Improved Zonal Isolation Through the Use of Sealants Before Primary Cementing Operations

Abstract

Zonal isolation in injection, geothermal, and producing wells is critical both to reservoir evaluation efforts and to operations, but often, cementing between the casing and the wellbore is still the only method of zonal isolation performed in many wells. Although zonal isolation and well diagnostics techniques have improved, primary cementing techniques may be inadequate for some reservoir conditions, resulting in undesirable water flows, gas flows, or both. If zonal isolation is inadequate, then expensive, often complicated remedial cementing work will be needed for improved well and reservoir performance. If conformance technology is applied during the drilling phase, it can improve the performance of primary cementing processes, making them safe and economical. Conformance treatments seal problem intervals before primary cementing, thereby alleviating potential well-control problems. These treat-ments also help ensure successful primary cementing operations and prevent isolation loss caused by the casing expanding and contracting during production-pressure cycles. The formation itself, rather that the cement sheath alone, becomes the zonal isolation mechanism. During the drilling phase, zonal isolation requires the use of coiled tubing, a hydrajetting device, a depth-correlation device, and a sealant. The sealant should be water-thin during placement, and should become an elastic polymer only after it is set in place. This zonal isolation technique requires an understanding ofdepth correlation to open hole logs, through the use of mud-pulse technology, andhydrajetting techniques that remove mud filter cake while placing the sealant. This paper presents examples of this technique for a 7 7 /8-in. wellbore and a 13 1/2-in. wellbore. Introduction The creation of an effective reservoir management strategy requires an operator to achieve zonal isolation, model past-production performance, and predict applied technology's influence on future production performance. Satter and Thakur1 state that the current challenges to effective reservoir management areimproved definition of reservoir characteristics,improved tracking of fluid movements through the reservoir, andimproved control of fluid movements in the reservoir. Reservoir diagnostics and performance predictions require the complete isolation of the intervals under evaluation, and conventional zonal isolation methods have proven inadequate for many reservoir-related conditions. Zonal isolation failure after primary cementing operations has often been attributed to the ineffective removal of drilling fluid filter cake. As early as 1940, large-scale displacement studies presented by Jones and Berdine2 discussed the difficulty of removing drilling-fluid filter cake from the face of a permeable formation. The studies concluded that a deposited filter cake was removed most effectively by scrapers, the hydraulic impingement of fluid jets, and acid treatments. Since 1940, the industry has spent millions of dollars studying the mud displacement process. Many industry-wide best practices have been established based on the results of this research. Even with these advances, some formations present significant challenges during drilling, primary cementing operations, and production or injection operations. These challenges often cannot be corrected when conventional best practices are applied. Sweatman et al.3 list the following challenges that the industry is currently facing:shallow water flowsgas- or water-cut drilling fluid and cementgas hydrates in the drilling fluidformation sloughing or hole collapse during drilling or open hole production