Particle Size Optimization of Lost Circulation Materials: A Comprehensive Experimental Study

Abstract

ABSTRACT Severe lost circulation in fractured formations is one of the most common and challenging problems in oil and gas drilling engineering. The lack of theoretical guidance on the selection of lost circulation materials (LCMs) for millimeter-level fractures results in unstable fracture bridging, which is prone to repeat failure. In this paper, a comprehensive laboratory experimental study was conducted with modified PPA equipment for millimeter-level fractures. The effectiveness of the LCMs is evaluated by fluid loss amount, pressure-bearing capacity, and the LCMs bridge location and integrity. Experimental results show that a single LCM size distribution cannot effectively seal millimeter-level fractures, while an optimized combination of bridging and filling particles can achieve successful sealing. It is recommended that the mass ratio of bridging particles to filling particles be 2:1. The D50 and D90 of the bridging particles should be 35-40% and 70-80% of the fracture entrance width, respectively. The D90 of the supplementary flaky materials, e.g., mica, should be slightly less than the fracture entrance width. The results of this laboratory study can provide guidance for selecting LCM size for effective fracture sealing and improved wellbore strengthening performance. INTRODUCTION During the drilling process, lost circulation happens when a lot of drilling fluid leaks into the formation. Lost circulation can result in the drilling fluid's depletion and prolong the drilling cycle. Improper management of lost circulation may also lead to complex situations, such as wellbore instability, blowouts, and stuck pipes, ultimately resulting in catastrophic engineering accidents. The lost circulation rate is estimated to account for approximately 20% to 25% of the total number of drilled wells. The annual cost of well plugging can reach $4 billion USD. In North America (the United States and Canada), the percentage of drilled wells that experience severe lost circulation in carbonate oil and gas reservoirs, shale oil and gas reservoirs, and other formations amounts to about 40% of the total. In the Middle East, the wells that experience severe lost circulation during the drilling of carbonate-fractured oil reservoirs account for more than 30% of the total, with the time loss due to lost circulation exceeding 50%.