Triaxial Creep Tests in Salt Applied in Drilling Through Thick Salt Layers in Campos Basin-Brazil

Abstract

Abstract The presence of salt structures in prospects for oil and gas exploration is, in itself, a factor that increases the probability of success due to favorable conditions for the hydrocarbons generation and trapping. However, many operational problems such as stuck pipes and casing collapse have been reported by the industry when drilling through those salt layers. Historically, in Campos Basin – Brazil, several deep wells have been drilled through thick salt intervals. Up to the 90', the lack of a reliable ways to predict salt behavior at high temperatures and high differential stresses led to very high costs and even loss of wells. In this paper we present a methodology for mud weight and casing design and also to define the drilling strategies employed for drilling through thick salt layers. The numerical simulations to evaluate the creep behavior of salt submitted to high differential stresses and high temperatures were done through the applications of an in-house finite element code developed. To calibrate the model, triaxial creep tests in salt samples were performed to evaluate and isolate reological properties to represent its creep behavior under different differential stresses and temperatures. It was verified that numerical and experimental results matched and had a fine conformity. A recent application of this methodology in a sub salt well in Campos Basin allowed us to drill the salt without a problem. Results obtained by numerical simulations were used to predict the evolution of the well closure with time for various mud weights and analyze several alternatives of casings capable of supporting salt creep. As a result, stuck pipe and casing collapse were avoided and drilling costs reduced.