Cementing of casings over weak formations has always been a challenge, especially in the case of multiple zone completions where the zones are spread over large intervals.
Devonian Shale wells in Appalachia model similar difficulties. Frac gradients as low as 0.30 psi/ft are common in these naturally fractured zones. Cement fill-ups of up to 3500 feet are required to allow for up to 7 stage completions. Commonly, these wells have been cemented using two stage cementing or by planning for a squeeze after the primary cement job.
A lightweight cement blend was introduced in 1997 to achieve the entire fill-up with the primary cement job only. This blend provides the flexibility of density alterations from 11.0 to 14.0 lbs/gal (ppg) and is extremely easy to mix and pump. Another advantage of this blend is its ability to achieve gellation very quickly in spite of its high water content. A silicate component in the slurry efficiently ties up the excess water.
The job design utilized to place a long cement column across the weak and naturally fractured shale has proven to be very critical. A minimal amount of water pumped ahead ensures lowest possible hydrostatic pressure for the bottom most shale zone.
A single stage cement job offers several advantages. Cost savings in excess of $ 10,000 per well can be realized if the desired cement fill can be successfully obtained with a single stage job. Elimination of the stage tool and the ability to run frac baffles are the major contributors to the cost savings.
Through February 1999, this lightweight slurry was used to cement 65 wells. The available data indicates that sufficient cement fill was obtained to cover all the desired zones in more than 60% of the wells. The success rate increases to 78% if certain avoidable factors are eliminated. Cement fill-ups of as much as 4300 feet have been obtained with this slurry.