High-Strength, High-Stability Pill System to Prevent Lost Circulation
Authors: Xu Cheng-yuan , Kang Yi-li , You Li-jun , Li Song , Chen Fei 
Published: May 2016

Abstract
A new high-strength, high-stability (HSHS) pill system for controlling lost circulation has been developed and optimized on the base of a physical model of stable plugged zone. This new HSHS pill system provides a stronger and more effective seal than traditional treatments.
Controlling lost circulation with plugged zone formed with lost circulation material (LCM) in the fracture has achieved tremendous success in the past years. However, investigation into the strength and stability of the plugged zone has not been reported. Ignorance of such knowledge often leads to excess costs from repeatedly fluid loss and rig time, 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 considering which the physical model of stable plugged zone is established. The pill system based on the model provides an engineered combination of rigid granules, fibers and deformable 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 described the laboratory-scale tests, which 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. This technology can also be used to guide the design of wellbore strengthening scheme and make sure the long-term effectiveness of wellbore strengthening measures.

Introduction
The formation pressure-bearing capacity is the comprehensive reflection of structural integrity and strength of formation, drilling fluid property and the interaction between formation and drilling fluid. Low strength of rock, high development degree of fracture, poor plugging ability of drilling fluid often lead to wellbore breakdown, natural and induced fracture propagation. This can be seen by formation low pressure-bearing capacity, massive losses of drilling fluid associated with other downhole troubles, which seriously hindered the development of oil and gas resources.
An API study published in 1991 includes data1 indicating that up to 45% of all wells require an intermediate casing string to prevent severe lost circulation while drilling to total depth (TD). Even when using these extra strings, lost circulation events still occurred in 18 to 26% of all hole sections. Some areas reported many more occurrences of lost circulation events ranging from 40% to 80% of wells. In recent years, these percentages have increased as the number of shallow, easy-to-find reservoirs has steadily declined and industry operators have intensified their search for deeper reservoirs and drilled through depleted or partially depleted formations. Conventional LCM including pills, squeezes, pretreatments, and drilling procedures employing equivalent circulating density (ECD) management have reached their limit in effectiveness and become unsuccessful in the deeper hole conditions where some formations are depleted, structurally weak, or naturally fractured and faulted.