Evaluation of Oil-Based Drilling Mud OBM and Spacer Contamination on API Class G Cement: Wellbore Challenges and Recommendations

Abstract Cementing operations, as a crucial job in the oil and gas industry, play delicate roles such as zonal isolation, preventing fluid contamination between producing zones, wellbore stability, and protection of steel casing against corrosion. With these unique roles, the contamination of cement slurries with Oil Based Mud OBM, Water Based Mud WBM, Brine, and Spacers during cementing operations remains a prevalent challenge that drastically affects the performance of slurries in a well. Primary Cementing operations are designed with the Lead Slurry to effectively displace spacers and mud from the well column based on density hierarchy and Yield Point (Ty). In contrast, remedial cementing operations such as Squeeze cementing or PlugToAbandon, slurries are designed to displace brine. However, slurries are contaminated with spacer or mud, which imposes some well problems such as Cement Left-In-Pipe CLIP, an undesired shorter and longer time for WOC, hole, and environmental problems. This research evaluates the impact of Oil-Based Mud OBM and Spacer contamination on the Thickening time (BC) and Compressive Strength CS of an API Class G Portland Cement Slurry. Adopting an innovative and systematic methodology in compliance with API guidelines for cement testing, cement slurries, and spacers were prepared with an Oil Based Mud OBM. Contamination of the cement slurry was done by percentage volume: 65% Slurry: 34% Spacer & 1% OBM for WELL A (primary cementing), and 90% Slurry:10% Spacer for WELL B (remedial cementing). Additionally, Thickening Time (TT) and Compressive Strength tests were performed for the neat and contaminated slurries. The results on the primary cementing job showed a TT of 06:52 hr:mn at 70Bc and a developed Compressive Strength of 2,060 psi in 24hrs for the neat and a CS of 1,104 psi in 24hrs with the contaminated slurry displaying a significant reduction of CS by 46.41%. WELL B (remedial job) gave a TT of 70Bc at 19:46 hr:mn compared to the neat slurry with a required TT of 70Bc at 09:06 hr:min. The CS developed for the contaminated and neat slurries were 500 psi in 24hrs and 2,170 psi in 24hrs respectively. WELL B demonstrated the adverse effect of contamination on a cement slurry which showed a 76.96% reduction in compressive strength compared to the neat cement slurry. Expository research findings explain the effects of cement slurry contamination in primary (Liner casing) and remedial (PlugToAbandon) cementing operations. It also provides recommendations on how contamination of wellbore cement can be mitigated to cut down on NPT, improve cementing operation performance, and proffer solutions to wellbore challenges such as CLIP.