Casing/cementing/zonal isolation

An Integrated Approach to Solving Sustained Casing Pressure in the Cana-Woodford Shale

Many wells in the Cana-Woodford shale suffer from chronic sustained casing pressure (SCP) because of poor cement-sheath bonding.

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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.

Introduction

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. Stage-to-stage isolation provided from cement around the entirety of the casing is important to ensuring virgin-reservoir stimulation and preventing communication between stages with the now significantly lower-stress environment. A reduction in drag force can also be seen by running centralizers. Centralizers can keep the casing off formation, thus reducing the points of contact.

The operator ran an aggressive casing-centralization schedule on these wells. The typical schedule was to run three centralizers per two joints of casing in the lateral to a few hundred feet above kickoff point (KOP). From KOP to 3,000 ft above planned top of cement, one centralizer per joint of casing was run.

Mud Properties. Drilling mud is a key consideration for a successful cement job. Typical horizontal-drilling muds have high yield points to ensure cuttings removal and hole cleaning. A thick mud with high yield is a requirement before cementing, but this high-yield mud can be detrimental to mud removal during cementing and can obstruct cement placement. Proper conditioning of a mud before a cement job is critical, although lowering the yield point too much can make the mud unstable.

Cementing Evaluation. Cement placement was evaluated with a wireline tool to evaluate the cement bond. Several evaluation approaches exist in the industry. A high-resolution logging tool that can help differentiate channels, solids, liquids, and gas should be used because these high-resolution images can provide causal information on failure mechanisms to provide feedback into the design to determine what to change on future slurries. It is recommended to use an ultrasonic-based logging tool that can measure acoustic impedance (AI) as a minimum. Given the nature of the tight pore and fracture gradients, the mud, spacer, mud-recovery fluid (MRF), and slurries can have very similar AI values. To distinguish between the fluids and low-impedance slurries better, having a log that also measures flexural attenuation (FA) is key when there are overlying or very similar AI values of different fluids and cements in the annulus. Having independent high-resolution maps of both AI and FA provides increased assurance in barrier evaluation, especially in lightweight or contaminated cements.

Proposed Solution

Log Evaluation of Mud Removal. All evaluations showed poor mud removal. Several factors must be taken into account when evaluating the ability to remove mud; centralization, density of the fluids, rheological properties of the fluid, and the spacer composition are all contributing factors. Through simulations, the centralization was determined to be optimized as much as possible. The pore and fracture gradients dictated that the optimum fluid-density hierarchy could not be achieved. Density is not a major concern in the horizontal section in regard to the ability of one fluid to displace another, but, as the fluid transitions from a horizontal orientation to a vertical orientation, the density hierarchy becomes a major contributor to channeling and poor mud removal.

Density hierarchy could not be improved because it is governed by the geology of the field. This also dictates the rates that could be achieved because the equivalent circulating density must be managed to prevent losses. The only two factors left that could be altered were the rheological properties of the mud, spacers, and slurries and the spacer composition. When designing a solution, the focus of the design in regard to mud removal will be on rheological properties and spacer composition.

Post-Fracturing SCP. Regardless of whether SCP is seen before fracturing, it was almost always seen after fracturing. A solution would have to take into account how the well will be stimulated. The forces that a typical Cana-Woodford stimulation puts on the cement sheath are significant, and an analysis of these forces must be completed by use of simulation software. Parameters such as treatment pressures vs. time, temperature changes vs. time, Young’s modulus of formation and cement, Poisson’s ratio, and thermal conductivity must be gathered for evaluation in a stress-analysis simulation. The software will use these parameters to simulate a fracture stage and evaluate the cement-sheath integrity in regard to compressional/tractional failures and the formation of a microannlus. A simulation was performed on a conventional slurry. The results showed that the cement sheath would fail in compression and traction and form a microannulus. This correlated to what has been seen on resulting SCP. A cement design that could account for the stimulation treatment had to be proposed.

Proposed Design. The objectives of this design were to account for OBM recovery and mud removal, to produce a good bond log, and to prevent pre- and post-completion SCP. An MRF was proposed to achieve effective OBM recovery. The MRF is a high-solid-content fluid that does not have significant cementitious content. This would be followed by a self-healing cement slurry placed in the casing-to-casing section. The self-healing cement cap was in place to close the fractures that may occur during a stimulation treatment in the presence of hydrocarbons. The self-healing cement cap was followed by a mechanically enhanced slurry that would better address the stresses that are seen during fracturing treatments and during the life of the well.

Conclusion

The process and methodologies discussed in this paper were shown to be successful at preventing SCP before and after stimulation in the Cana-Woodford shale. The following attributes have resulted in the success of this project:

  • Evaluation of cement with high-resolution tools provided key insights into the root causes of the SCP and drove engineered solutions to the issues.
  • The engineered approach using modeling tools to predict cement performance under expected conditions against the observed failure mechanisms on past wells allowed an effective solution to be developed faster and more economically than did trial-and-error methods.
  • Aggressive centralization schedules can be used in horizontal wells effectively, and centralization can greatly improve the ability to place critical cement barriers strategically.
  • Use of an ultrasonic imaging tool with FA can allow for the analysis of third interface echoes to evaluate the actual standoff achieved.
  • Rheological hierarchy must be evaluated on a per-well basis and is critical when operating in challenging placement environments.
  • Cement-slurry designs must take into account the stress that cement slurry will see during stimulation and during the life of the well. Mechanically enhanced slurries and self-healing cement slurries are a vital solution to SCP.
  • At the time of the writing of this paper, the engineered approach had achieved a 95% success rate at eliminating SCP in the Cana-Woodford over the past 2 years.

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.