As the global demand for energy increases, operators are being forced to drill in increasingly challenging environments. In the northeastern United States, that often means exploring unconventional reservoirs such as the Devonian Shale or lower-pressured sandstone reservoirs such as the Big Injun or Berea sands (and drilling through the associated Sunbury Shale). Economic limitations in these areas often require operators to attempt single-stage production cement jobs with associated fills of 3,000 ft or more. Achieving zonal isolation above these low-pressure zones requires low-density cements that are pushing the limits of current cementing technology. If the cement is made light enough to circulate, but cannot withstand the stress of fracturing or other downhole conditions, the initial seal will only be temporary. If cement cannot be successfully circulated or fails to function shortly after placement, this can have a great impact on drilling costs usually designed with very tight economic constraints and can cause delays in production delivery because of required remedial work. Most of these wells require stimulation treatments to become economical producers. If zonal isolation is not initially achieved, remedial work may not be enough to effectively isolate the downhole formations, resulting in a complete loss of the well or zones within the well. Historically, lightweight slurries have been applied in these reservoirs in an attempt to successfully achieve zonal isolation in both single- and multiple-stage cement jobs; however, previous cement solutions were found to be lacking in one way or another. Problems ranged from price to poor bonding results, poor zonal isolation, or more typically designed cement fills not being attained.
As cementing solutions become more advanced in technology, the associated costs often increase. When choosing a cementing solution, the operator should balance price with the potential well productivity and the cost of using a cheaper solution. In this case study, we examine various examples where an effectively priced, high-quality, low-density cement was used to overcome numerous cement challenges, delivering zonal isolation that can allow effective post-cementing stimulation treatments that result in a positive return on investment (ROI).
Past experience cementing unconventional reservoirs such as the Devonian Shale or lower-pressured sandstone reservoirs such as the Big Injun or Berea sands (and drilling through the associated Sunbury Shale), has shown that lifting cement to the designed height has been difficult if not impossible. Typical wells in this area are produced through 4 ½-in. casing set between 3,500 and 5,000 ft. A 7-in. intermediate casing string is commonly set to a depth between 1,100 and 1,400 ft. To seal the annulus, the cement is designed to fill the entire openhole annulus plus 300 ft back into the 7-in. intermediate casing.
If the cement is too heavy, the resulting circulating pressure can exceed the fracture pressure in the weaker zones and cementing fluid can be lost to the formation. If cement is lost to the formation, the resulting top of cement (TOC) will be lower than planned. If cement is lost to one of the producing zones, typically naturally fractured shales, then production out of these could be reduced. If TOC is not high enough above the shallowest producing zone, remedial work may be required. Achieving effective zonal isolation by remedial cementing is never as effective as performing a good primary cement job.
Remedial work in the simplest case is achieved with a top-out job. Here, cement is dumped into the annulus or pumped into the annulus through tubing. Alternatively, the casing is perforated and either squeezed or circulated. Remedial cementing, required to isolate all producing intervals, not only costs money that should not have to be spent, but wastes precious rig time and manpower as well. If the cement density is reduced sufficiently, exceeding the fracture gradient can be avoided. When the circulating pressure remains below the fracture pressure during the entire cement job, the design TOC can be achieved and the wasted time and expense of top-out jobs can be eliminated.