Semester of Graduation

Spring 2024


Master of Science in Petroleum Engineering (MSPE)


Craft and Hawkins Department of Petroleum Engineering

Document Type



In the ideal plug-flow mode viscoplastic drilling fluids display uniform frontal velocity due to wall slippage (at very low flowrate limited by the fluid’s yield strength). The flow rate could be increased by raising the yield strength that would make the fluid best candidate for frontal displacement. Based on this concept the study qualifies wall slippage effect for wellbore displacement. Using a conceptual model of fluid motion between two parallel plates defines maximum velocity as a function of shearing rate and yield stress. Conversion to coaxial geometry gives practical formulas for fluid testing with rotational viscometer. A new analysis of the fluid’s rheogram quantifies two measures of displacement performance: Maximum Slip Velocity and Plug Displacement Index.

The study evaluates variety of viscoplastic fluids used for water-based drilling muds such as high-molecular-weight polymer solutions, clay suspensions, and suspensions flocculated with different salts. The results show superiority of fluids best-represented by the Bingham plastic model – bentonite suspensions flocculated with NaCl or CaCl2 - rather than solutions following the Herschel Buckley model.

Also considered is the wall roughness effect on wall slippage in real wellbores – by testing the flocculated bentonite suspension in a modified viscometer with the bob and sleeve surface roughness ranging from 23 μm to 205 μm. The study shows insensitivity of displacement performance to the wall surface’s roughness as the rheograms for each value of roughness are identical. Thus, the bench-tested performance of displacement should be expected in actual wellbore.

Also shown in the study is an optimized formulation of displacement fluid for maximum performance vs bentonite concentration, salt type (NaCl and CaCl2), and salt concentration. A distinct maximum is reached at the bentonite concentration 3.0% v/v and the peak flocculation value of the salt treatment.

The results were demonstrated by designing displacement operation of the gas influx (kick) removal from an unconventional well in the Permian Basin. The operation involved pumping of 80 bbls of bentonite suspension designed in this study at the rate required for plug flow with maximum wall slippage. After one hour of pumping, when the displacement fluid reached the well’s bottom, the gas influx was completely removed from the well.



Committee Chair

Dr. Andrew Wojtanowicz