Structural Reliability: Assessing the Condition and Reliability of Casing in Compacting Reservoirs
Authors: Chantose Prasongsit , Gardoni Paolo , Schubert Jerome Jacob , Teodoriu Catalin 
Published: March 2015

Abstract
Casing has a higher likelihood of failure in compacting reservoir than in a typical reservoir. Casing fails because reservoir compaction induces compression and shear stresses onto it. The compaction occurs as reservoir pressure depletes during production. High compacted reservoirs typically are composed of unconsolidated, overpressured rocks such as chalk, diatomite, and sandstone. Pore pressure depletion increases effective stress, which is the rock matrix stress pushing upward against overburden pressure. Effective stress may exceed rock compressive strength, inducing compaction. Wells in compacting reservoirs are likely to fail and to have high deformation rates.
This paper introduces the concept of structural reliability to quantify the probability of casing failure in compacting reservoirs. Probabilistic models for casing capacities are developed using current design methods and a reservoir compaction load observed using finite-element model simulations. The proposed probabilistic models are used to define two limit-states functions used in predicting the probability of casing failure for two possible modes of failure: axial yielding and buckling failures. A limit-state function describes the casing condition as the casing experiences a reservoir compaction load. The limit state function is the input in component and system analyses for estimating the probabilities of failure as reservoir pressure is depleting. Sensitivity and importance analyses are also performed to determine the role and significant of parameters and random variables affecting the casing reliability.
Applying the knowledge produced from this research to casing design can improve design reliabilities and forecast the probability of casing failure in compacting reservoirs.

Introduction
Casing failure and deformation are becoming problems in mature oilfields. The number of these oilfields has been increasing since 1980, where failures are a major problem to cause wells to stop producing or preventing necessary well services. The number of failure reported from Shengli oilfield, Ekofisk chalk field, and Belridge diatomite field combined to over 10,000 casing failures (Yodavich et al. 1998, Fredich et al. 1998).
A major cause of casing failure in these mature fields is reservoir compaction. Reservoir compaction causes casing to experience large additional stresses that casing may not be designed for. Assessing and predicting casing conditions in mature fields where reservoirs tend to compact are important in order to prevent casing failures. Many papers have shown casing failure mechanisms and reservoir compaction mechanisms using methods such as geomechanical simulation, analytical formulation, and probabilistic modeling.
Geomechanical simulations are widely used to determine high stress zones in the formation around the field. Earlier geomechanical models depend on 2D modeling to find crucial depths and compaction rates related to the amount of production (Chia et al. 1989, Fredich et al. 1998, Hansen et al.1995, Wooley et al. 1988). Today technology and software allows users to create 3D reservoir models or use an existing model from reservoir simulation to compute compaction rates and stresses in the formation (Bruno 2001, Hilbert et al. 1998). Modeling provides answers to casing failure mechanism as reservoirs compact and high compaction location. Although these models prove to be very useful, they cannot account for uncertainties from model parameters or methods used in simulation. Also, modeling results do not quantify the accuracy and correctness of its results. Geomechanical simulations also take time to build and run.