Reading Shut-In Pressures and Rising SIDPP
A kick is taken while drilling at 3,420 m TVD with 1.32 SG mud. The pumps were running at 480 gpm with 3,150 psi standpipe pressure when the pit gained 15 bbl. The driller shut in and the pressures were read: SIDPP 380 psi, SICP 520 psi, stabilised for about ten minutes. The derrickman then reports the SIDPP creeping up to 420 psi and the SICP correspondingly higher, with no change on the pit level. The toolpusher wants to start circulating immediately at the original 480 gpm to bring the gas out, arguing that waiting wastes time and that the pressure rise proves the well is getting worse.
1. What do the SIDPP and SICP values tell you, and why is it normal for SICP to be the higher of the two?
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SIDPP is the difference between formation pressure and the hydrostatic pressure of the mud column inside the drill pipe. With a known mud weight and depth, it is the input used to calculate the kill mud weight. SICP is measured on the annulus, which contains the same mud plus the influx.
SICP is normally higher because the influx is lighter than the mud it replaced. A column of gas or gas-cut mud exerts less pressure per metre than drilling mud, so more surface pressure is needed to hold the same bottom-hole pressure. A gas influx gives a large SICP-SIDPP difference; a small water influx gives a small one. A difference of 140 psi is entirely consistent with a gas influx of this size.
Neither number on its own is a problem. What matters is that they are read, recorded with the time and the pit gain, and kept unchanged until the responsible person decides how to proceed.
2. What is the cause of the slowly rising SIDPP with a static pit, and what does it mean for the toolpusher's plan?
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A rising SIDPP and SICP together, with no gain in pit volume, is the classic signature of gas migration. The influx is moving up the annulus under its own buoyancy, and because the well is closed and the gas cannot expand freely, it carries pressure with it. The volume at surface does not change, but the pressure does - the gas is not being compressed into a smaller space, the whole trapped system is being squeezed.
If nothing is done, the pressure will keep climbing and will eventually be applied to the weakest point in the well, commonly the shoe. That is what makes 'wait and see' unacceptable.
The toolpusher is right that the pressure rise is a warning, but wrong to read it as a reason to circulate immediately at the original rate without a plan. It is a reason to act according to the approved procedure - bleed pressure in a controlled way as directed, or start circulating on the correct kill schedule - and to keep recording pressure, pit volume and time throughout.
3. The toolpusher argues the pressure rise means the well is deteriorating, so circulating at the original rate is safest. How would you assess that reasoning?
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The conclusion may be right but the reasoning is incomplete, and the specific proposal carries risk.
Circulating at the original 480 gpm will bring the gas to surface faster, but it also applies additional circulating pressure to the open hole and, importantly, the gas will expand as it rises. The choke must be adjusted to hold the correct casing pressure while the gas expands; if the choke is left alone, the expanding gas takes pressure off the bottom and invites a second influx. If the choke is over-adjusted, the formation is fractured. That is a job for the person trained in the approved method, working to a calculated schedule.
The pressure rise is not evidence that the well is 'getting worse' in the sense of more influx - the pit is static, so no new volume has entered. It is evidence of migration. The correct behaviour at Level 1 is to report, record and follow instructions, not to invent a circulation plan under time pressure.