Shale has been known to be the source of wellbore instability during the drilling process. Organic rich shales are anisotropic due to their laminated structure and chemical properties. The goal of this study is to evaluate anisotropic mechanical properties of shale by triaxial tests, and predict shale anisotropic properties by well logging data interpretation. Shale samples were prepared with bedding plane inclination angles equal to 0 degrees, 45 degrees, and 90 degrees. Young's modulus, shear modulus, and Poisson's ratio in different directions were measured for a sample with 0 degrees bedding plane inclination angle. Parameters of the stiffness tensor were calculated by mechanical properties. Compressive strength was measured under different confining pressures of 0 psi, 500 psi, 1000 psi, and 1500 psi. The strength properties of shale samples were evaluated by both compressive strength and tensile strength. Simple Plane of Weakness and Modified Cam Clay failure criteria were applied to describe shear failure mechanisms. A scanning electron microscope method was used for the comparison of micro structures between the intact shale sample and failed sample with different bedding plane inclination angles. Well logging data was used to connect experimental lab data and field data. Compressional wave velocity was predicted with different inclination angles by stiffness parameters. The predicted compressional wave velocity for a 45-degree inclination angle showed a perfect fit with the field logging data. Steps of inverse sonic log data to stiffness parameters were shown by a flow chart. The UCS strength for 0 degrees and 45 degrees was predicted by several empirical relations using sonic logging data. The safe mud window for this special shale formation is predicted by experimental data. As shown in experimental results, our shale sample has a weak direction for both failure criteria. Well logging data and experimental data can be connected, especially by sonic log data. However, to predict shale anisotropic strength through well logging still requires more effort. The novelty of the process which connects experimental results and well logging data will be helpful for solving instability problems occurring in shale formation.

Creep, the time-dependent deformation of rock, will increase the pressure applied on the interface between the cement and formation. The objective of this paper is to study the influence of the formation creeping effect on the cement sheath integrity and zonal isolation. It focuses on the failure behavior of the cement sheath in the long period after drilling. The paper also investigates the changing of mechanical properties of cement to avoid loss of zonal isolation. The interface pressure between the cement and formation cannot be measured directly in the field, so it will be valuable to predict this pressure through alternative methods. A Casing-Cement -Formation System (CCFS) analytical model based on linear-elasticity and Cam-Clay plasticity model was built. The CCFS model includes four layers, casing layer-cement layer- plastic creeping layer and the formation layer. This plastic- transition layer is formed because of formation creeping. The axial stress and tangential stress distribution of the cement sheath were calculated by the CCFS model. The contact pressure between the cement sheath and formation was calculated. Mohr-Coulomb yielding criterion was applied to estimate failure behavior for the cement sheath. Two case studies were performed with the new CCFS model and previous CCFS model that do not consider the formation creeping effect. The comparison between two models showed that without considering the formation creeping effect, we might underestimate failure of the cement sheath. The simulation result by our CCFS analytical model indicated that the creeping effect would make the interface between the casing and cement vulnerable to shear failure. We changed the Young's modulus and Poisson's ratio for the failed case to investigate the influence of mechanical properties of the cement material. The result showed that a lower Young's modulus and higher Poisson's ratio were preferred for improving zonal isolation. Instead of pursuing how creeping happens, this paper accepts formation creeping as a fact in the whole life of the well. The geomechanical impacts of the plastic-creeping formation, although undetectable from the surface observations, may cause detrimental consequences to cement integrity.

Zonal isolation is significant for safety operation of the well. Failure to keep wellbore integrity can lead to sustained annulus pressure (SAP), and gas migration (GM), which may cause long non-productive time. Losing zonal isolation can cause severe environmental issue, which is irreversible and detrimental. However, cement sheath is exposed to temperature and pressure changes from the beginning of the drilling process to the whole life of the well. These cyclic changes can lead to fatigue failure of the cement. The objective of this study is to investigate the fatigue failure that caused by cyclic changing of temperature and pressure during life of the well. The scope of the study is based on the laboratory fatigue failure cases in previous literatures. Instead of using mechanical failure models, support vector machine (SVM) model is used to predict the fatigue failure of the cement sheath. The data is gathered from six papers of One-Petro, which includes 325 laboratory cement fatigue failure cases. The model has fourteen inputs. Seven cement related factors were selected, which include cement type, additive material, Uniaxial Confining Strength (UCS), curing temperature, curing pressure, curing age, and Young's modulus. Seven experimental related factors, which include highest inner pressure, loading increment rate, frequency of loading, experimental temperature, confining pressure, existence of outer confining part, and cycles to reach failure. The SVM model is implemented by Python. We investigated 240 combinations of input groups and selected the best performance SVM model. The classification result is zero for no fatigue failure, and one for failure. The accuracy for the SVM model is 72.7%, which shows that SVM can be an acceptable model for cement fatigue prediction. The SVM model we proposed is more applicable for real implementation. Because we used real wellbore geometry data (thick wall geometry). Although the data were based on laboratory result, the SVM model provides a helpful method in predicting cement-sheath-failure. This study provides a data based method to predict cement fatigue failure under cyclic changing pressure and temperature. The result will be instructive for the cement design and wellbore operation optimization.

Wellbore pressure gradient in gas wells is significant in designing deliquification technologies and optimizing production. At present, no model has yet to be established specifically for gas wells at a wide gas flow rate range. When calculating pressure gradient in a specific gas field, engineers must evaluate these widely used models and get the best-performance model at a certain range. To establish a more comprehensive model in horizontal gas wells, an experimental study was conducted to investigate the flow behavior of liquid-gas two-phase flow at different gas and liquid velocities and inclined angles in a 50-mm visual pipe. The evaluation of these widely used models against the experimental data shows that no model can predict liquid holdup at different gas velocity ranges, and huge deviations due to several reasons can be observed. After conducting a comprehensive analysis, a new liquid-holdup correlation was proposed based on the Mukherjee-Brill model by correlating from the experimental results, which have parametric ranges closer to the production of gas wells. This new model adopts a new dimensionless gas velocity number to characterize flow similarities and better scale up pressure from the experiment to the gas wells. By validating against experimental data and field data, the results indicate that the new two-phase flow model has stable performance and can accurately predict pressure gradient at different ranges of pressure and gas/liquid velocities.

Zonal isolation is key to the safety of drilling, injection, and production. If zonal isolation fails, significant financial losses or environmental damage can occur. Cement integrity is critical to maintain zonal isolation. The objective of this paper is to study the influence of wellbore shape on cement sheath integrity and zonal isolation. To diagnose cement sheath integrity, cylindrical wellbore models which contain circular-shape casing, cement, and formation layers have been built. However, the wellbore geometry in real drilling process is elliptical instead of circular. In this study, we built an elliptical-geometry Casing-Cement -Formation System (CCFS) analytical model by applying complex variable method. Two case studies were performed with the elliptical-geometry CCFS model and circular-shape CCFS model that does not consider the wellbore shape factor.