Effect of P– ρ– T behavior of muds on loss/gain during high-temperature deep-well drilling

Abstract

High-pressure and high-temperature deep-well drilling is associated with several unusual problems. A recent report on one such well in the North Sea indicated an apparent partial mud loss during circulation and a pit gain after stopping the circulation. When the well was shut-in on gain, a pressure buildup was observed at the surface. In the present study, an attempt is made to explain these observations quantitatively with the help of pressure–density–temperature ( P– ρ– T) behavior of mud and the temperature changes in a static mud column. An analytical method is developed to estimate the maximum pit gain when the linear temperature profile of wellbore changes from that immediately after circulation to geothermal. To estimate the maximum rise in pressure if the well is shut-in during these temperature changes, an analytical method is outlined. An approximate relationship between the gain and lapsed time after circulation is proposed. Estimated maximum gains for an example well are of the order of a few tens of barrels for both water- and oil-based muds. In general, estimated maximum gains in the case of oil-based muds are nearly twice that for water-based muds of identical densities. It is also shown that the maximum rise in surface pressure under shut-in condition which is of the order of a fraction of one ppg, is approximately equivalent to the decrease in static bottomhole pressure under open condition.