Many wells in the Cana-Woodford shale suffer from chronic sustained casing pressure (SCP). With more stringent SCP regulations on the horizon, operators need a way to prevent SCP effectively during primary cementing because remediation can prove costly or detrimental to business. At the start of a recent project virtually all cement bond logs showed poor coverage and a lack of zonal isolation. Due to the poor cement sheath bonding, SCP was occasionally observed before fracturing and almost always occurred after fracturing. This paper discusses a method that has resulted in a high success rate in preventing SCP - where previously, failure was common.
An integrated approach between multiple service company operations and operator was used to address the cement bonding issues and the long-term integrity of the cement sheath. The ultimate objective was to not have any SCP on any annulus of the well for the lifetime of the well. Initially, high-resolution ultrasonic bond logs were run to evaluate slurry placement and methodologies. Placement of the slurry around the casing was then optimized using laboratory testing and modeling software based on lessons learned from log evaluation. The success of the engineered approach was evaluated with ultrasonic logs utilizing flexural attenuation, thereby validating simulations. Finally, the long-term set cement properties were evaluated through the use of stress analysis software, which simulated stimulation treatments and their effect upon the mechanical properties testing.
Cement placement was optimized with an oil-based mud (OBM) recovery in mind. Using simulation software, centralization of the production casing was evaluated, and an optimized frictional pressure hierarchy then designed. A cement bond log was run to evaluate the proposed solution. The results proved that zonal isolation was achieved throughout the curve and into the casing-to-casing section. The logs showed excellent coverage in this well relative to its peers and the best results to date. Continued monitoring showed no casing pressure after stimulation and 1.5 years of production. The operator's other seven wells in this section, which did not have the engineered approach applied, showed SCP immediately after stimulation.
The engineered placement method ensured complete cement coverage around casing through an optimized frictional pressure hierarchy. This multilayered approach using mechanically optimized slurries with different mechanisms of action, including self-healing, provided a comprehensive cementing portfolio that contained layers of contingency. To date, the method has a high success rate in the prevention of SCP in the Cana-Woodford shale. Unconventional gas/liquid plays have changed our industry. The methodology outlined in this paper has been successful, and the lessons learned can be applied to many other unconventional gas/liquid plays that include fracturing stimulation for development.