Annular Pressure Build-up Analysis and Methodology with Examples from Multifrac Horizontal Wells and HPHT Reservoirs

Abstract

Abstract Annulus pressure build-up (APB) remains an important design consideration for many wells, not just deepwater or subsea wells. This paper outlines a step-by-step methodology for analysing APB issues applicable to any type of well. Analyses of APB scenarios for a tight chalk oil reservoir and an HPHT gas-condensate reservoir in the Danish Sector of the North Sea are used to demonstrate the methodology. APB is a potentially serious issue with HPHT wells created by annuli that heat up during production. The increased temperatures cause fluid expansion that can potentially over-stress the casing and tubing if not mitigated. Specific issues for HPHT wells are presented. The significant increase in the use of multi-stage horizontal fracturing systems with open or cased hole packers and ball or intervention operated sliding sleeves creates a fluid contraction threat. Overpressure through annulus fluid contraction caused by cooling has been rarely analysed. A case is shown to disprove a common belief that the fluid external to the sleeves equalizes with the reservoir over the time frame of the stimulation operation which prevents over-pressurization. Failure cases are presented along with the design calculations required to assess the combination of tubing ballooning, fluid contraction / expansion and transient reservoir flow. It is demonstrated that with cases of toe-to-heel stimulation combined with low reservoir permeabilities, significant transient drops in pressure external to the sleeves can occur. This can lead to tubing, sleeve or packer failures.