Low-Pressure Wellhead for High-Pressure Stimulation
- Tom Garvin (Total Tool Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- May 1994
- Document Type
- Journal Paper
- 383 - 383
- 1994. Society of Petroleum Engineers
- 3 Production and Well Operations, 4.2.3 Materials and Corrosion, 5.3.4 Integration of geomechanics in models, 4.1.2 Separation and Treating, 1.7 Pressure Management, 1.14 Casing and Cementing, 2.2.2 Perforating, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2.1 Phase Behavior and PVT Measurements
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Maturity of U.S. producing wells and constrained economics dictate carefulplanning and strict attention to details to optimize workover and recompletionoperations. One aspect of operations that can significantly reduce costs isplanned preparation of a wellhead for high-pressure stimulation. Historically,there have been four basic approaches to solving old wellhead configurationproblems to prepare for high-pressure stimulation down casing.
Deteriorated casing and/or inadequate casing cement may dictate the methodchosen. An adequate wellhead without competent tubulars will not permithigh-pressure stimulation down casing.
Purchase of a high-pressure wellhead with all components rated abovetreating pressures generally provides the least economic solution. Conventionalwellhead seals in casing hangers and conventional valve designs do notadequately address pressure require ments during stimulation even if properlypressure-rated. Case histories adequately describe the weaknesses of usingconventional production casing hanger seals during fracturing of a well.Several cases are documented where the seal was released, lodged against thecasing hanger slips, and caused the casing to fall into the well below thecasing head. Residues from stimulation (such as acid or proppant) often arefound behind the seals and weaken the system so that long-term service ischaracterized by leaks and failures.
Conventional production gate valves of proper pressure rating exhibit theirown special set of performance problems and are quite expensive. These valvescan become packed with fracturing proppant and fail to close. This conditionoften leads to broken shear pins or twisted stems, depending on the design ofthe gate/stem mechanism. Loose-fitting gates and seals have low-pressuresealing problems and permit fracturing fluids to enter the operating mechanism.Stimulation fluids corrode and damage as well as permit freeze-ups duringhigh-rate gas production or in cold weather.
The wellhead isolation tool (WIT), or tree saver, was developed tocircumvent these problems by retaining high-pressure stimulation fluids insidethe tool during fracturing. This device isolates the wellhead from the harshenvironment of the stimulation processes. The WIT must be lifted above theexisting wellhead and "stabbed" into the treating string. Many of theold wells have "egged" or "burred" casing that makesinstallation of the WIT an expensive, major operation. The presence of the WITmandrel restricts flow during stimulation and interferes with staging packersand perforating devices when multiple zones are to be stimulated.
Because the WIT must sustain and resist damage caused by corrosive andabrasive fluids, repair costs are often significant. Erosion of the WIT mandrelmust be considered when using this tool. Close tolerances, check valves, andseals must be maintained in the WIT for high-pressure service. Case historiesof WIT failures usually reflect exposure of the low-pressure wellhead tostimulation pressures and proppant invasion.
A lower-cost and often-used approach to configuring for high-pressurepumping is to rent a high-pressure valve and BOP. Fracturing connections andlubricators for packers or perforating equipment are exchanged between stagesby breaking connections above the BOP. The rented high-pressure valve issubject to erosion, corrosion, and proppant packing as described previously.The BOP is not designed to hold pressure from above and is especially exposedto fluid erosion and proppant damage. Low initial costs are often overshadowedby expensive BOP repair charges when this approach to well preparation isused.
Some recent technical developments may provide a lower-cost solution. Acasing spool is now available that features an external casing seal that cannotbe pumped out. This seal is extremely reliable and has functioned on manyextreme-pressure (15,000-psi) jobs without a single failure. Having the sealinstalled in a "frac spool" is very important because it providesconvenient adaption from the low-pressure casing hanger to high-pressurecontrol equipment with safety and reliability. Casing hanger seals are isolatedfrom the harsh stimulation environment.
While the newly available spool discussed above provides a partial solutionto the low-pressure-wellhead/high- pressure-stimulation problem, valveperformance must also be considered. Recent improvements in gate-valve designhave produced a valve that meets all required performance criteria forefficient stimulation operations. The new "frac valve" features lowmaintenance with close tolerances on gate/seal surfaces, low operating torque,high corrosion and erosion resistance, sealing in both directions, andgenerally 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 usedthroughout. The components of this fracturing tree have been used on more than600 high-pressure, high-rate, high-proppant stimulation treatments without asingle performance failure.
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