Abstract
Cement bond quality is imperative for all phases in the life of a well. Achieving a superior cement bond can be an engineering challenge in areas with naturally fractured formations. Often there is a driving force to lose the cement slurry to the fractures or for the slurry to be contaminated by formation fluid flowing into the wellbore from the fractures. Failure to effectively isolate these zones can be extremely costly over the life of the well. An inadequate cement bond can result in interzonal gas and liquids migration within the annulus, negatively impacting the recovery of hydrocarbons and increasing downhole scaling and corrosion tendencies. In severe cases, subterranean fluids can flow along the annulus all the way to the surface of the well, resulting in environmental contamination and expensive remediation.
This paper addresses how adjustments to pre-flushes and spacers have improved cement bond quality in the Appalachian basin. When drilling in the Appalachian basin, encountering hard sandstone, fractured shale, and coals with low pore pressure gradients is very common. The fractured shales and coals may present problems when cementing the well. These problems include cement dehydration due to excessive fluid loss or formation "breakdown," in which whole cement slurry is lost to a created hydraulic fracture. When this situation is encountered, it can be difficult to achieve proper cement tops and cement bond quality can be poor.
Field data was collected from Gilmer County, West Virginia. In the study area, the wells were drilled to the Alexander formation with compressed air. Cement jobs were historically pumped with the following schedule: gelled water, cement slurry and displacement. A re-engineered pump schedule now includes a dual chemical-spacer system ahead of the cement slurry. The leading spacer was a mixed cleaner/surfactant solution, and the trailing spacer was a filtrate-reduction solution that also enhances cement adhesion to pipe and formation. Approximately five days following the completion of both cement jobs, cement bond logs (CBL) were run on the wells and the results of each were compared.
As evidenced by the comparative analysis of the CBLs, the adjustment in the pre-flush/spacer design resulted in a marked improvement in the quality of the cement bond. These results are documented in the paper. The resulting impact of the modified design on well completion and operating costs are also reviewed. Mitigation of any environmental risks are quantified.
