Simulation Experiment Study on Lost Circulation Control by Borehole Wall Strengthening

Laboratory simulation study was conducted on the strengthening of borehole walls to try to find a way of dealing with lost circulation under pressure and to reveal the micro-mechanisms of fracture plugging to strengthen the borehole walls. By comprehensively considering the effects of the closure pressure of fractures on their openings during borehole wall strengthening, a set of experiment apparatus was developed and a method for evaluating the plugging of fractures with variable openings established. Used with the apparatus and the evaluation method, two quantitative evaluation indicators, which are maximum plugging differential pressure and equivalent plugging position, were proposed. Compared with the commonly used particle size matching criterion, using the revised normal distribution particle size matching criterion, the pressure bearing capacity of a borehole wall can be increased by 2.36 times at most. Equivalent plugging position is inversely proportional to pressure bearing capacity, and the revised normal distribution continuous particle size matching criterion can form at the entry of fractures a thin and dense plugging layer. Increase the strength of the plugging materials to a level as high as possible, the effect of the well pressure fluctuation can be reduced; increase the elasticity of the borehole wall strengthening plugging agents, the adaptability of the plugging layer to the dynamic fractures can be improved. Moreover, appropriately increasing the suspension stability of the plugging system and reasonably reducing its injection rate both are beneficial to the formation of the pressure bearing plugging layer and to the improvement of borehole wall strengthening.