Investigation of Long-Term Casing Strength Degradation Prediction Method Under "Thermal Stress – Corrosion" Combined Effect in Cyclic Steam Injection Wells

Abstract

ABSTRACT Steam huff and puff is the main production mode of heavy oil, but the casing damage problems during the cyclic thermal injection process are very serious. The casing damage of thermal recovery wells is an important factor affecting oilfield production and economic benefits. The conventional casing strength calculation method does not consider the effect of corrosion and thermal degradation in terms of long-term casing strength designing. The establishment of a long-term casing strength prediction method to check and optimize casing strength is an important guarantee for the safe production of heavy oil thermal recovery wells. In this paper, the strength degradation characteristic of two kinds of casing steels (N80 and TP100H) are tested under the cyclic thermal effect using a High Temperature Tensile Testing Machine. The heat temperature is from 30˚C to 350˚C, and heating cycles are 1, 3, 5 respectively. The results show that the N80 casing strength decreases by 18.2% in the first thermal cycle, and the average damage of each subsequent thermal cycles are about 0.88%. However, the TP100H decreases by 13.7% in the first cycle and subsequently with 0.31% degradation averagely. The long-term corrosion rates of N80 and TP100H were measured in HTHP autoclave, which are 0.43m/a and 0.3m/a respectively. Combining the obtained experimental results with the standard casing strength calculation formulas, a newly prediction method of casing long-term strength for cyclic thermal injection well is presented. This method has been successfully used in Lvda 21-2 block of Bohai oil field CNOOC, which provides a technical basis for casing strength design in cyclic thermal injection wells. INTRODUCTION Casing damage in oil fields has now become a pressing problem worldwide, and many fields have encountered casing damage during development. The factors affecting casing damage are extremely complex. Steam injection is the most widely used and effective method in the exploitation of heavy oil reservoir. However, in the process of steam huff and puff, the high temperature steam injected in circulation makes the casing undergo great alternating stress, and the residual stress increases constantly, reducing the casing strength. In addition, after steam injection, thermal cracking of heavy oil produces acidic gas and the casing is in a corrosive environment. This leads to the casing strength decay and reduces the casing service life. Therefore, casing damage is a difficult problem in the development of heavy oil reservoir.