Drilling massive salt sections in the deepwater Gulf of Mexico is becoming a frequent occurrence. The abundance of salt and the ability of seismic to image under salt have made drilling massive salt sections common place. It is estimated that over a hundred wells in the deepwater Gulf of Mexico have penetrated salt at both shallow and deeper well depths. As the industry gains experience drilling deepwater, shallow allochthonous salt bodies, several advantages of drilling through, rather than around, salt have emerged. Fracture gradients in shallow salt intervals have proven to be much higher than in non-salt sediments at a comparable depth. As a result of the increased fracture pressure, massive salt sections have been used to extend casing points and eliminate casing strings, resulting in greatly reduced well costs through reduced rig time, well tangibles, underreaming, cement volumes, and mud volumes. Many operators are now choosing to drill massive, shallow salt sections to take advantage of these benefits. In some cases these cost savings have the potential of making marginal deepwater reserves economically feasible to develop and produce. A reliable estimate of the fracture gradient in shallow, massive salt sections is needed to design casing strings and plan mud weights for safe and cost effective wells.
Upon exiting a shallow salt zone, often the formation pore pressure and fracture pressure are very close. Typically this condition results in drilling problems including lost returns, well control events, etc. The proper choice of a mud weight to exit a shallow, massive salt section can be a critical factor for both well integrity and cost.
This paper describes a method to estimate the fracture gradient for a salt formation below a casing shoe. The method is based on experience gained while drilling wells which penetrated substantial salt sections. Also presented are guidelines that can help in selecting a mud weight when exiting the bottom of a massive shallow salt section.