A Risk Analysis Approach Using Stress Analysis Models to Design for Cement Sheath Integrity in a Multilateral Well
Authors: Laidler Allan , Taoutaou Salim , Johnson Carl Robert , Quisel Natalia , Desroches Jean 
Published: February 2016

Abstract
Technical risk analysis was used during the planning phase of a critical cement job on a North Sea operator's platform to ensure complete zonal isolation across the production zones. The well was a multilateral with two legs, with heavy emphasis on the quality of interzonal isolation. The critical risk in this cement job was that the well would become uneconomical if either main-bore or lateral isolation failed. This well was chosen as a first-time application for a new stress-modeling and risk-analysis methodology using a complementary suite of software tools. The cement job optimization included analysis of all critical parameters to achieve optimum mud removal and mitigate the risk of cement sheath integrity failure.

Introduction
This paper describes the key steps of a design-execution-evaluation (DEE) process based on a technical risk analysis approach. In particular, the risk analysis process includes analysis of both the cement placement and stress analysis of the cement sheath. Mud-removal modeling was used in conjunction with a centralization program and fluids design software to achieve optimized placement. Stress-analysis calculations were used to identify possible cement failure mechanisms and propose an appropriate solution. Each step of the risk analysis approach will be illustrated by the steps taken during the design of the multilateral well.
A particular challenge that this well also presented was the drilling of the openhole section and successful running of the 7-in. liner to total depth with a special drilling assembly. The presence of the drilling assembly in the string required special simulations to determine the effect of increased shear on the slurry when pumping it through the bit, the first step is to determine the pressure drop across the motor, and the drilling bit nozzles, the second step is to simulate this extra shear in the wearing blender when mixing the cement slurry.
The thickening time is then performed and compared with the base case, in the event there are significant changes in the thickening time then retest until good results are obtained.
The multilateral well was cased and cemented using a novel engineered sealant material. The sealant incorporated flexible and expanding components within an optimized particle-size distribution blend. The blend was a customized design based on the stress and risk analysis results. Since placement, the well has been logged with sonic and ultrasonic tools and has yielded excellent bonding response, confirming that hydraulic isolation between the main bore and the lateral has been achieved.
At the end of the paper, a comparison of results obtained from two independent stress-analysis models a (semi-analytical and a finite element model) is also provided. This comparison illustrates the reliability of the semi-analytic stress analysis approach vs. the more complex and time consuming method of finite element analysis.

Design-execution-evaluation process
The basic DEE of a cementing operation can be regarded as the basic foundation for the entire zonal isolation process, which includes 3 main steps:technical risk analysis during the design phasecontrol of the cementing operation's critical parameters during the execution phaseevaluation of the integrity of the hydraulic isolation across the annulus and re-evaluation of the execution data to understand where the whole cycle can be improved
It should be appreciated that this process takes place from the early planning phase to the evaluation of the integrity of the sealant behind the casing and the re-evaluation of the execution data, in order to complete the 'feedback' loop as a means for continuous improvement. The aim of this paper is to demonstrate how this basic concept can be combined with the power of computer modelling tools and techniques to optimize each step of the process and minimize all the potential risks.

Tools in the Technical Risk-Analysis Approach
The ultimate goal of the approach presented in the paper is to ensure that the well achieves its full production potential through ensuring complete hydraulic zonal isolation.