This paper presents the design challenges and improvements made to cementing long lateral wells in the Duvernay formation. Low-rheology thermal cement slurries were designed and successfully pumped in more than 30 production jobs. These new designs were coupled with a lower-modeled pumping pressure at higher flow rates that were previously unattainable with conventional designs.
A long lateral production-casing design was used in the Duvernay formation to maximize hydrocarbon production and net present value (NPV). Generally, the lateral horizontal length of a well is increased to enhance wellbore contact with the reservoir. In many cases, the total depth is over 6,000 m, with a lateral section of approximately 3,000 m. Additionally, the typical wellbore diameter is reduced with the goal of decreasing the well cost and, consequently, the unit development cost (UDC).
Sufficient zonal isolation is essential for production casing. Designing low-rheology, thermal cement blends is complex and must meet all slurry-property requirements from both the regulatory board and those required to achieve a competent zonal isolation. These requirements include:
Adequate silica content to prevent thermal retrogression
500-psi compressive strength in 48 hours
Zero free fluid
Fluid loss of less than 100 cc/30 min
This paper discusses the primary design changes that led to the improved cement slurry designs, including:
Loss of circulation during cementing and displacement
Poor cement quality owing to channeling caused by low flow rates
Failure to bump the top dart for cemented sleeves
A computational fluid dynamics and finite-element simulator was used to evaluate the impact of conventional and improved designs. Improvements led to two new single-slurry designs. These single-slurry designs replaced the previous two slurry jobs (lead and tail), while maintaining the same, or higher, overall well hydrostatic pressure. The newly developed, field-trialed slurries with two different densities (1,675 or 1,750-kg/m3) and low rheology were designed to control the ECD below an acceptable value (fracture gradient = 21 kPa/m) at effective pump rates during both the placement and displacement of the cement slurries. The cement design also allowed for more cost-effective completions, further increasing the well NPV.