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Abstract

With the growing trend of pre-salt discoveries, some salt formations cannot be simply bypassed to get to the reservoir. A considerable thickness of salt of up to 15,000-ft is usually encountered in the deepwater areas. Salt structures embedded within the hydrocarbon prone formations have contributed to the complexities of well trajectory design and operations. Salt domes are impermeable cap rocks for petroleum accumulation but the stress distribution and geo-mechanical dynamics within and around them limit the success of directional drilling. Creeping is the major behavior of salt formations.

This study presents a new stress analysis model for rock salt formations based on the octahedral shear stress theory and von Mises failure criterion. The model determines the second deviatoric stress invariant called octahedral shear stress. The stress model also includes the Maxwell’s Creep Model in the formulation of the time-dependent response of salt formations to changes in octahedral stresses within salt structures due to the inherent visco-elasto-plastic properties of salt. Results obtained from the stress model show that the octahedral shear stress values in salt formations prior to drilling are dependent on the three principal stresses exerted by neighboring formations while the octahedral shear stress values in the salt formations during or after drilling are dependent on the minimum horizontal stress, wellbore pressure, wellbore radius, and the radial distance from a selected stress center.The stress model can be used to estimate the extents and distribution of the octahedral shear stresses within and around the salt structures prior to drilling and after drilling using the available stress data from geo-mechanical models. The position and inclination of the wellbore can be selected in a less-stressed zone within the salt formation. The case study for this study is the 3D-geomechanical model of a salt diapir from the Officer Basin, South Australia, subjected to long period of deformation. This paper concludes with the description of the suitable bottom-hole assembly (BHA) and drilling practices for successful drilling through salt structures.

Introduction

Rock salt is an evaporite known as halite and being impermeable is a good candidate for reservoir cap rock.  Rock salts have a unique mobile nature, which varies in different areas and requires different practices. Rock salt formations have become of great economic importance because they possess the ability to form diapirs that become suitable locations for trapping petroleum deposits.