This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 174525, “Bridging the Gap: An Integrated Approach to Solving Sustained Casing Pressure in the Cana-Woodford Shale,” by G. Landry, R.D. Welty, SPE, M. Thomas, M.L. Vaughan, and D. Tatum, Schlumberger, prepared for the 2015 SPE Well Integrity Symposium, Galveston, Texas, USA, 2–3 June. The paper has not been peer reviewed.
Many wells in the Cana-Woodford shale suffer from chronic sustained casing pressure (SCP) because of poor cement-sheath bonding. Using simulation software, centralization of the production casing was evaluated, and an optimized frictional pressure hierarchy was then designed. The engineered placement method ensured complete cement coverage around the 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.
One of the major operators in the Cana- Woodford field experienced poor bond logs and SCP both before and after stimulation, with the greatest number of issues appearing after stimulation. SCP can be defined as the presence of pressure in the annulus of nonstructural strings. The presence of SCP is an indication of a path of flow of hydrocarbons to the surface. SCP is typically caused by poorly placed cement or by not taking into account all factors in a well, such as stimulation conditions. The cement is placed in the annulus for many reasons, but one of the more significant is to provide zonal isolation. The zonal isolation is needed for stage-to- stage isolation in fracturing; isolation of the fracture treatment from shallower formations, which are typically in a lower-stress environment; and for prevention of hydrocarbon flow to surface.
This paper presents the methodology and specific considerations that were taken into account to integrate all known information and causal learnings from logging to design a system to provide zonal isolation in the challenging Cana- Woodford shale.
Field and Design Evaluation
Borehole Geometry. Drilling practices can have a significant effect on cement-placement results. Several factors must be examined when evaluating a wellbore (e.g., washouts, hole size, dogleg severity, azimuthal gradients, curve build rates). All these factors can affect the ability to remove cuttings properly, to place centralizers effectively, and to remove mud efficiently. The operator placed emphasis on controlling these factors to help improve the likelihood of success. An oil-based mud (OBM) was used throughout the openhole section to maintain wellbore stability in the shale formation.
Centralizer Program. The use of centralizers in horizontal wells is not a common practice because of a perception that they might impede the ability of casing to reach the bottom. With improvements in casing hardware technology and modeling software, effective centralization can be achieved in horizontal wells. Without proper casing centralization, the casing will lie on the low side of the hole. This contact with the wellbore will make mud removal impossible, and placement of cement around the pipe will not be achieved. This can also have an effect on a stimulation treatment.