A New Approach to Characterize Dynamic Drilling Fluids Invasion Profiles in Application to Near-Wellbore Strengthening Effect

Ezeakacha, Chinedum Peter (University of Oklahoma) | Salehi, Saeed (University of Oklahoma) | Bi, Hongfeng (Grace Instrument Co.)

OnePetro 

Abstract

Mud cake evolution and plastering have been identified as important wellbore strengthening mechanisms. They serve to reduce losses through pore throats and fractures, while impeding the growth of induced fractures. Recent experimental and analytical studies have also revealed the complexities in drilling fluids’ invasion profiles and mud cake buildup. These complexities arise from the changing wellbore conditions observed in an actual field scenario. It is important to investigate the effects of these conditions on drilling fluid invasion for near-wellbore strengthening application.

To achieve this goal, some dynamic wellbore conditions which are close-to-real field conditions were simulated in a controlled laboratory setup. The following conditions were investigated: rotary speed, temperature, type of lost circulation material (LCM), concentration of LCM, differential pressure, eccentricity, rock permeability, and fracture width. In the experimental setup, the geometry of the shaft that simulates drill pipe rotation allowed for mud cake evolution and plastering around the inner diameter of the thick-walled cylindrical porous media. Water-based mud (WBM) recipes were formulated for different porous media types. The rheological profile for each mud recipe was investigated for operating temperature limit. Dynamic drilling fluid invasion experiments were conducted with thick-walled cylindrical Buff Berea sandstone, Upper Grey sandstone, and fracture slots with varying widths.

The results indicate that temperature, rock permeability, fracture width, and LCM type and concentration are the most influential factors that control dynamic fluid invasion profiles. Increase in granular LCM concentration at elevated temperature is not completely effective in reducing pore-scale fluid invasion. Spurt invasion, rock porosity, permeability, and fracture width are important determinants of mud cake evolution. Increase in fiber LCM concentration showed effective mud cake evolution in the fracture slots. The results from testing mud cake stability revealed mud cake rupturing on three experiments out of the nine that were performed. The novelty in this approach is the use of thick-walled cylindrical cores and fracture slots to profile dynamic fluid invasion of different fluid recipes. Pressure, temperature, and pipe rotation were combined to simulate wellbore conditions under which fluid loss, cake growth, and plastering occur. This approach can be used in drilling fluid design for minimizing fluid loss, cost, and selection of operating conditions.