Research of rock salt creep and him application in engineering calculation of well design

The combination of the conditions (stress- strain state and temperature) and rheology (creep) of rocks in certain horizons, especially for rock salts, leads to stress concentration during the construction and / or operation of wells, which leads to the destruction of the well design. Therefore, the selection and study of the rheological characteristics of rock salts from the East Siberian deposits should be put into practice. The paper presents the results of laboratory studies of rock salts in the context of one of the field in Eastern Siberia and the construction of an elastic- visco-plastic model for calculating the dependence of the creep pressure on the casing on time. According to the results obtained, recommendations are made for choosing a casing string at a given service life of the well design.
Materials and methods
Creep of rock salts was investigated in the laboratory of LLC "NovosibirskNIPIneft" on full-size core samples with a diameter of 100 mm and a length of 200 mm. To create a complete pseudo-tri-axial loading, a loader was used, which allows to develop a crimping pressure in the range of 0-50 MPa and an axial force in the range of 0-90 MPa in terms of the cross section of the sample. For a long time (7 days), the specified pressures were maintained with an accuracy of not worse than ± 1 atm, and the temperature was maintained with an accuracy of ±1°C. The axial strain of the sample was measured by an incremental linear displacement transducer with an absolute measurement error of 10 ?m. Based on this data, conventional creep models were calibrated and as result kinetic constants were calculated. These constants were used in calculating the strength of casing strings, according to Sarkisov's method.
Results
In the course of the experiments, the rates of stationary deformation were measured for a range of diff ential stresses of 12-33 MPa and temperatures of 15-50 °C. The obtained values are in the range 10-9-10-6 sec-1. According to the above models, the strength characteristics of casing strings were calculated. Later this result was compared with the analysis of incidents with the crushing of casing in two areas of the field in Eastern Siberia.
Conclusions
The authors of the article obtained the results on the investigation of the rheological properties (creep) of rock salts of the Angara and Usolsky horizon in the range of diff ential stresses of 12-33 MPa and temperatures of 15-50 °C, the established rates of relative deformations in the range 10-9-10-6 sec-1. The resulting strain curves were processed according to the phenomenological model. The kinetic constants in the creep model take the characteristic values ? = 0.20 ±1 and ? = 0.33 ± 0.06, which agrees with the literature data. And with increasing diff ential stress, ? decreases, and ? increases. The obtained results made it possible to calculate the life span of the borehole. Thus, to ensure a service life of 30 years of the well design, a casing with a nominal diameter of 168 mm with a wall thickness of 12.1 mm of strength class E) or a wall thickness of 10.6 mm in class E is required.