A novel method to couple well bore flow to reservoir flow to understand the impact of a well bore failure during CO2 sequestration operation
Creators: name: Pawar, Rajesh
name: Zyvoloski, George
name: Pawar, Rajesh
Publisher: name: XVI International Conference on Computational Methods in Water Resources
Year: 2006

description: One of the major concerns of geologic sequestration is consequences of a failed 
well bore and subsequent release of CO2.  The release can be slow or catastrophic 
depending on the mode of well bore failure.  It is important to predict the release 
characteristics of a failed well bore in order to develop monitoring plans, to 
understand potential impact and to develop strategies to mitigate the leak. In 
order to understand the total amount of CO2 that can escape due to release through 
failed well bores, it is necessary to perform numerical simulations coupling well 
bore flow to reservoir flow.  These are complex simulations, requiring solution of 
coupled equations describing mass and heat transfer processes occurring between the 
well bore and surrounding rock.  In addition, it is important to capture the CO2 
phase change behavior as it escapes through well bores as it can impact the flow 
behavior through the well bore.  Traditionally, adding well bores to reservoirs to 
capture short and long-term dynamic behavior in the vicinity of the well bore has 
required refining the computational grid.  In addition, to accurately capture the 
thermodynamic effects within the well bore (such as density changes) requires 
representing the well bore in multiple segments.  The traditional approach results 
in an extremely large computational grid for a reservoir with multiple well bores.  
In addition, the traditional approach has limited flexibility when introducing new 
well bores to an existing computational grid.  
We have developed a computationally efficient method to couple well bores to an 
existing numerical grid using FEHM (Finite Element Heat and Mass), the Los Alamos 
National Laboratory’s numerical code for simulating heat and mass flow in porous 
media.  FEHM uses a control volume finite element approach to solve numerical 
equations for heat and mass flow.  The equations are generated using an 
unstructured grid approximation and are solved using a multiple degrees of freedom 
Krylov space method.  This approach has the advantage where new connections to an 
existing (primary) grid can be easily added or modified, without needing to 
regenerate a completely new grid or create a large computational grid.  We use this 
capability to add well bores to an existing grid.  The well bore is added by 
connecting well bore nodes to the primary computational grid nodes.  The addition 
is done in a way that does not require modifying existing grid block 
connectivities.  The equations representing the physics of well bore flow are 
coupled to the reservoir flow equations through an implicit formulation.  The 
approach can be used to connect multiple well bores, each with multiple segments, 
at any desired locations within an existing grid.  This approach does not lead to 
large computational grids and results in a computationally efficient method to 
simulate coupled well bore and reservoir flow.  In this paper we provide details of 
the above-mentioned approach.  We also provide results of the simulations comparing 
the solutions obtained using the new approach to the solutions obtained using the 
analytical approach as well as the numerical approach where a highly refined grid 
is used to represent the well bores.