GRI and its contractor CTES LC of Conroe, Texas, have developed a straightforward technology to improve the quality of cement jobs in gas wells.
Hydraulic integrity of the seal between the casing and the formation is critical in gas wells. As cement surrounding a well casing hardens, it undergoes physical changes that can lead to fluid migration. This weakens the integrity of the hydraulic seal between the casing and the formation. One way to lessen this problem is to create movement among the cement particles during the setting process.
The construction industry routinely vibrates concrete slurries to improve the quality of the set concrete. Several methods related to this approach were attempted in the oil industry without success. GRI has investigated an alternative strategy to improve cement bonding and prevent fluid migration. The new technique is called cement pulsation (CP). The cement slurry is vibrated directly by sending cyclic, low-pressure (about 100 psi) pulses down the well annulus. Using relatively simple equipment at the surface, CP introduces pulses of water directly into the annulus above the slurry. The annulus serves as a wave guide to transmit the pressure pulses through the slurry. The cyclic pressure pulses keep the cement gel structure broken until it sets. This in turn maintains the hydrostatic pressure of the cement column and prevents the influx of formation fluid in the annulus.
The technique is carried out with skid-mounted equipment that must be rigged up and operational prior to pumping the cement. The CP skid can sit up to 100ft from the wellhead and must have a source of low-pressure water (50 to 70 psi). Once engaged, the system repeatedly pressurizes the annulus with water to about 100 psi and releases the pressure back to the supply tank. The amplitude and period of these pressure pulses are adjustable.
The annulus response to CP is shown in Figure 1. Increasing the pressure in the annulus with water compresses the cement column alongside the casing. Releasing the pressure allows the cement column to decompress. The cycle period represented at Dt gives an indication of the state of gelation in the cement.
Testing indicates the CP process could be adapted to deep wells with large volumes of fluid in the annulus. If it works as expected, it could be an economical solution to improving gas well cementing. Field experiments will help determine the CP's effective depth of influence and its range of application.
GRI and CTES are actively seeking test wells for applying CP. Gas migration problems are not a prerequisite, and oil wells would be acceptable. The companies also are looking for opportunities to test the technique during a plug and abandonment operation. The test wells can be anywhere in mainland United States, but the Gulf Coast region is preferred. Candidate wells should be at least 5,000ft measured depth, and directional wells are acceptable.
If you have a well or group of wells you would like to volunteer for a free trial of this technology, contact Scott Quigley at squigley@ctes.com, or call 1-936-521-2209.
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