Study On Casing Damage Mechanism And Prevention Technology Of Complicated Horizontal Wells

Horizontal well technology is wide used in the oil and gas exploitation at home and abroad for its unique advantages. And many new types of horizontal wells are derived continuously to meet the needs of different type reservoirs exploitations, such as short-radius horizontal wells, multi-lateral horizontal wells and thermal recovery horizontal wells. The implementation and promotion of these new type horizontal wells greatly improve the oil and gas extraction rate, and significantly reduce the tons of crude oil production costs, but which also take higher requirements and challenges for the design and safety assessment of horizontal well completion casing strings. Some of the horizontal well casing completion technologies have become one of the key influencing factors restricting the use of horizontal well technology in certain areas.The main aims of this dissertation is to obtain the stress distributions and safety level of horizontal well casing completion strings during completion, operation and thermal recovery processes. Many existing researches, engineering experiences and theories are used during the study, which include modern mathematics, mechanics, computer and oil machinery theory.The stress and deformation calculating equations of horizontal well casing string under different work stations are deduced and verified by the results of experiments and numerical simulations. The computer software for horizontal well completion casing strings design and check is compiled basing on the calculating equations which can afford reference and data to take measures and recommendations preventing casings failures. The detailed studies of this dissertation can be made up by the following parts.(1) The stress distributions and the safety factors of horizontal well casing string are obtained according to the characteristics of loads and impact factors on the casing strings during completions and operations (such as fracturing). The safety of horizontal well casing strings is evaluated basing on the results. The recommendations of improving casing safety factor are also given.(2) The horizontal well casing strings collapse model is established and deduced the calculating equations, which locates at the built-up parts of horizontal well. The theoretical calculated results are tested by laboratory test and three-dimensional finite element simulation results. Various factors on the horizontal well casing strings collapse strength are discussed. These research results can serve as the references to the casing strings design at the built-up parts.(3) The collapse pressures of casing subjected to bending loads are studied. The calculating results show that the stress produced by bending loads is the main reseaon leading the casing failed. The ovality produced by bending loads has little effect on the collapse pressure of casing, which can be neglected in the actural engineering problems. According to the neglection, the formula of casing collapse strength subjected to bending loads has been simplified to facilitate on-site use and promotion.(4) Basing on the above findings, stress analysis and strength check software of horizontal well completion strings are compiled by VB computer language. Many horizontal wells completion casing strings were simulated by the software, their safeties were evaluated.(5) The plastic hinge model of perforated liner casing is built up to calculate the collapse pressure according to the strength and deformation behavior of perforated liner casing subjected to external pressure. The calculation equations of collapse pressure of perforated liner casing with initial ovalization are deduced by energy method. The influences of phrase angle, diameter of perforation, shot density, initial ovalization, diameter-thickness ratio and yield stress on the collapse pressure of perforated liner casing are studied.(6) The failure types of thermal recovery horizontal wells casing strings are classified as strength destruction and instability. The force model of completion casing during thermal recovery process is established the study the influences of different factors on the casing string damages. The recommendations and measures to prevent casing damage are proposed.(7) According to the contents of the above studies, different comprehensive measures to protect horizontal wells casing strings are carried out catering the different casing damage mechanisms, which are beneficial to extend the life of horizontal well casing strings.