Zonal Isolation Achieved in Kashagan Field Through Integrated Approach

Abstract

The Kashagan Field is located in the northeast end of the Caspian Sea (Kazakhstan), and its carbonate reservoir is characterized by high pressure and high content of H2S and CO2. The primary purpose of the production liner is zonal isolation to contain reservoir and injected fluids. The challenges are magnified in these wells because of the presence of corrosive fluids, varying properties of rock, and high pore-pressures. Carbonation of neat Portland cement systems in CO2 environments is well-known. Carbonation is a significant concern if CO2 enters the cemented annulus. The surface area of the cement sheath that contacts CO2 should be minimized to help prevent carbonation. This can be achieved by reducing permeability, preventing formation of cracks and microannuli, and reducing the components in the cement sheath prone to attack from CO2. Zonal isolation was achieved by designing and deploying a cement sheath that had 1) high structural integrity, 2) low permeability, and 3) was stable when exposed to downhole conditions of chemicals, temperature, and pressure. The structural integrity helped prevent the formation of microannuli and cracks during well operations. This feature, combined with low permeability, prevented the formation fluids from entering the annulus. The chemical stability helped prevent any deterioration of the cement sheath if it were to come in contact with downhole fluids such as CO2. The production liner was set at ±4,500 meters and the mud density was ±1.95 g/cc. When mud was swapped with seawater during the completion operation, it resulted in significant pressure decrease inside the casing. The cement sheath was designed to withstand completion and subsequent operations during the well-life to maintain structural integrity and help secure zonal isolation. The cement system was formulated to withstand well operations by improving the elasticity, reducing hydration volume shrinkage, and engineering optimum expansion of the cement sheath. The potential for carbonation was decreased by lowering the permeability and reducing the components in the cement sheath that could react with CO2. The cement system was tested in the laboratory for elasticity, resistance to CO2, and expansion under downhole conditions. If the well were to be operated beyond its design limits, then there would be a risk of cement sheath failure leading to creation of microannuli and small cracks, and potential influx of formation fluid into the annulus. The cement system is designed to react with formation fluids and respond in such a way to seal small cracks and microannuli automatically, without well intervention. The cement system has been successfully deployed in six wells. The cement bond logs were excellent and verified the successful cement slurry placement on all jobs, and subsequent successful well operations confirmed zonal isolation. The same cement system is being deployed in additional wells. Cement slurry design, field deployment, and reservoir performance is discussed. The results presented in this work should help in the design and implementation of solutions to achieve zonal isolation, even in a corrosive environment. Introduction The challenges to zonal isolation in the Kashagan field are both chemical and mechanical, resulting from exposure of cement sheath to reservoir fluid containing CO2 and H2S and stresses from extreme well operations. After detailed study of the challenge, it was evident that an integrated approach was needed to achieve zonal isolation. The geology, reservoir fluid characteristics, drilling parameters, and details of well operations, such as production and injection, are all important and need to be considered to design and deploy a reliable cement system. A multidisciplinary team was assembled to solve this challenge.