Abstract

This paper gives effective procedures to drill wells in a depleted reservoir. From the fact that depletion result in decreased horizontal stresses and fracture gradients, the working hypotheses is that a depleted reservoir should be drilled by keeping the Equivalent Circulating Density (ECD) as low as possible. This have been achieved by using a low mudweight and applying the downhole pressure sub actively; which imply focus on the combination of hole cleaning, lower pump rates, avoiding backreaming, and careful breaking of circulation before pumps are started.

The pressure sub and theoretical calculations show that bottom hole pressure give peak values under the following circumstances: Too quick start of pumps/breaking circulation, too high mudweight, too high mud viscosity, too high tripping velocity, poor hole cleaning, too high pump rates, and pressure pulses. Other factors which may have some influence on the bottom hole pressure are: Well stability problems leading to excess fill and loading of the well, and unstable mud. An additional method to reduce the ECD is to increase the fluid column diameter by installing liner instead of casing or by drilling holes with a larger diameter. Also, an exact calculation of the ECD is useful.

Three cases from three different wells are discussed, and theoretical calculations and observations are compared. We also observed that the fracture gradient of the borehole wall increased with time, which is useful if it is not possible to go below the minimum fracturation gradient.

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