A Transient Flow Model for Predicting Pressure Buildup in Closed Annuli
Authors: Udegbunam John Emeka , Sui Dan , Moeinikia Fatemeh , Lage Antonio C. V. M. , Fjelde Kjell Kåre , Arild Øystein , Rabenjafimanantsoa Herimonja A. , Nygaard Gerhard H. 
Published: October 2017

The annuli between two casings can be closed or open to formation. After completion, the temperature of the annular fluids will be close to the formation temperature. This is because it will take some time for the well to begin to exchange heat with the produced fluids from the reservoir. If the well is located in deep water, the wellhead temperature will be equal to the bottom seawater temperature.
At the start of production, warm reservoir fluids will be transported upward to the surface. This will transport heat to the upper part of the well and heat up fluids in the closed annuli. Because of the space limitation, the fluids are not allowed to expand. As a result, the pressure in each annulus will build up with time. In the worst case, this can lead to casing failures. A similar situation may occur when a high pressure, high temperature (HPHT) well is shut in and the well temperature approaches the formation temperature. If there is a net heating of the well, a pressure buildup will be exerted on the blowout preventer (BOP) system.
An example well consists of several casings and producing tubing. Each annulus is filled with a fluid, which can be a water-based or an oil-based fluid. In this study, a simple fluid, which comprises water and barite with no additives, is considered. The monodisperse suspension will be used to investigate the barite settling in a closed A-annulus. It is expected that the settling process will be much faster than it would have been if the fluid is a gel. The production casing and packers serve as barrier elements in the well.
This paper gives an overview and challenges associated with temperature-induced pressure increase in closed annuli. The paper also considers barite settling — another effect responsible for annular pressure buildup (APB) in closed annuli. In addition, remedies for APB will be reviewed.
The dynamic density and volume behavior of a fluid depend on pressure and temperature. To predict APB, a transient flow model is required. The main objective is to demonstrate that the AUSMV scheme can simulate APB due to barite settling and temperature increase. The AUSMV is a hybrid explicit numerical scheme that combines advection upstream splitting method (AUSM) and flux vector splitting (FVS) method.
A transient hydraulic model should be able to capture the dynamics of the settling process where settling particles force compressible liquid upward, resulting in pressure buildup in a sealed annular space. The proposed model captures these effects and takes the compressibility of the annular fluid into account. Another advantage of the transient model is that it can also predict the local concentration of barite in the annulus. This is important for a closer integration of the scheme with a temperature model, because heat flux depends on the spatial and time-varying compositions of the fluid in the annulus.
The AUMSV scheme is one out of many numerical methods that can be used for solving mass transport problems. It has been chosen due to its simplicity with respect to implementation. In the present work, the annular temperatures predicted by a separate transient temperature model will serve as inputs to the numerical scheme.