# A New Prediction Model of Annular Pressure Buildup for Offshore Wells
## 1 Introduction  
Annular pressure buildup is caused by fluid expansion due to temperature rise, which in turn leads to an increase in pressure in the annular space between the layers of the casing [1; 2; 3]. For onshore and shallow water wells, annular pressure buildup can be reduced by wellhead pressure relief. However, for deepwater wells where the wellhead is located underwater, the annular pressure cannot be released or reduced [4; 5]. Therefore, the accurate prediction of annular pressure buildup is crucial for the casing design and safe production of deepwater oil and gas wells [6; 7; 8]. After several hours of production in Well A-2 of the Marlin Oilfield in the Gulf of Mexico, the high-temperature fluid caused the annular pressure to rise, resulting in the collapse of the production casing [9; 10; 11; 12]. During the drilling of Well Pompano A-31 in the Gulf of Mexico, the high circulating temperature of the drilling fluid caused the high annular pressure, resulting in the collapse of the 16''casing [13; 14]. [PERSON] [15] studied the design and strength analysis of multi-layer tubing-casing and proposed the use of SLA (Service Life Analysis) based on the wellbore string system to check the overall loading of wellbore strings. The method takes into account the additional loads generated by the thermal expansion of annular fluid for the first time. [PERSON] [16] applied the SLA casing design method to study and analyze the casing strength of HPHT wells in the field. [PERSON] and [PERSON] [17] took into account the fluid nonlinear PTV relationship and elastic deformation of the casing and performed a rigorous mathematical derivation. [PERSON] [18] proposed that the annular pressure buildup is important to the wellbore. The thermal expansion of confined annulus fluid can cause packer leakage, and a high wellbore profile temperature can cause tubing bending. [PERSON] [19] and [PERSON] [15], respectively, proposed that the annular fluids generateannular pressure buildup through being heated. [PERSON] [20] proposed that the annular pressure buildup is mainly caused by the thermal expansion/compression characteristics of the fluid trapped in the annulus, the deformation of the casing, and the migration of the fluid into the annulus, and established a calculation model for annular pressure buildup. In previous studies, only the impact of casing-free section deformation on annular pressure buildup was investigated. However, the impact of casing sealing section deformation on annular pressure buildup has not been studied [21; 22; 23; 24; 25]. [PERSON] [26] simplified the calculation of casing deformation in the PVT equation and gave suggestions on how to prevent annular pressure. In 2006, [PERSON] [27] proposed an iterative model to predict annular pressure. Considering the coupling effect of annular pressure and volume, the calculation result of the iterative method is 10% smaller than that of the traditional method, mainly due to the influence of annular volume change. [PERSON] et al. [28] established a calculation model of annulus pressure by using the plane strain equation and analyzed the influences of reservoir temperature, bottom-hole temperature, fluid properties, production flow rate, and well mechanism on annulus pressure buildup model for the wellbore-reservoir system. The annular temperature calculated by this model was more accurate than that calculated by the traditional semistable method. In 2009 and 2010, [PERSON] [23] proposed two models, semi-steady state and transient, to calculate the annular pressure in the production process, in which the thermal expansion of the fluid and the leakage effect of the fluid were strictly considered. In order to illustrate the influence of casing sealing section deformation on annular pressure buildup, in this paper, a coordination deformation model of casing-cement sheath-formation was established, and then a new multi-string annular pressure buildup model was proposed. The influence of casing sealing section deformation on annular pressure was analyzed.