot say that because the gas has the reservoir pressure at the bottom. This pressure affects gas cap pressure.

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.

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.

5750 is the bottom of the gas cap in the well No2 which must be taken into account.