Design Procedure for Cementing Intercalated Salt Zones
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 26310, “Design Procedure for Cementing Intercalated Salt Zones,” by S.R.K. Jandhyala, SPE, and K. Ravi, SPE, Halliburton, and J. Anjos, Petrobras, prepared for the 2015 Offshore Technology Conference Brasil, Rio de Janeiro, 27–29 October. The paper has not been peer reviewed.

Wells often require being drilled through and cemented across salt formations. In many parts of the world, salt sections consist of multiple salt types. This analysis shows that intercalated salts subject cement sheaths to a series of tensile and compressive loads whose magnitude depends on the size and relative position of different salts. The salt/salt-interface effects dominate the general tenet of increasing creep rate with increasing depth. This work demonstrates cement design that includes evaluating cement-sheath mechanical integrity in intercalated salts.

Introduction

Drilling and cementing challenges associated with salt formations are well-known. One of the more significant of these is the plastic deformation of salt attributed to the existence of deviatoric (shear) stress. This deformation is known as creep.

To determine the role of creep in the mechanical integrity of a cement sheath, it is necessary to analyze the thermostructural model of the well-construction process using the creep constitutive relationship. The outcome of the analysis is the stresses experienced by the cement sheath. It is possible to quantify the risk posed by salt creep and other operational loads to the cement-sheath integrity by comparing these stresses to the failure properties of the cement. To provide an accurate quantification of the risk, it is necessary to verify the creep constitutive relationship with data from experimental creep tests.

The type of failure of cement sheath depends on the type of loads exerted on it. For example, compression loads are likely to result in shear failure. Similarly, extensional loads are likely to result in tensile failure. In a typical well- construction process, the in-situ stresses attributed to overburden are greater than the hydrostatic pressure offered by wellbore fluids, such as drilling mud, cement, and completion fluid. Hence, the deviatoric stress on salt formations will lead to wellbore closure. The cement sheath thus will experience compression loads caused by creeping salts. However, the compression load is not uniform along the axial direction. This is because the creep rate of salt varies with depth. The variation is severe when multiple salts with different creep rates are intercalated. Fig. 1 shows an example analysis of openhole closing in the presence of intercalated salts. In such scenarios, the salt/salt- intercalation junctions exert extensional loads on the cement sheath. The closure is not to scale, and the displacements are magnified for a better view. Thus, in the presence of intercalated salt formations, the possibility of tensile failure of a cement sheath should be investigated in addition to shear failure. Evaluating for both shear and tensile failure requires analyzing a longitudinal wellbore model.