|
|
Technical Paper: Real-Time Formation Integrity Tests Using Downhole Data
Paper 59123
Abstract
Because of shallow water flow concerns in deepwater wells with narrow stability margins, pressure differences of a few tenths of a lbm/gal can make the difference between straightforward drilling and the need for an extra string of casing to protect shallow intervals. Accurate leak off tests (LOTs) / formation integrity tests (FITs) are essential to enable efficient management of the equivalent circulating density (ECD) within the safe pressure window.
The rate of downhole pressure build-up is traditionally estimated from the standpipe pressure, but can now be monitored directly with a downhole annular pressure measurement. We discuss two methods that can be applied to give a real-time LOT/FIT profile.
The first method is a real-time FIT from a deepwater well in the Gulf of Mexico. The low flowrates used during an FIT/LOT preclude the use of traditional MWD systems to transmit data to the surface. However, with the use of a wireline coupling, downhole pressure was transmitted to surface in real-time, allowing the operator to simultaneously view the surface and downhole pressure build-up, and evaluate both the formation integrity and mud compressibility.
The second method uses both surface and downhole data from a North Sea well. We describe an algorithm and procedure whereby two downhole pressure points are used to calibrate the hydrostatic and compressibility offsets between surface and downhole. A complete LOT profile is then created at surface as soon as conventional pumping resumes.
Advantages of the real-time annular pressure measurements are:
- Instantaneous data to correctly stop the test, thus avoiding unnecessary overpressuring of the formation.
- Removing the uncertainties in the compressibility of the drilling fluid; this is particularly true of synthetic muds.
- Avoiding the need for additional circulation(s) to condition the mud.
- Increasing the accuracy of the LOT/FIT, allowing more precise casing point determination.
This page contains additional information that requires a premium content account to view. Sign in to view the rest.
|
|
|
