Abstract
This paper aims to quantitatively explore effects of effective radial stress change (???'??) around a wellbore because of cement shrinkage in a 2D elastic isothermal state. As cement shrinks, the formation moves inward and the radial stresses diminish. This induced change can be problematic if the total radial stress falls below the formation pore pressure in the outside-the-casing environment (the hydraulic fracture condition). If this occurs (???? < po), fluid migration risk is massively enhanced because a connected vertical microannulus (fracture) is a pathway for buoyant upward gas movement. To model stress change, we use simple Kirsch equations and a computer code based on a radial shrinkage percentage in the cement to calculate the new effective radial stress. Similarly, using ABAQUS software, the possibility of surface detaching between the formation and cement bond has been modeled. Numerical results show that, based on the geomechanical properties of a formation and the cement shrinkage percentage value in the cement sheath, upward movement of fluid is possible, even likely in many circumstances.
1. INTRODUCTION
Gas Migration (GM), gas invasion, or gas seepage (in a more general form "fluid" migration) has been an oil and gas industry issue for decades. This problem is important for the oil and gas industry because in cases of formation fluid migration, the integrity of a wellbore will not be sustained completely and can lead to environmental issues [1]. Even if the environmental impacts are minimal, leakage events have a large social impact, and this aspect of the risk involved must be properly managed.
The term gas migration is commonly used, but a better understanding of the seepage mechanisms is essential. During a wellbore’s life, from well construction (drilling and completion) to decommissioning (well abandonment) and thereafter, the possibility of migration remains present. Not surprisingly, many researchers have tried to identify and explain the mechanisms [2, 3] from their perspectives, but a general scientifically supported consensus still seems unachieved.
