Alternative Materials for Plug and abandonment of Petroleum Wells: Assessing Mechanical Performance of Hydrated Bentonite Plugs

Abstract

Hydrated bentonite plugs may be used as barrier elements in plug and abandonment operations of petroleum wells. To this end, the way applied to dislocate the pellets inside the well is different for onshore and offshore wells. This study aims to evaluate how the operational procedure adopted for the pellets placement in the well and the exposure to subsurface conditions influences the mechanical integrity of hydrated plugs. Two methodologies were used to obtain the plugs: from the first methodology, the pellets were immersed in diesel or olefin, which are proposed as displacement fluids in offshore operations, and then, hydrated in deionized water; in the second methodology, representative of onshore operations, the pellets were directly hydrated in deionized water. After that, the plugs were tested by compression and adhesion tests. These mechanical tests were also carried out for test specimens obtained from plugs exposed to four formulations of synthetic formation waters. The results obtained demonstrated that the previous contact with olefin adversely affects the mechanical stability of bentonite plugs, while plugs formed from pellets immersed in diesel presented satisfactory mechanical properties. For plugs directly hydrated in deionized water, following the methodology proposed for onshore wells, the mechanical properties have demonstrated a highly successful performance of these elements, once its compressive strength was minimally impacted by exposure to formation waters and more pronounced reductions were observed only in the adhesion properties after the contact with higher salinity formation waters.