| Summary: | Over 8,000 wells in the Gulf of Mexico exhibit sustained casing pressure (SCP). SCP is defined as any measurable casing pressure that rebuilds after being bled down, attributable to cause(s) other than artificially applied pressures or temperature fluctuations in the well. The Minerals Management Service (MMS) regulations consider SCP hazardous and, in principle, require its elimination. In some cases the agency may allow continuing production at a well with SCP by granting a temporary departure permit.
The departure permits are based on diagnostic tests involving pressure bleed-down through a 0.5-inch needle valve followed by closing the valve and recording pressure buildup for 24 hours. Presently, analysis of testing data is mostly qualitative and limited to arbitrary criteria. This work provides theory, mathematical models and software needed for qualitative analysis of SCP tests.
SCP occurs due to the loss of wells external integrity causing gas inflow from a high-pressure formation into the wells annulus. Then, the gas migrates upward through a leaking cement sheath, percolates through the mud column and accumulates above the liquid level inside the gas cap. The study identified two scenarios of gas flow in the liquid column: rapid percolation through low-viscosity Newtonian fluid; and, slow ascendance of gas bubble swarms in viscous, non-Newtonian mud. The two scenarios have been mathematically modeled and theoretically studied.
The first model assumes rapid percolation and ignores gas entrainment in the liquid column. Simulation showed that early pressure buildup was controlled by mud compressibility, annular conductivity, and gas cap volume while formation pressure controlled the late pressure buildup. Mathematical simulations matched pressure buildups recorded in two wells, showing that the model had physical merit.
The second mathematical model fully describes gas migration by coupling the variable rate gas flow in cement with the two-phase flow in liquid column. The model was used to study typical patterns of bleed-down and buildup from SCP diagnostic tests. It showed that analysis of pressure bleed-down gives properties of gas-liquid mixture above the cement, while a sufficiently long pressure buildup may give values of the annular conductivity, the depth and pressure of the gas-source formation.
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