Deep water well design and drilling is challenging due to the narrow separation between pore pressures and fracture pressure. Comparing the fundamentals of deep water well design to shallow water / onshore well design is dramatic. Once the pore pressures and fracture pressures have been modeled, the shallow water / onshore well design begin with choosing the desired casing size at total depth. Establishing the desired casing size at total depth for shallow water / onshore wells for the most part determines the casing points from total depth to surface. This is often referred to as the "Bottom-Up" well design.
On the contrary, deep water well design begins with the selection of the 20″ conductor depth. Once this depth has been selected, it determines subsequent casing points. This is commonly referred at the "Top-Down" well design. The deeper this string is set, the deeper the subsequent casing strings can be set. Extending this depth can eliminate entire casing strings. The challenge becomes to safely set the 20″ conductor as deep as possible.
A major deepwater drilling program has successfully drilled more than 19 wells in deep water in the Krishna-Godavari Basin. The setting depth of the 20″ casing has been extended using 3-D seismic, seal capacity analysis, shallow pore pressure prediction, shallow fracture pressure prediction and "pump and dump".
Once the 20″ conductor has been set, which is predominately in the very plastic Godavari clay, determining the maximum mud weight is also a challenge. The proper analysis will allow the next casing string to set deeper.
Although deep water well design in the Krishna-Godavari Basin should not be considered routine, the appropriate pre-drill analysis has significantly reduced the risks and costs of deep water drilling.