Safety Evaluation For Down-hole Casings During Well Completion In Their Service Life

Well completion is an important process of production in petroleum exploration and exploitation. The casing load condition during well testing is more abominable than that during drilling since the casing load is mutative. The survey found that collapsing strength and internal pressure strength of casing may be reduced because of wear and perforation and many of them were collapsed even ruptured for they can’t bear reservoir pressure outside of casing and differential pressure of water column pressure inside of casing. So the remained strength of wear casing and perforating casing of testing well and the load and security during well completion were analyzed in this text, and several aspects of the research work about the text was mainly completed as follows:(1) Analysis of wear mechanism, influence factor and wear depth of casing.The wear-efficiency model was chosen on the basis of wear mechanism of casing and the analysis of influence factor. The analysis method of wear extent of casing was given according to parameters offered by history of drilling. The wear-efficiency of S135drill pipe with N80casing was test through the10pairs worn samples and the influences of contacting force, revolution of table, wear time to the wear-efficiency were observed and studied, which were basic data of analysis of wear depth of casing.(2) Analysis of worn casing strength under non-uniform in-situ stressAssumption that the advanced degree of worn casing equaled to ideal circular tube, when collapsing strength under non-uniform in-situ stress compared with uniform in-situ stress. According to the collapsing strength calculation formula of the ideal pipe, the collapsing strength calculation formula of the worn pipe under non-uniform in-situ stress was created. The stress of worn casing was analyzed by using finite element analysis software of ANSYS, and the changing regularities of the worn casing maximum equivalent stress were obtained, with the factors of worn location, depth, coefficient of degree of uniformity of load and temperature.(3) Analysis of mechanical property of perforated casingThe residual strength of perforated casing which influenced by perforating parameters and stress concentration coefficient was analyzed by using finite element analysis software. The conclusions were as follows:as the same aperture and perforating density, the stress concentration coefficient around perforation changed significantly with the increasing of perforating phase; as the same aperture and perforating phase, with the increasing of perforating density, the maximum equivalent stress of perforated casing increased and the stress concentration coefficient around perforation decreased; as the same perforating density and phase, with the increasing of aperture, the maximum equivalent stress of perforated casing increased, but the stress concentration coefficient around perforation changed slightly. Meanwhile, considering the temperature loads, the hot structure coupling of perforated casing under acidification condition was analyzed in this paper. The bigger the temperature difference between inner wall of perforated casing and layer was, the bigger the growing rate of the maximum equivalent stress of perforated casing was.Based on the above conclusions, allowable worn depth of casing and reasonable perforated parameters under the non-uniform stress were given, which can provide reference of deep well casing string strength design.