A Hybrid Model for Predicting the Likelihood of Differential Sticking in Offshore Wells
Authors: Sampaio Jorge H B , Lourenco Affonso 
Published: June 2014

Abstract
An innovative theoretical model to quantify the risk of differential sticking is presented. The proposed risk assessment is based on the concept of likelihood versus consequence. The likelihood of the problem's occurrence in a given wellbore segment (case) is evaluated from a knowledge-based model and translated by a similarity measure of relevant operational conditions between the target case and historical cases with known outcomes. This standalone module performed satisfactorily and predicts the likelihood of occurrence by more than a chance probability, demonstrated by a rate of sixty eight percent (68%) correct predictions against field data from forty four wells drilled by different operators in several fields. The consequence assessment is performed through one-dimensional mechanistic model that predicts the downhole overpull (differential sticking force), which is not detailed in this article. Together, the models serve as a risk assessment tool able to correctly describe risky operational trends while designing or drilling wells, with critical situations being defined as high likelihood plus high potential overpulls. Both models utilize unique experimental data about mechanical properties of drilling fluids filtercakes (hardness, torque resistance and adhesion-cohesion strength) under simulated downhole conditions, raised through the HTHP Mudcake Characterization Equipment (HPHT MCCE) developed during the course of this research work.
Moreover, the study contributes towards the development of modern predictive models aiming at combining large amount of available operational drilling data (LWD, PWD, mud logging, survey, drilling reports, etc), expert's knowledge, laboratory data and phenomenological models in order to optimize drilling operations.

Introduction
Stuck pipe is a general term used, particularly among drillers, to describe the problem of losing the ability to move the drillstring assembly. The causes of stuck pipe are many and they can be classified into two categories: mechanical and differential sticking. The classification is based on the physical mechanism causing the problem, and therefore mechanical sticking refers to the cases where the movement of the drillstring is prevented by mechanical means. The most common causes of mechanical sticking are accumulation of drilled cuttings (inadequate hole cleaning), borehole instability (pack off), the closing of borehole by reactive formations (clay swelling), the drilling of unsymmetrical sections of the borehole (key seating) and the presence of junk (expression referring to any undesirable object or part of drillstring left in the well).
Differential sticking is different. The locking mechanism is caused by a force, acting radially, that pushes the drillstring against the borehole wall. This force is generated whenever a positive differential pressure exists between the borehole and the formation being drilled. The drillstring becomes exposed to the differential force whenever it is embedded in the filtercake formed by solids present in the drilling fluid and deposited on the borehole wall during filtration through a permeable formation. The problem resurged with more significant consequences due to the expansion of the horizons of hydrocarbon exploration from onshore shallow vertical wells to deep offshore wells with complex trajectories. The time to free a stuck pipe can range from hours to weeks, rapidly becoming a very expensive exercise with disastrous consequences to the cost of drilling campaigns.