Experimental Evaluation of the Impact of Oil-Based Mud Residuals on Cement-Formation Bonding Strength

Open Access
Author:
Alghamdi, Anwar Salah
Area of Honors:
Petroleum and Natural Gas Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Arash Dahi Taleghani, Thesis Supervisor
  • John Yilin Wang, Honors Advisor
Keywords:
  • Well cementing
  • Shear bond strength
  • Cement-formation interface
  • Muds removal
  • Oil-based muds
Abstract:
Mud removal is a key operation to ensure successful well cementing and, consequently, zonal isolation through the lifetime of the well. If mud is not properly removed, a thin layer of mud may remain in place and contaminate the cement slurry and prevent cement of strong bonding with the formation and the casing string. Without having strong bonding at cement interfaces, the annulus becomes susceptible to gas migration, especially under extreme conditions, such as hydraulic fracturing treatments, that involves large pressure fluctuations. Oil based muds (OBMs), in comparison to water-based muds (WBMs), are improving drilling by enhancing lubricity, temperature and borehole stability, and greater penetration rates in certain formations. However, OBMs have counter effects on the cement performance by oil-wetting the formation. The objective of this work is to develop an experimental procedure to quantify the impact of wettability alteration caused by mud residuals on cement bonding in three different formations: sandstone, limestone, and shale. To this end, a modified push-out test is developed, and the effect of different OBM compositions and spacer fluids are examined. The modified push-out setup comprises a rock-core set by cement in the center of a casing-grade steel pipe. Class-G cement slurry is prepared and placed inside the pipe and cured under high pressure and high temperature conditions of 200 °F and 3000 psi for 24 hours to mimic borehole conditions. To mimic the real conditions, the rock-cores are preconditioned with OBM for 24 hours to examine the effect of mud residues on the cement-formation rock bonding. Finally, the specimen was loaded axially at a low displacement rate of 0.3 mm/min for measuring the shear strength along the cement interfaces. Additionally, the effects of OBM oil/water ratio (OWR) and mud removal agent (spacer) on the cement-formation interface are analyzed. Based on our findings, OBMs have negative effects on the cement-formation bonding strength inversely proportional to the OBM’s OWR. While the lithology had a negligible effect on the cement performance. Therefore, higher OWR OBMs are recommended for improving cement bonding with the casing and formation rock. In addition, a proper removal of OBM by spacer fluid increases the shear-bond strength of the cement significantly. However, the improvement is still 50% below the original shear strength measure in the absence of mud-residues. The proposed experimental setup can provide an effective tool to measure the impact of mud removal by different spacer fluids.