A New Approach to Prevent Lost Circulation While Drilling in Fracture-Cavity Type Carbonate Reservoirs: Theory, Numerical Assessment and Experimental Study

Abstract

Lost circulation often occurs during drilling and completion processes of geothermal and hydrocarbon resources development. There have been mature plugging techniques for preventing drilling fluid loss in high-permeability and fractured formations. However, the plugging materials are so hard to reside that effective solutions of preventing serious karst cave-type leakage still need to be improved. This paper presents the application of a novel technique, known as energy-gathered bundle type nesting plugging and wall reinforcing device, in fracture-cavity type carbonate reservoirs using both numerical simulation and experiments. Firstly, the composition and function of this new technique are presented. The invented device comprises external slotted metal pipes and an internal nesting explosion tool. Either spiral slits or straight slits are cut on the slotted metal pipe. And the nesting explosion tool is a mandrel with several presetting grooves, in which explosives are placed. The plugging can be achieved by forming a crossed metal bridge-structure, which is to prepare small spaces for the refilling and better residing of lost circulation materials. Secondly, a coupled wellbore and cavity reservoir model was established to study the impact of the generated downhole explosion on wellbore stability. The explosion load was added at the fixed locations of limestone and dolomite formations. And the stress analysis was performed on both wellbore and cavity formation. Next, another coupled explosive-mechanical model was established. The deformation of this device induced by explosion was simulated in order to analyze the performance of different kinds of slotted metal pipes and to optimize the bridging material and the slotting structure. Finally, several explosion experiments were conducted in air, submerged water, and downhole environment respectively to verify the feasibility of this technique. This study indicates that explosion impacts on limestone and dolomite formations are close. Although the downhole explosion has effects on stress distribution of the borehole wall and the formation in both vertical and horizontal directions, the wellbore collapse would not be caused. The series 5 aluminum alloy pipe with straight slotting patterns, exhibits a suitable tensile deformation without failure under the loading explosion of detonating cord, and can be utilized as the plugging tool. In conclusion, the presented technique can solve the problem that the lost circulation material does not readily remain to form an artificial borehole wall, and can be an effective solution to improve the plugging results in fracture-cavity type carbonate reservoirs.