Well Design for Drilling Through Thick Evaporite Layers

Abstract

Abstract The presence of evaporite sections in prospects for oil or gas exploration is, in itself, a factor that increases the probabilities of success in the area due to favorable conditions for the hydrocarbons generation and trapping. However, many operational problems such as stuck pipe and casing collapse have been reported by the industry when drilling through those salt layers. Historically, in the Campos Basin - Brazil, several deep wells have been drilled through thick salt intervals. Up to the 90's, the lack of reliable ways to predict salt behavior at high temperatures and high differential stresses led to very high drilling costs and even loss of wells.[1] This paper presents a methodology for drilling fluid and casing design and drilling strategy for drilling at great depths through thick salt layers. The numerical simulation to evaluate the creep behavior of salt rocks (halite, carnalite and taquihydryte) submitted to high differential stress and high temperature was done through the application of an in-house finite element code developed. Results obtained by the numerical simulations in prospect with 2000 meters thickness of different evaporite rocks with high creep rate was used to predict the evolution of the well closure with time for various drilling fluid and analyze several technically feasible alternatives to drilling strategy. The casing design was accomplished with several failure scenarios of cement the casing/borehole annulus through the salt, and drilling fluid in annulus to determine the nonuniform loading and timing of salt loading on well casings deformation or ovalization. The casing was design to be capable of supporting high creep rate of carnalite and taquihydryte. Introduction The Santos Basin, offshore southeast Brazil, is one of the Brazilian basins that is receiving considerably industry attention nowadays. Active drilling in the this area in recent years has not yet yielded discoveries comparable in size to fields in Campos Basin, the most prolific oil provinces in Brazil (Fig. 1). However, due to its size and for being relative under explored, it continue to attract interest.[2] The object of this study is planning an exploratory deepwater well (WD = 2140 m) to a depth of 6000 meters TVD in Santos Basin. This prospect is expected to drill through almost 2000 m of salt rock - halite, carnalite and taquihydryte (Fig. 2). This is the first of a toal of four wells to be drilled up to the end of 2006. The presence of evaporite sections in prospects for oil or gas exploration is, in itself, a factor that increases the probabilities of success in the area due to favorable conditions for the hydrocarbons generation and trapping. However, many operational problems such as stuck pipe and casing collapse have been reported by the industry when drilling through those salt layers. Before planning to drill long salt sections, a through knowledge of salt and its properties is necessary. Salt rocks belong to the group of sedimentary rocks, called evaporite, deposited by evaporation of saline water. Salt is an unusual geologic material for under sustained constant stress, significant deformation can be expected as a function of time, loading conditions and physical properties.[3] This behavior is called creep. The rate salt will creep is dependent on some parameters: temperature, differential stress and salt type are of the paramount importance.