Junction stability is one of the major concerns while drilling multilateral wells. Conventional wellbore stability analyses are usually assumed under isothermal conditions. However, thermal effect due to mud circulation might have direct consequence on the stability of the wellbore. If formation rock is cooled significantly by mud, stresses around the wellbore will change considerably which may lead to failure. The computed temperature profile around the wellbore was validated with the analytical solution. The results showed that the cooling would lower the fracture gradient around wellbore and its influence could reach eight times of the wellbore radius after circulation mud for 24 hours. Excessive reduction of fracture gradient will reduce safe drilling mud weight window and may lead to significant mud loss. Such effect of cooling is an important stability factor for the multilateral junctions where initial mud weight window is narrow, reservoir temperature is high, or completion operations require longer time.

1. INTRODUCTION

Problems occurring while drilling oil and gas wells result in non-productive drilling time. Wellbore instability is caused by the changes of the in-situ stresses due to rock removal, temperature change during mud circulation and other factors. The literature is rich in addressing wellbore stability analysis involving single wellbore (Frydman and da Fontoura 2000; Karstad and Aadnøy 2005; Hawkes 2007; Pasic et al. 2007; Ahmed et al. 2009). Compared to single wellbores, multilateral junctions exhibit greater risk to wellbore instability because of their excavation geometry. It is worth to mention that most of the previous work on multilateral junction stability focused on instantaneous drilling and isothermal conditions (Mohamad-Hussein and Ni 2018; Aadnøy and Edland 1999; Bargui and Abousleiman 2000; Manríquez et al. 2004; Plischke et al. 2004).

The impact of cooling on multilateral junction stability is rarely addressed by researchers. Thermal cooling due to the differential temperature gradient between the mud and the formation fluid can have a large impact on stress changes around the wellbore and the junction. Such considerable changes should not be ignored, otherwise the wellbore model won't be reliable for stability purposes. This paper is devoted to examining the impact of cooling on multilateral junction stability. A workflow is proposed for modelling the changes of stresses around the wellbore over a period of mud circulation using finite element solvers. The workflow involves creating a high-resolution 3D near wellbore model that contains full description of mechanical properties, in-situ stresses, pore pressure and formation temperature.

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