Weak formations encountered while drilling in central Alberta, Canada makeadequate cementing of casing difficult. These zones can cause lost returns andcement fall-back when trying to circulate slurry on a primary cement job. Thiscan result in having to perform one or more squeeze Jobs in an effort to fillan uncemented annulus, isolate zones, and protect casing. This added expensehas led operators to try various low density cementing systems. A system thathas proven to be effective where others often fail is foam cement. Not onlydoes this low density, high strength cement solve circulation problems, it alsohas properties that stop annular invasion form gas and water Zones.
A presentation of case histories will be given from wells having weakformations. Included will be previously attempted solutions along with examplesof foam cement's ability to successfully deal with the problem.
Areas of Alberta have formations which make it very difficult to achievesatisfactory cementing results with standard materials and job procedures. Weakformations have been the primary problem, causing lost circulation andincomplete annular fill in addition to this, large washed-out zones aresometimes created while drilling which make it difficult to calculate necessaryvolumes of cement- Conventional cementing practices, such as the use of waterextended low density cement slurries, were usually found to be inadequate tosolve the problems in these areas. Formations still broke down, resulting inloss of cement slurry to the formation and incomplete annular fill.
To solve this problem a cement slurry was needed which would have adequatecompressive strength at densities lower than what are typically obtained withstandard water extended cement slurries. Foam cement was chosen because itprovides an ultralow density cement system with good compressive strengthcharacteristics. It also has other properties which make it a good choice notonly for fighting lost circulation problems, but also for helping stop annulargas invasion.
A discussion of foam cement's physical properties, and a presentation ofsignificant job case histories are given to show how foam cement is uniquelysuited to the drilling and completion problems in Alberta. Details of problemareas are presented along with some past attempts which were made to obtain agood primary cement job. Successful applications will be presented, includingjob design, job procedure, and final results.
Foam cement is chosen for use in wells with lost circulation problems mainlydue to its high strength/low density characteristics. While standard waterextended slurries can only yield a minimum slurry density of 1260kg/m3, foam cement slurries can be generated with measurablecompressive strength at densities as low as 420 kg/m3. 1Along with the extended density range, foam cement will yield much highercompressive strength values than water extended slurries at the same givendensity. 2
To understand why this is so one must understand that the main controllingfactor for the maximum compressive strength of cement slurries is the water tocement ratio.