Estimating Fracture Gradient in Gulf of Mexico Deepwater, Shallow, Massive Salt Sections

Abstract

Abstract 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. Introduction The U.S. gulf coast basin contains the largest known deposits of salt in the world. It has been estimated that 80 percent of the proven gulf basin reserves are likely related to salt structures.1 Drilling through massive salt sections has been achieved along the gulf coast since the 1940s and is common place on land, on the Gulf of Mexico (GOM) shelf, and in the Gulf of Mexico deepwater today. In the mid 1980s the industry began to drill in the deepwater Gulf of Mexico. Salt walls, diapers, and allochthonous salt bodies are common in the deeper water depths of the GOM as shown in Fig. 1.2 In many cases shallow salt sheets cover entire GOM block areas. In the early 1990s deepwater GOM shallow salt sheets began to be drilled in order to reach deeper geologic objectives. New seismic acquisition techniques and depth migration processing advancements resulted in improved imaging of subsalt clastic formations. In 1990 Exxon spudded the Mississippi Canyon Block 211 No. 1 well in 4,352 ft of water depth with the objective of exploring subsalt formations. The well was the industry's first find of significant subsalt, deepwater hydrocarbons. An apprasial well was drilled in 1997 and the discovery was subsequently developed with three subsea completions. In 1995 Texaco discovered a subsalt field at Mississippi Canyon Block 292 in 3,400 ft of water and developed this field with three wells in 1999. 3 Today operators routinely drill shallow salt layers to explore and produce deeper objectives. Drilling shallow salt intervals has become a preferred option because two issues have been overcome in recent years. First, studies have concluded that salt loading on well casings due to salt creep is manageable for salt formations encountered along the US gulf coast.4 Second, operational problems experienced while drilling salt sections in early wells have been overcome. Shallow GOM salt is typically very hard which leads to low drilling penetration rates, directional control problems, and excessive vibrations. Early experience with drilling shallow salt zones resulted in average penetration rates in salt of only 10 to 20 ft/hr. New tools and operational practices have permitted drilling salt today at over 50 ft/hr with minimal directional problems and drillstring vibrations.5,6