Computing safety drilling fluid density of ultra-deep rock salt formation

Extremely thick rock salt formation distributes in the depth range 1000-6000 m in Kuqa depression, Tarim Basin. Under the condition of high geostress and geotemperature, ultra-deep rock salt shows remarkable creep feature. Wellbore shrinkage frequently occurs during drilling in rock salt formation. In order to reduce this type of drilling accident, a new method to compute safety drilling fluid density of ultra-deep rock salt formation is developed. Conventional triaxial creep experiments are conducted under the following conditions: (1) axial compression is 125 MPa; (2) confining pressure is in the range 120-85 MPa; (3) temperature is in the range 100-140 degrees C. Undetermined parameters in constitutive relationship of rock salt are obtained on the basis of the analysis of experimental data. Computational formula of safety drilling fluid column pressure is established through stress analysis of rock salt around wellbore and constitutive relationship. Principal stress and geotemperature in the formula are determined through well logging. Finally, the computational formula is applied to calculate the equivalent circulation density (ECD) of two rock salt formations distributed in the depth ranges 6008-6055 m and 5705-5770 m. Results indicate that the computational values of ECD are accurate compared with the values of practical density. The new method provided in this study can be employed to design safety drilling fluid density of rock salt in ultra-deep formation.

Automated summary

Under high geostress and geotemperature conditions in ultra-deep rock salt formations, a new method to compute safety drilling fluid density has been developed and applied successfully. The method utilizes computational formulas and analysis of experimental data to accurately calculate the equivalent circulation density of the rock salt formations. This method has the potential to reduce drilling accidents.