Abstract
For high-pressure low-permeability wells, wellbore temperature drops drastically in high-rate and multistage acid fracturing process. Under the combined action of the swelling of tubing string and the contraction of annular fluid between packers, annular pressure between packers undergoes violent transient change in staged acid jobs, thereby deteriorating loading on the tubing string and packers. Based on the principle of energy conservation and wellbore heat conduction, the transient prediction of two-dimensional (2D) wellbore temperature field under pumping injection condition was established by considering the effects of heat generated by friction and convection heat exchange. Moreover, the effects of wellbore temperature/pressure changes on the annular volume between packers were analyzed. Furthermore, in combination with the transient prediction model of wellbore temperature, PVT state equation of annular fluid, the calculation model of tubing string radial deformation and the transient seepage equation of the formation, the transient prediction model of annular pressure between packers in high-pressure low-permeability wells was established. Finally, by taking a high-pressure low-permeability well as an example, annular pressure between packers was calculated and the forces on the packers and tubing string were analyzed. According to the prediction results, the tubing string, which was regarded to be safe using conventional design method, exhibited an extremely high risk of failure after taking into account the decrease in annular pressure between packers. Therefore, the decrease in annular pressure should be fully considered in the design of tubing string for high-pressure low-permeability wells in multistage acid fracturing process. In combination with sensitivity analysis results, it can be concluded that formation permeability, injection rate and formation pressure all affected the change in annular pressure between packers.
Highlights
For some considerations in design and construction, one or more enclosed fluid space may exist in oil-gas well structures such as annulus between tubing and casing with packer strings and various casing annulus including free casing segment
During well testing and production, fluid temperature in various enclosed annulus within the wellbore increases with the rapid increase in wellbore temperature, which can lead to the increasing pressure in the annular enclosed space and may trigger a series of problems mainly including the failures of tubing string and casing string as well as the elevation of wellhead (Hasan et al, 2010; Yang et al, 2013b)
Since the 1980s, scholars all over the world have conducted a great deal of research on the annular pressure in wellbore enclosed space, examined the annular pressure induced by the rise in wellbore temperature, established the related theoretical calculation models (Halal et al, 1994; Hu et al, 2012; Oudeman and Kerem, 2006; Yin and Gao, 2014) and raised some risk mitigation measures in accordance with different well conditions and requirements (Ezell et al, 2010; Sathuvalli et al, 2005; Tahmourpour et al, 2010; Williamson et al, 2003)
Summary
For some considerations in design and construction, one or more enclosed fluid space may exist in oil-gas well structures such as annulus between tubing and casing with packer strings and various casing annulus including free casing segment. Scholars have mainly focused on the increase in annular increase induced by pressure/temperature. A typical technology in the development of low-permeability oil/gas reservoirs is high-pumping-pressure, high-rate and multistage acid fracturing jobs, during which the wellbore temperature drops drastically. The loading conditions in the operating tubing strings and the packer deteriorate, thereby causing the failures of tubing string and packers At present, both tubing string and packers are generally designed without considering the transient change in annular pressure between packers. The transient prediction model of annular pressure between packers is constructed and the related prediction method is validated by an application example
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