New Well Design and Drilling Practices Changes Ensured Well Integrity Across Highly Fractured Carbonate Formations in Total Losses Area

Abstract

The low success rate of curing complete loss of circulation across fractured carbonates in the field, where all kind of loss circulation material and unconventional plugs were tried without success, required better understating of the fractures and changes to the well design and drilling practices to ensure proper zonal isolation and well integrity. This paper describes the successful planning and implementation of these changes and how they improved overall well construction performance. As most of the total losses occurred in the intermediate section, which combines pressurized water injection formation with depleted oil-bearing reservoir, curing process was very often never achieved. Understanding that some areas of the field are naturally highly fractured, but specially characterized by carbonate karst, dissolution, and mega fractures, was critical to map the high-risk area and modify the well design and practices. The innovative risk-based well design was deployed to a series of wells across the field. The isolation between low- and high-pressured layers to prevent casing-to-casing annulus (CCA) sustained pressure[NMF1] throughout well life and, possibility of drilling ahead with no returns with one of layers already secured by previous casing were implemented. Suitable casing seat was selected in such way that critical high pressure and depleted zones were identified and isolated. Water-bearing layers in the upper section was efficiently balanced with enough mud weight and casing point set above risky zone with addition of double mechanical barrier with gas tight feature. Highly fractures formations in the lower section were drilled with significant lower mud weight at minimum overbalance, surge stresses and best possible fluid rheology. Casing running practices were adjusted to avoid inducing losses, cement slurries redesigned for optimum properties at minimum equivalent circulating density (ECD) and cementing jobs were conducted with full returns. The total loss events were significantly mitigated in the campaign, with overall 15% well performance improvement, ensuring zonal isolation, and reduction of future CCA occurrence, which can compromise the production casing integrity after fracking job, involves high remedial work costs, longer shutdown phase and possible production loss. This solution balances performance and costs can serve a technical reference for future application in the basin or other regions with loss-prone environment across large and fractured formations, and well integrity is a concern for operators and service companies.