Low-Pressure Wellhead for High-Pressure Stimulation
Authors: Garvin Tom 
Published: December 2007

E&P Exchange
Maturity of U.S. producing wells and constrained economics dictate careful planning and strict attention to details to optimize workover and recompletion operations. One aspect of operations that can significantly reduce costs is planned preparation of a wellhead for high-pressure stimulation. Historically, there have been four basic approaches to solving old wellhead configuration problems to prepare for high-pressure stimulation down casing.
Deteriorated casing and/or inadequate casing cement may dictate the method chosen. An adequate wellhead without competent tubulars will not permit high-pressure stimulation down casing.
Purchase of a high-pressure wellhead with all components rated above treating pressures generally provides the least economic solution. Conventional wellhead seals in casing hangers and conventional valve designs do not adequately address pressure requirements during stimulation even if properly pressure-rated. Case histories adequately describe the weaknesses of using conventional production casing hanger seals during fracturing of a well. Several cases are documented where the seal was released, lodged against the casing hanger slips, and caused the casing to fall into the well below the casing head. Residues from stimulation (such as acid or proppant) often are found behind the seals and weaken the system so that long-term service is characterized by leaks and failures.
Conventional production gate valves of proper pressure rating exhibit their own special set of performance problems and are quite expensive. These valve scan become packed with fracturing proppant and fail to close. This condition often leads to broken shear pins or twisted stems, depending on the design of the gate/stem mechanism. Loose-fitting gates and seals have low-pressure sealing problems and permit fracturing fluids to enter the operating mechanism. Stimulation fluids corrode and damage as well as permit freeze-ups during high-rate gas production or in cold weather.
The wellhead isolation tool (WIT), or tree saver, was developed to circumvent these problems by retaining high-pressure stimulation fluids inside the tool during fracturing. This device isolates the wellhead from the harsh environment of the stimulation processes. The WIT must be lifted above the existing wellhead and "stabbed" into the treating string. Many of the old wells have "egged" or "burred" casing that makes installation of the WIT an expensive, major operation. The presence of the WIT mandrel restricts flow during stimulation and interferes with staging packers and perforating devices when multiple zones are to be stimulated.
Because the WIT must sustain and resist damage caused by corrosive and abrasive fluids, repair costs are often significant. Erosion of the WIT mandrel must be considered when using this tool. Close tolerances, check valves, and seals must be maintained in the WIT for high-pressure service. Case histories of WIT failures usually reflect exposure of the low-pressure wellhead to stimulation pressures and proppant invasion.
A lower-cost and often-used approach to configuring for high-pressure pumping is to rent a high-pressure valve and BOP. Fracturing connections and lubricators for packers or perforating equipment are exchanged between stages by breaking connections above the BOP. The rented high-pressure valve is subject to erosion, corrosion, and proppant packing as described previously. The BOP is not designed to hold pressure from above and is especially exposed to fluid erosion and proppant damage. Low initial costs are often overshadowed by expensive BOP repair charges when this approach to well preparation isused.
Some recent technical developments may provide a lower-cost solution. A casing spool is now available that features an external casing seal that cannot be pumped out. This seal is extremely reliable and has functioned on many extreme-pressure (15,000-psi) jobs without a single failure. Having the seal installed in a "frac spool" is very important because it provides convenient adaption from the low-pressure casing hanger to high-pressure control equipment with safety and reliability. Casing hanger seals are isolated from the harsh stimulation environment.
While the newly available spool discussed above provides a partial solution to the low-pressure-wellhead/high- pressure-stimulation problem, valve performance must also be considered. Recent improvements in gate-valve design have produced a valve that meets all required performance criteria for efficient stimulation operations. The new "frac valve" features low maintenance with close tolerances on gate/seal surfaces, low operating torque, high corrosion and erosion resistance, sealing in both directions, and generally high performance in functionality.
As shown in Fig. 1, a competitive solution for high-pressure operations maybe a compact wellhead configuration with the most recent advances used throughout. The components of this fracturing tree have been used on more than600 high-pressure, high-rate, high-proppant stimulation treatments without a single performance failure.
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