Natural Sealing of Leaky Wells by Creep of Rocksalt
Authors: TerHeege J. H. 
Published: August 2024

ABSTRACT:
Wells drilled through salt formations can be subject to leakage if a (micro)annulus is present between the salt formation and well casing that allows significant fluid flow over large well sections. Laboratory experiments were performed to assess if natural sealing by rocksalt creep can close flow paths over such a salt-casing annulus. The experiments were performed on a scaled down well system consisting of a casing in a hollow salt sample that was placed in a triaxial deformation apparatus. The applied experimental conditions were equivalent to conditions of Zechstein salt at ∼2.5 km depth in the Netherlands. Sealing of flow through the salt-casing annulus was observed. Salt seals could sustain more than 35 bar of pressure difference over the 100 mm casing length for both brine-filled and oil-filled annulus after 5-8 days of creep, up to a maximum of 148 bar after 7-10 days of creep. The timing of sealing barrier formation critically depends on temperature and difference between axial/radial stress and annulus pressure. The experimental results show that salt creep can effectively close a salt-casing annulus at conditions relevant for leaky wells drilled through salt overlying reservoirs used for hydrocarbon production or storage of CO2 or H2.

1. INTRODUCTION
Wells drilled through salt formations can be subject to leakage if an annulus between the salt formation and well system is present that allows significant fluid flow over large well sections. Development of a salt-casing (micro)annulus can have different causes including (1) issues with cementation of casings and (2) casing deformation due to variations in well pressure and temperature during operations. Wells drilled through salt are particular prone to annulus formation, for example due to washouts of soluble salts. Moreover, cyclic loading of wells during cyclic injection and withdrawal of hydrogen streams in salt caverns used for hydrogen storage increase the risks of annulus formation and associated well leakage.
Natural sealing by salt creep has been suggested to be able to form a permanent sealing barrier that prevents fluid flow (Hou et al., 2012; Orlic and Buijze, 2014). The main interests of this process within the framework of subsurface energy applications are: (1) mitigation of upward migration of fluids or gases along well sections with poor zonal isolation at salt formations, and (2) well decommissioning involving plugging of wells by the formation of a permanent, impermeable and continuous seal at milled-out well sections in salt formations (Fig. 1). If natural sealing of salt-casing annulus is effective, it may reduce measures required to monitor and mitigate salt-casing annulus formation and associated fluid/gas formation. Besides sealing migration pathways along salt-casing microannulus at poorly cemented well sections, creep of rocksalt may also be used to seal well sections that are milled-out at the salt formation. Natural sealing of these sections may be effective in permanently sealing decommissioned wells penetrating salt formations. Thereby, it may be considered as a cost-effective and reliable alternative to sealing abandoned (salt) wells by cement plugs. Well plugging involving the formation of natural sealing barriers around wells by creep of rocksalt has the advantage that it relies on natural occurring rocksalt formations and on creep processes that are known to occur in rocksalt formations. Therefore, (1) introduction of extraneous material for sealing barrier formation is not required, (2) formation of a permanent sealing barrier is based on natural occurring creep processes that occur without human intervention, and (3) acceleration of barrier formation can be performed relatively easy using known temperature dependence of salt creep.