Mechanism Of Formation Slippage And Shear Casing Damage Of The Marker Bed In Sazhong Development Area Of Daqing Oilfield

In the process of oilfield development, casing damage always exists and is an important factor affecting oilfield production. Especially, casing damage caused by water flooding has been an important topic studied by scholars from home and abroad, but it has not been effectively solved.The casing damage in the marker bed of Sazhong area in Daqing Oilfield has distinct features compared with previous damages caused by water flooding, and a major feature of casing damage is that the casing damage form gives priority to the shear deformation and the fault and also has the regional focus and time concentration characteristic when it occurred. Besides, the layer of casing damage is consistent. Therefore, the corresponding experimental study and the finite element numerical simulation on casing damages in the marker bed of Sazhong development area in Daqing Oilfield were carried out, and it was expected to reveal the mechanism of the casing damage in the marker bed, and to provide a theoretical method for damage prevention and control methods in the process of oilfield development.Firstly, the casing damage form, the layer etc. in Sazhong development area were counted; the cause and the layer of casing damage were acquired, and the geologic and lithologic characteristic of the casing damage layer according to the previous research were studied; the study object of the rock mechanical experiments and the numerical simulation was defined.Then, the rock mechanical experiments of the shale and the mudstone in the marker bed were carried out; the shale and the mudstone tension strength, shear strength, uniaxial compressive strength, triaxial compressive strength, elastic modeling quantity, Poisson's ratio and other mechanical parameters were obtained through rock mechanical experiments. The experiment results show that the shale and mudstone mechanics parameters show obvious differences for different coring direction, and obvious anisotropic characteristics; the shale and the mudstone are easier to be broken along the bedding direction than others, the shale cohesive force among bedding is obviously lower than that of the mudstone. Therefore, the shale is easier to break along the bedding and thus leads to horizontal fractures.The finite element mathematical model is established according to the formation lithology characteristics of the marker bed. According to the finite element numerical simulation, we conclude that the formation elevation leads to stress concentration in marker bed; the marker bedding architecture develops and the shear strength is low. Therefore, the marker bed is inclined to become stress free bed and generate formation slip, leading to casing shearing deformation and fault. The effect of the thickness of upper reservoir, the marker bed relative position and other parameters on the formation slip were analyzed, and then the functional relationship between the formation slip and effective factors to the numerical simulation was obtained; and finally the calculation model of maximum formation slippage was established.Based on the fluid-solid coupling theory, using the actual formation parameters of the experimental area, and combining the results of rock mechanics experiments, the finite element numerical model of the marker bed formation slip was established. With numerical simulation, the formation elevation value, the relative formation slippage of the marker bed and other parameters were calculated from 2009 to 2010, the calculation results illustrate that the longitudinal displacement located in the high pressure block is larger, but the marker bed formation slip in the low pressure block is larger, and coincidence rate between the calculation results and actual situation results is 86.76%.Finally, using the established three-dimensional finite element model of the experimental area, the variation distribution of formation slip on the condition of various regional pressure differences was calculated; the critical regional pressure difference of the marker bed casing damage was calculated by using the method of the inverse distance weight, and the calculation results illustrate that the lower regional pressure difference is, the smaller the corresponding marker bed slip is; and the upper value of the regional security pressure difference in experimental area is 0.67MPa.