A major operator has used a foamed-cement system to cement lateral sections of four wells. The lateral sections all retained zonal isolation after being hydraulically fractured.

Throughout the production history of the Rangely field in Colorado, deviated sections of the wells have been difficult to seal on a long-term basis. Poor sealing in these well sections has required several squeeze jobs. Foamed cement in the lateral sections proved to be the solution.
The operator's drilling and completion program required cementing liners in four sidetrack wells deviated 50° to 70° from vertical. These jobs, which included cementing 4½-in. casing in 6½-in. wellbores, required near-complete drilling-mud displacement. Since two producing horizons were being targeted for zonal fracturing treatments, the cement had to provide good zonal isolation in the deviated wellbores.
Foamed cement
During pumping operations, foamed cement develops higher dynamic-flow shear stress than conventional cements, increasing its mud-displacement capabilities. In addition, foamed cement, which consists of a cement slurry injected with nitrogen gas, can be optimized for individual well conditions. Slurry density, determined by gas content or quality (the porosity in the set cement), depends on the pump rate of the base slurry, foamer and stabilizer injection rates, and nitrogen rate. Computer programs help optimize slurry design and predict job placement pressures.
The gas used to foam the cement continues to expand while the cement volume reduces, allowing slurry pressure to remain almost constant during the cement's transition period. Consequently, foamed cement can control gas migration and formation-fluid influx better than any other type of cement, and it resists temperature- and pressure-induced cement-sheath stresses.
Foamed cement is at least one order of magnitude more ductile than other cements. Testing has shown that foamed, 18- to 35-quality cement remains more ductile than other cements, allowing the cement sheath to withstand higher internal casing pressures. This feature allows the cement sheath to "give" as the well's casing expands, helping prevent cement-sheath cracking on a long-term basis. Above about 35%, the cement is too porous to provide isolation, and below about 18%, it becomes brittle.
Although flexible cements offer longer life in sour-well applications, foamed cement is less expensive. Because the Rangely field was mature, investment plans had to be compatible with the field's remaining productive life.
Rangely field
The Rangely field consists of 11 horizons with six sandstone producing horizons. The productive area consists of 19,153 acres, with a vertical closure at 950 ft (290 m).
When it was discovered in 1933, the Rangely field contained 1.9 billion bbl of oil. However, the field was not developed until 1944. A pilot waterflooding project was initiated in the mid-1950s. Later, 20-acre wells were infill drilled for improved waterflooding efficiency; then the operator began infill drilling several wells on a 10-acre well spacing.
A pilot project that involved sidetracking three wells was initiated to provide a better understanding of deviated wellbores in the Weber formation. The first well was a vertical sidetrack on an injection well. The second well, a producer, was deviated 70° from vertical. The third well, also a producer, was drilled as a 600-ft (183-m) horizontal lateral. All three wells were openhole completions. Four additional sidetrack wells were drilled at a deviation of 70° from vertical.
Job design, execution
The design of the foamed cementing job was based on available drilling fluids and cementing materials. A fluid sample was laboratory-tested for drilling-fluid mobility, which determines the shear rate or amount of force required for displacing the drilling fluid from the wellbore. Laboratory personnel generated rheology values for the foamed system and used cementing simulation software to predict job results. This software generated values for required nitrogen rates, equivalent circulating densities, wellhead pressure and fluid-pumping rate (based on the weight of the drilling fluid).
The cementing equipment used for this job included:
• truck-mounted recirculating cement mixer;
• cement batch mixer;
• cement bulk truck;
• nitrogen pumper;
• chemical truck; and
• water trucks.
A high-pressure line that incorporated a foam generator and a squeeze manifold was linked to the flow stream. Once the 4½-in. liner was run in the well, the rig pump circulated the well with drilling fluid, which was monitored for the lowest possible plastic viscosity and yield point. Controlling these properties helped ensure the best possible fluid movement in the small-diameter pipes used for this job. When the drilling fluid was in condition, the cementing job began.
The premium cement base was premixed in a batch mixer until a weight of 16.4 lb/gal was reached. The spacer was batch-mixed for the first well only. When it became clear this mixture was compatible with the wellbore conditions for this job, the spacer was pumped on the fly for the following three wells. After the spacer was pumped, the base cement slurry and nitrogen were pumped. A wiper plug designed to land with 1,000 psi over displacement pressure then was pumped. Next, the pressure was bled off, and operators checked for indications of flowback. Finally, the drillpipe was pulled above the estimated cement top, and the wellbore volume, plus 20 bbl of drilling fluid, was reversed out.
Results
After the four wells were cemented, two fracture treatments were performed on each well. During the procedure, the liner tops were exposed to fracture pressure, and none required remedial squeeze jobs. No subsequent failures have occurred. Based on the variance in individual zone fracturing pressures and post-fracture analysis, the operator determined zonal isolation was accomplished during the fracturing treatments.
Because of the foamed cement's poor acoustic properties, a cement bond log only indicated marginal zonal isolation. This phenomenon is not unusual, and alternative means of evaluating foamed cement have therefore been developed.