Study on the mechanism of ENI action on preventing drilling fluid overflowing in HDD

When pipe laying in a clay or sandy stratum by Horizontal Directional Drilling (HDD), instability issues in the borehole wall are common due to low cohesion. Not only can such instability lead to the collapse of the upper ground and the destruction of the adjacent structures on the ground, but it can also cause drilling fluid overflows. In this paper, the authors proposed another additive using in horizontal directional drilling to prevent ground surface movement and drilling fluid overflowing. Based on the comparable method, the variety tests including bentonite yield, core dispersion test, expansion rate test, shear strength test, microstructures of mud cake, and rheological properties were performed between the ordinary drilling fluid and ENI solutions. The test results indicate that ENI solutions can provide enough positively cations such as Al3+ to decrease the cation exchange activities and keep surrounding soil stability in the borehole. As a result, comparing to the ordinary drilling fluid, the rate of bentonite yield in ENI aqueous solution is decreased by about 50%. In addition, ENI solutions will product complex materials to fill the space between the soil particles to prevent fluid intrusion in the borehole. The shear strength test indicates that appropriate concentration of ENI can enhance the mechanical strength of the surrounding soil in the borehole. Then it can provide sufficient strength to balance the fluid pressure in horizontal directional drilling. At the same time, ENI can create a modified microstructure in the layers, so that the connecting force between particles is enhanced, and then the surrounding soil stability can also be improved. ENI also can keep the rheological properties of the drilling fluid system constant to meet requirements in the fieldwork.