Losses, Flow Check and a Narrow Window in an 8½-Inch Section
An 8½-inch section of a 62-degree development well is being drilled at 3,380 m TVD (4,050 m MD). Pore pressure at the 9⅝-inch shoe at 2,980 m TVD is 1.62 SG equivalent, and the shoe was tested to 1.71 SG. The programmed mud weight is 1.66 SG. After a connection, the shakers show 7 m³ of loss over 40 minutes, followed by 3-5% gas cut and angular cavings. At the next connection, equivalent circulating density at the shoe reaches 1.745 SG and losses resume. The mud logger reports a 12-minute flow check with pumps off, gaining 1.8 m³ before the well stabilises. The directional driller wants to continue to section TD at 4,150 m MD because running casing now would cost four days. The rig has LCM, a spare liner hanger, and a 36-hour weather window before a tropical wave arrives at the location.
1. Interpret the flow check, losses and cavings. Is this ballooning, a kick, or wellbore instability, and what is your immediate sequence of actions?
Read the worked answer
The data are ambiguous, and the correct first response is to stop and resolve the ambiguity rather than label it. A 1.8 m³ gain that stabilises after 12 minutes, preceded by losses and followed by losses when ECD rises, is classic ballooning: the well breathes as induced fractures open and close. But gas cut and angular cavings mean a real influx and mechanical instability cannot be excluded, and a stabilised flow check does not prove there is no influx.
Immediate actions: stop drilling and control the annulus; shut in and record SIDPP and SICP after a proper stabilisation period; compare SIDPP with the shoe MAASP of (1.71 - 1.66) x 0.0981 x 2,980, or about 14.6 bar. If SIDPP is clearly positive and builds, treat it as a kick and kill. If SIDPP is zero and pressures bleed to zero, treat it as ballooning, but still stop losses. Circulate at reduced rate, record ECD, and inspect cavings. Do not drill ahead to TD while the well is taking losses and flowing. Cure the losses with LCM and reduce mud weight toward 1.64 SG if pore pressure allows, then set casing through the unstable interval before the weather window closes.
2. Assume a shut-in drillpipe pressure of 6 bar is confirmed. Calculate the kill mud weight and the shoe margin during circulation, and state the operational implication.
Read the worked answer
Kill mud weight = current MW + SIDPP / (0.0981 x TVD). That is 1.66 + 6 / (0.0981 x 3,380) = 1.66 + 0.018 = 1.678 SG.
At the shoe, hydrostatic with kill mud is 1.678 x 0.0981 x 2,980 = 490.5 bar. The shoe fracture pressure is 1.71 x 0.0981 x 2,980 = 499.9 bar. The static margin is 9.4 bar. While circulating, annular friction at the shoe adds roughly 0.03 SG equivalent, or about 8.8 bar, leaving only about 0.6 bar of margin. That is not a safe operating envelope; any surge, slug or pump-rate variation will break the shoe.
The implication is that the well cannot be killed at the programmed circulating rate with 1.678 SG mud. Options are to kill at a much lower rate and accept a long kill, to first cure losses and lower the base mud weight, or to set casing and drill the next section with a designed window. The decision should be recorded and communicated to the rig floor; the four days saved by drilling ahead are trivial compared with an underground blowout.