The Study Of Stress And Its Application Of The Salt And Gypsum Formations Of Paleogene In Yubei Area

Yubei area in Tarim Basin is under a complex geological conditions, many underground anomalies encountered during drilling. Especially in the salt gypsum rock in paleogene which is above the target bed, there was a serious creep deformation,resulting casing deformation in 4 wells. In this paper,7 groups of samples’ rock mechanics are tested i under formation conditions. In addition, 2groups of samples’ stress are tested. Based on this, the character of rock mechanics and stress in salt and gypsum are studied.The relation between stress and casing deformation are also analyzed.The experiments of rock mechanics which is completed under formation conditions show that strength parameters, elastic modulus, bulk modulus, shear modulus of salt rocks is relatively low, the gypsum rocks has the highest level.However, there is the largest Poisson ratio in salt rocks.The salt rock’s plastic is the strongest. Based on this, the impact of natural gamma, density, etc.are used to establish the formulation between P-wave and S-wave data in different lithologies Considering the influence of shale content and Wave Impedance on the compressive strength in cream mudstone and gypsum, the logging interpretation formulas established. The existing empirical formula of cohesion and internal friction angle was amended with the measured data. Hoek-Brown strength criterion and the tested data are used to established the formulas in salt mechanics parameter.Acoustic emission data, differential strain datad and ADS method are used to calculate the stress, acoustic emission and differential strain data are used to check the results,and accurate stress profile is established. In cream mudstone and gypsum,stress state is normally that is σV> σH> σh, however, stress state in salt is changed,that is σH> σV> σh. The maximum horizontal stress and differential stress in salt are at a high level. By the differential strain experiments, paleomagnetic testing, wall caving method and other methods, the results shows that the main direction of the maximum horizontal stress is nearly north-south, the position which closed to the main fault zone, the stress direction may be affected by faults.Since the strong plastic, the salt is easily to creep deformation under a high temperatures and pressures condition, at the same time, the salt section of the maximum horizontal stress and differential stress are at a high level, superimposed the effect of salt creep, the casing(outer diameter 250.83mm) is easily deformed. Finally,the relation between casing deformation and stress are analyzed. The threshold which casing deformation occurs is that the maximum principal stress is more than91.46 MPa,the differential stress is more than 17.8MPa and minimum level of the maximum principal stress ratio is less than 0.8.In this paper, the character of salt and gypsum rock mechanics,stress, the relation between stress and casing deformation are also analyzed.And many research results are obtained. It can provide a theoretical basis for checking casing strength and drilling safety.