Study and Application of a Drilling Fluid Plugging Agent Based on Pressure Transfer Inhibition

The size distribution of pores in shale formations in Nanpu oilfield is in a level of micrometers or even nanometers. Conventional micrometer-sized solid particulate plugging agents used in water base drilling fluids are unable to effectively plug these pores. Laboratory experiments have been conducted with several nanometer-sized particulate plugging agents, such as FT-3000, HLFD-1 and Green Seal, to ascertain the effects of type and concentration of these plugging agents on HTHP filter loss, filtration through sand disc, pressure bearing capacity of mud cakes, and their performance in plugging nanometer-sized pores. Experiments have also been done on core samples taken from Nanpu oilfield to test the changes in core permeability before and after plugging of cores, pressure transfer and membrane efficiency. The experimental results showed that Green Seal had HTHP filter loss of 17 mL and filter loss through sand disc of 13.8 mL, the minimum values among the three plugging agents. With Green Seal, the pressure bearing capacity of mud cakes was increased to 8 MPa or even higher. Analyses of permeability of the cores before and after plugging experiments indicated that Green Seal had the best efficiency in pore plugging, effectively inhibiting pressure transfer. Membrane efficiency of Green Seal was 0.0990, the highest value among the three plugging agents. These experiments have proved that Green Seal, an environmentally friendly deformable plugging agent, is the best plugging agent evaluated. Green Seal has been applied on the well Nanpu36-3652 and the well Nanpu36-3701. The borehole wall was stable, and gauge holes were obtained. Recovery of permeability was high and filtrate invasion was low, realizing the multiple goals of stabilizing shale borehole and protecting reservoir formations from damaging.