Case Study: Zonal Isolation Challenges in the Midway-Sunset Field of the San Joaquin Valley
Authors: Valadez Juan José , Todd Larry , Cleveland Matthew , Clark Travis , Kruse Matt 
Published: November 2015

Abstract
Effective and verifiable zonal isolation is an increasing industry concern, especially in the strict regulatory environment of California. Unconsolidated formations in the Midway-Sunset field of the San Joaquin Valley have proven to be a challenge for conventional well construction designs to meet job objectives. Several root causes for a failure to achieve verifiable zonal isolation were found, including losses due to weak formation, logging issues associated with low-density cement, and technical challenges of logging in what is known as the air sands, an unconsolidated sandstone formation with dry pore space.
Incremental improvements in the well construction design of the Midway-Sunset field developed several best practices. Low-density thermal-resistant cement systems with lost circulation materials were implemented to improve top-of-cement and cement-to-surface success rates. In addition, modifications to the cement system design were made to improve rheological properties to reduce equivalent circulating density (ECD) while maintaining a high solid volume fraction (SVF). The focus of this paper is the addition of a spacer train with a specialized silicate solution that was applied to solve two concerns in the field: losses across the unconsolidated formations and the poor log response in the air sands.
Since the implementation of the best practices, significant improvements in verification of effective zonal isolation have been achieved. In problematic wells, those that incurred heavy losses prior to the cementing operation, the success rate of achieving cement to surface increased from 45% to 75%. This is the result of using lost circulation materials, reducing the density of cement systems while maintaining a high SVF and low fluid loss properties, and reducing ECDs through modified rheological properties. In addition, the specialized silicate spacer train improved the response of the cement bond log (CBL) run in these wells. Interpreting a CBL in the Midway-Sunset field has been historically difficult due to the air sands. The dry pore space of this formation is known to dehydrate cement slurries. Dehydrated slurries may be one factor leading to the formation of a dry microannulus which affects the response from a CBL. The specialized silicate spacer train performs two important roles in solving this problem. First, the specialized silicate spacer train enters the pore space and unconsolidated matrix of the formation. Upon mixing with the chloride activator, the silicate solution reacts quickly forming a rigid gel and thus preventing the bulk losses of cement slurry to the formation. In addition, this silicate spacer inhibits fluid loss from the cement slurry that could dehydrate the slurry and cause a microannulus. Over 200 wells have been drilled and cemented in the Midway-Sunset field since the implementation of these best practices, and improvements have been seen throughout the well construction process from the initial placement of the slurry through the logging evaluation.