Impact of Change in Mud Weight & Offset Well Injection Pressure on Cement Sheath Integrity; Case Study for UAE Offshore Field.

Abstract

Abstract As drilling horizontal wells became more complex with multiple production drains from various reservoirs, zonal isolation has become a key challenge to be achieved. Many studies have shown the impact of change in mud weight during drilling activities on wellbore stability, without considering the consequences on the set cement. This case study will be focusing on drilling activities on offset wells, evaluating & exploring how mud weight changes can affect wellbore instability and damage the cement sheath despite the fact that the cement evaluation logs showed good results days after the cement job and before the mud weight reduction. The wells being studied have been analyzed using Finite Element Analysis (FEA) modeling after the cement was set and exposed to a change in mud weight as well as being exposed to the offset injection well. FEA computer simulations have enabled this modelling to determine the optimal mechanical properties required for the cement slurries to withstand those load limits (pressure testing, pressure reduction due to change of mud weight or an increase of the pore pressure due to an offset injection well). After opening the window and drilling ahead through the production drain wellbore, the wells have the same tendency of instability and require an increase in mud weight to control the inflow. This paper presents the impacts of the offset wells and the change of mud weight on the cement sheath failure in debonding, cracking, or sheath deterioration leading to gas channeling due to a micro annulus. The considered wells present the same behavior that has been highlighted above. The question to ask is why did a well that has been cemented and recorded a good cement bond fail to provide proper zonal isolation? The injection pressure of the offset well was evaluated and then used in the simulations, considering the conventional cement used during the primary cement job. The analysis has shown a complete failure of the cement integrity: cement debonding from the casing or from the formation; cement cracking and sheath failure. The cement slurry with improved mechanical properties was employed for the study under the same pressure conditions from an offset well and mud weight change. The model results demonstrate that the new cement's ability to withstand stresses has improved on the wells cemented after this study. The modelling effort presented in this paper allows for a barrier to be designed that has enhanced mechanical properties compared to the already placed barrier on the offset wells in the same field. Aside from improving the set cement mechanical properties, the mud weight change design has been improved in such a way that the mud weight must be anticipated, decreasing its impact on the barrier.