The Haynesville shale presents special challenges when designing cement systems. Extended-reach, horizontal wellbores create a high-pressure/high-temperature (HP/HT) environment not conducive to conventional cement slurries. Tight annular clearances and a narrow pore-pressure/fracture-gradient window have forced the industry to push the boundaries of cementing theory. Cement designs with a latex additive have improved rheological characteristics that yield lower equivalent circulating densities (ECDs) and reduce the frictional coefficient necessary for manageable surface pressures. The latex-based designs are thermally stable up to 400°F and provide excellent fluid-loss control, while still improving surface mixability. Most of the desired properties are observed when the cement is liquid; the set cement sheath can also provide corrosion resistance, annular bonding, and elasticity for well cycling. The development, testing, and case histories of latex-based cement slurries are discussed in this paper and compared to conventional cement designs for horizontal applications. By improving the physical properties of the cement while it is still in a fluid state, the cement can be properly placed, thus decreasing the potential for job failures. With the fluid viscosities enhanced, the pump rates can be optimized using hydraulic modeling to obtain increased mud-displacement efficiency. Laboratory testing has shown the latex additive to be effective from 1 to 2.5 gal per sack of cement. The latex additive enhances several cementing properties and reduces the need for additional fluid modifiers. By reducing the additives in the blend, the testing variability is decreased and repeatability is increased. Similar designs have been used in the past, but failure to maintain system stability created limitations. With new technology and theory, a heavyweight, thermally stable cement can be pumped during the most adverse well conditions.
Horizontal wells can present special challenges for cementing operations. Extended-reach lateral sections, unconventional mineralogy, and high-pressure/high-temperature (HP/HT) environments can cause failures during and after the cement job. The high clay content and ductility of these formations force operators to use oil-based mud (OBM) to drill the curve and lateral section. A narrow pore-pressure/fracture-gradient window and tight annular clearance create higher than usual equivalent circulating densities (ECDs) and can affect mud displacement efficiency and cement placement. As unconventional reservoirs become more prevalent, increased focus on the cementing process will be vital to the long-term success of the well. New theories and proper planning will help decrease the probability of an irregular cement job and increase the chances of having zonal isolation for the life of the well. In this paper, many aspects of the cement job design are discussed and recommendations are provided for horizontal cementing in unconventional reservoirs. New testing methods and additive composition help ensure the compatibility and thermal stability of the cementing fluids while creating more realistic wellbore hydraulic modeling and job optimization. Improved casing attachments and plug sets help increase displacement efficiency, while formation-specific cement designs increase effectiveness of the stimulation method. As multistage sliding-sleeves completion tools become more prevalent and increase efficiency, the cementing operation should align with the overall goal of the completion procedure. With the entire lateral section in one formation, it is not only critical to isolate the target formation from shallower zones but to also create a good annular seal between fracture stages. Without isolation, communication between stages can cause fracture treatments to migrate to unwanted areas. Modeling the expected temperatures, forces on the formation, displacement efficiency, and stress on the set cement sheath allows for a design that meets the expectations during the life cycle of the well. Complex temperature profiles create unfavorable design criteria, so accurate temperature determination is an important element to the cement design. With a renewed focus on the fundamentals of cementing, sufficient planning, and new technology, these unconventional reservoirs can become a manageable and sustainable resource for many years.
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