Underbalanced Drilling with Casing Evolution in the South Texas Vicksburg

Abstract

Abstract Drilling with production casing while underbalanced is improving the drilling performance dramatically in South Texas fields. This approach allows drilling of depleted and high pressure sands intermingled within one hole section, resulting in significantly less expensive well plans. Drilling cost reductions of 30% have been realized. Smaller reserve targets are viable, a key advantage in the mature South Texas Vicksburg play. Introduction Shell has developed and operated gas fields in South Texas for the past 50 years. These 10,000 ft - 16,000 ft high pressure, high temperature (HPHT) wells normally have initial shut in tubing pressures approaching 10,000 psi when virgin pressure sands are completed. The bottom hole temperatures range from 280–400°F. Most wells have multiple low permeability pay sands, which require massive hydraulic fracture treatments to produce economically. Each pay interval is fracture treated in a separate stage and the production from all the sands is commingled. Most current drilling activity is in and around mature fields where large volumes of gas have been produced. Severe reservoir pressure depletion intermingled with high pressure sands is often encountered. The presence and level of pressure depletion is difficult to predict due to complex geology, low permeability and production commingling. The underbalanced drilling with casing approach was first applied to a slim-hole reentry program that began in 1995. These reentries were either sidetracks to replace wells that failed due to casing damage or wells that were deepened to new objectives. The reentries normally cost half what a new well would allowing smaller reserve targets to be drilled economically. They were sidetracked out of the existing 5"or 5–1/2" casing and a new string of 2–7/8" casing was run and cemented. By 2000, remaining reentry candidates were difficult to drill with lost circulation and well control problems more common. The program was becoming uneconomic due to the inability to set liners in the small hole size. Drilling with casing1,2 while underbalanced was applied to resolve these problems and ten reentries have been drilled this way since 2001. The low permeability Vicksburg sands allow operations with a higher underbalance than would be possible in most other applications. The learnings from the reentry program have been transferred to the drilling of new South Texas wells. Drilling new wells with casing while underbalanced enables the casing size programs to be reduced, eliminates liners and reduces trouble cost resulting in cost savings of up to 30%. The majority of Shell's new South Texas wells utilize drilling with casing while underbalanced. Vicksburg Well Design Evolution A typical South Texas well plan prior to 1990 (figure 1a) required 13–3/8" surface casing and 9–5/8" protective casing, with the option for a 7–5/8" liner when needed. Wells were completed with 5" or 5–1/2" production casing, 2–7/8" tubing and a permanent packer. The sands were produced one at a time from bottom to top. A workover rig was used to plug back and recomplete to the next zone. Well lives needed to be 30 years or longer which was beyond what a typical Vicksburg well would last before failure. In 1990, the new completion philosophy was to commingle production from all sands (figure 1b). Commingling accelerated production and reduced the necessary well life, providing an opportunity to improve drilling economics. In 1994, a change to a 3–1/2" tubingless (or "cemented completion") design (figure 1c) was made. The 9–5/8" protective casing was designed to withstand loads under producing conditions in addition to being designed as protective casing. This design change resulted in a 15% decrease in well cost and the resulting monobore made multi-stage completion operations much simpler.