High-Strength, High-Stability Pill System To Prevent Lost Circulation

Abstract

Summary A high-strength, high-stability (HSHS) pill system for controlling lost circulation has been developed and optimized on the basis of a physical model of a stable plugged zone. This HSHS pill system provides a stronger and more-effective seal than traditional treatments. Controlling lost circulation with a plugged zone formed with lost-circulation material (LCM) in the fracture has achieved tremendous success in past years. However, investigation into the strength and stability of the plugged zone has not been reported. Lack of such knowledge often leads to excess costs from repeated fluid loss and rig time; it increases the difficulty and complexity of loss-zone diagnosis. The new HSHS pill system addresses these shortcomings. Surface-friction coefficient, LCM volume fraction, and amount of contact deformation are the main influencing factors of the strength and stability of the plugged zone. The strength of the plugged zone is enhanced with the increase of the above factors. A physical model for stable plugged zone is established considering the three factors. The pill system that is based on the model provides an engineered combination of rigid granules, fibers, and resilient particles. The sealing efficiency and the pressure-bearing capacity are greatly enhanced. It was validated in several field trials in west China. Operational practices that facilitate the safe use of the HSHS system with overbalance exceeding 2,174 psi are discussed. In addition to the field-trial results, this paper also describes the laboratory experiments, that were used for developing the new system. With the development of the physical model and the HSHS pill system, it is now possible to optimize and select the types, properties, and matching relations of the LCM. One can also use this technology to guide the design of the wellbore-strengthening scheme and to make sure of the long-term effectiveness of wellbore-strengthening measures.