Research On Mechanical Integrity Of Cementing Casing And Cement Ring

With the progress of oil-gas exploitation, as an important design concept of the oil and gas field exploitation technique, wellbore integrity can not only ensure the security of well drilling, but also ensure the long-term stability and exploitation of oil and gas well production the seal integrity of casing and cement is an important content of wellbore integrity, its failure will cause pressure in the annular gas migration which is a serious threat to the security and economic exploitation of oil and gas wells. Currently, the casing and cement sheath seal integrity study mainly focus on its body stress, less research on the emergence and development of micro-annulus on the interface has been carried. therefore, it is necessary to do the relevant research work and evaluate the seal integrity of casing and cement at different production condition, which is useful for setting reasonable production condition of oil and gas wells exploitation.Based on the two aspects of the temperature effect and the mechanical performance of the material, the foundation of the integrity model is determined in this thesis. The change of temperature will lead to thermal stress and increase the risk of body failure to the casing and cement sheath. At the same time, it can also cause the mechanical parameters change of casing and cement sheath, which will then impact the stress state of casing and cement sheath. The triaxial stress curves of casing and cement rock show that the casing and cement stone will gradually transition from elastic state to plastic state during the loading process, Therefore, the model should be considered as elastic-plastic body material. The foreign equivalent physical test and micro CT scanning test show that interfacial micro-annulus can emerges and can lead to gas migration when no body failure has happened. Therefore, we should establish interfacial micro-annulus model of downhole casing and cement sheath to research its seal integrity.Based on the thermodynamics and elastic plastic mechanics theory, the thermal stress distribution model of temperature change and the elastic plastic body model of composite have been established in this thesis. Combining casing failure criterion and Tresca criterion, the thesis has built special case model Ⅰ,special case model Ⅱ and special case model Ⅲto study the body integrity of casing and cement sheath according to the material condition of the cement sheath under different internal pressure. At the same time, according to the mechanical behavior of the combination under the internal pressure loading and unloading, the interfacial micro-annulus model has been established. This thesis has analyzed the impact of internal pressure change, temperature change, cement elastic modulus and temperature change on the stress state and micro-annulus of the combination, which clearly shows that we should use the casing and cement body stress model combining its micro-annulus model to evaluate the combination’s underground sealing characteristics.based on the mechanical model,a Casing-Cement Integrity evaluation software has been Established. This thesis has use it to evaluate the body integrity and interface integrity of XX block XX well in the process of pressure test for application analysis and then get the reasonable construction conditions to ensure the well casing and cement sheath Seal integrity.