Overpressure or Gas?
Learning objectives
- State the ambiguity: a sand can read slow on velocity because it is overpressured or because it holds gas, and the two verdicts could not be more different
- Anchor the numbers on the soft sand at porosity 0.25: an overpressured brine sand (effective pressure 10 instead of 20 MPa) drops Vp 4.9 percent while its Vp/Vs rises to 2.03
- Contrast the gas case: gas at normal pressure drops Vp 18.4 percent while its Vp/Vs crashes to 1.48
- Read the separator: unloading softens the whole frame so both velocities fall and the ratio rises, while gas softens only the bulk modulus so the ratio collapses
Two Ways to Be Slow
A slow sand on a seismic velocity is one of the most important anomalies an interpreter can meet, and one of the most dangerous, because two completely different rocks produce it. The first is overpressure: pore fluid trapped and unable to drain, carrying part of the overburden, so the effective pressure on the frame is low and the grain contacts stay soft. The second is gas: light hydrocarbon in the pores softening the fluid. Both read slow. One is a drilling hazard that can kick a well; the other is a prospect. Reading velocity alone, you cannot tell which one you are looking at, and guessing wrong is expensive in both directions.
Put numbers on the two. Start from the standard soft brine sand at porosity 0.25 and a normal effective pressure of 20 MPa, running at km/s with a velocity ratio of 1.91. Now make it slow two different ways.
The Numbers, Side by Side
Overpressure. Drop the effective pressure to 10 MPa, as ten megapascals of overpressure would, holding the brine in place. The Hertz-Mindlin frame softens with the pressure law of Part 5.2, and the velocity falls to 2.743 km/s, down 4.9 percent. But look at the ratio: it rises to 2.03. Unloading softens the frame, and softening the frame lowers the shear modulus along with the bulk modulus, so drops even harder than and the ratio climbs.
Gas. Now leave the pressure at its normal 20 MPa and fill the pores with gas instead. The velocity falls to 2.353 km/s, down 18.4 percent, a much larger dimming. And the ratio crashes to 1.48. Gas softens only the pore fluid, so it lowers the bulk modulus and leaves the shear modulus untouched, which drives down while if anything rises on the lighter density. The ratio falls off a cliff.
The Velocity Ratio Splits Them
The two cases are twins on alone, both slow, but opposites on . That is the whole diagnosis. Unloading is a frame effect: it moves both velocities the same way and nudges the ratio up (2.03). Gas is a fluid effect: it moves the bulk modulus alone and sends the ratio down (1.48). The velocity ratio, carried by shear information from converted waves or from AVO, is the single measurement that separates a pressure hazard from a hydrocarbon prospect, the same shear-versus-fluid logic that ran through the 4D pressure-or-fluid discussion of Part 8, now pointed at a drilling decision rather than a time-lapse one. Get the shear, and the ambiguity dissolves. The final section of this part carries the same detect-versus-quantify tension into a monitoring problem, watching injected CO2, where the plume is easy to find and its saturation is hard to pin down.
References
- Bowers, G. L. (1995). Pore pressure estimation from velocity data: Accounting for overpressure mechanisms besides undercompaction. SPE Drilling & Completion, 10(2), 89-95.
- Mavko, G., Mukerji, T., & Dvorkin, J. (2009). The Rock Physics Handbook (2nd ed.). Cambridge University Press.