Evaluation of a Tubing Stress Analysis Method to Design an Acid Fracturing String in a Complex Horizontal Well, Tarim Oilfield, China

Abstract

A carbonate reservoir in the Tarim oilfield has a deep burial depth (5000 to 7000 m), high temperature (130 to 150°C), and long horizontal section (600 to 1600 m). It is a heterogeneous formation with obvious interlayer physical differences. The requirements for using a multistage acid fracturing process (5 to 12 stages) in the stimulation program for this complex horizontal well completion highlight the challenges associated with ensuring tubing string integrity. Multistage acid fracturing string design is more difficult than conventional string design because the tubing string consists of multiple packers and sliding sleeves, which can allow for uneven fluid inflow distribution through the variable cross section. Thus, to understand the pressure decline profile resulting from localized heat loss, an accurate pressure and temperature simulation is important. The horizontal well process of running in—setting—release—tie-back complicates the calculation of initial conditions for string mechanics analysis. The tubing string operation process can be very intricate; as such, a proper tubing string stress analysis should consider the operation characteristics adequately. Based on technological characteristics, this paper introduces, in detail, a string design flow for the staged acid fracturing of a horizontal well. This paper provides detailed descriptions of the anticipated loads encountered during multistage acid fracturing, using a wellbore thermal and flow simulator to perform wellbore temperature and pressure simulation under different operation scenario conditions. In a standard tubular design study, a comprehensive model that includes the well configuration with the tubing string structure and operation characteristics is essential to adequately simulate the potential stress placed on tubing, packers, and sliding sleeves. This also would help to optimize the design of the tubing string composition and operational parameters. In view of the technological characteristics of acid fracturing strings for horizontal wells, this paper focuses on the introduction of design methods and considerations. An example horizontal well is used to describe the analytical process of string mechanics for staged acid fracturing.