Wellbore associated problems due to gas injection into non-horizontal structures

The rapid growth in natural gas consumption has increased the need for gas storage, in particular in the form of injection into depleted reservoirs. Also, CO2 sequestration into the depleted reservoirs has attracted a large attention recently. However, it is important to ensure that the injection pressure is maintained below a certain limit to avoid unsealing the cap rock or reactivation of any existing fracture planes within or above the reservoir rocks. in particular, it can be thought that gas injection into formations with non-horizontal structures, such as anticlines, is more problematic than horizontal formations due to the development of shear zones in such structures. This could potentially result in long term wellbore problems such as casing collapse or shearing along a fault or fracture plane intersecting the wellbore. in this study we compare the stress profile changes before and after gas injection into three structures: a horizontal and two anticlines with different slopes at their flanks. For this purposes a 3D numerical simulator was used. The program was developed using finite element method (FEM) and the code was written in Fortran.The stress magnitudes along curved profiles were compared for three structures at a similar depth. A limited extension of a porous zone was assumed in this study. The results indicate how as structure becomes more curvy in its geometry the likelihood of shear displacement increases. This is significantly important for well design purposes as it could reactivate any existing fault or fracture planes and result in cutting the wellbore or causing casing collapse.

Keywords:

Well design , CO2 sequestration , gas injection , reservoir , Geomechanics , fracture reactivation