Lost circulation is one of the great challenges during the drilling process as it can not only increase the risk of drilling operations but also cause an increase in drilling costs, thus greatly affecting the drilling efficiency.In order to prevent this problem, it is necessary to strengthen the wellbore.
Wellbore strengthening is a technique implemented to keep formation integrity and control losses when drilling challenging formations, which can increase the maximum sustainable pressure for a wellbore through fracture sealing or fracture plugging and bridging.
The mechanism of the nano plugging agent drilling fluid to strengthen wellbore is shown as down. Under the action of pressure difference,nanoparticles with a small particle size and uniform distribution are pressed into the micropores and microcracks on the rock surface, reducing the intrusion of the filtrate and forming a physical plugging layer. In addition, the bitumen grain in the nano pluging agent on the shale surface and produces strong adsorption, which thickens the hydration film .The formation of chemical seals in fractures can alter the pore pressure and tangential stress around the wellbore in shale formations, and the hydration swelling caused by surface water absorption can be suppressed after nanomaterials form a protective film on the surface of shale.
The mechanism of the bridging cross-linking plugging agent strengthening the wellbore is shown as upper. The bridging and cross-linking plugging agent is added to allow high-strength particles of diferent particle sizes to bridge the cracks. The fiber material is filled between the particles, and the chemical cross-linking consolidation material tightly cements and solidifies the particles,fibers, and the rock body; stops the propagation of the fracture; and eventually cures the losses.
MEdrilling developed a rigid nano plugging agent fits in those two mechanism perfectly. It prepared from nanoscale material, mineral fiber and cationic emulsifier. The mineral fiber has fiber structure, which can bridge the pores and fractures of formation rock, nanomaterial will seal the pores left by bridging and cationic emulsified can form the dense film on the shale surface.It not only can play the role of “preventive” strengthening,but also can effect as “remedial” strengthening.
