A triaxial rock creep testing apparatus has been set up, which is capable of providing confining pressure 2 Kbar and temperature 200°C. A series of natural rock salt samples were tested in multiple stages of creep with monotonically increasing temperature and stress level to determine the viscoelastic constitutive equation for S3–2 salt. By means of high temperature triaxial test with stepwise changes of constant strain rate the viscoplastic constitutive equation was also obtained. Based on experimental data the boundary between the viscoelastic and viscoplastic states was established. As an application in petroleum engineering, a differential equation for numerical calculation of the drilling mud density for controlling the borehole closure rate was derived and the graphs of mud density vs. depth were plotted. Field practice indicated that the research results agreed fairly well with the field data.
It is well known that rock salt is a kind of viscoelastic and plastic material. Many drilling and completion problems, such as pipe sticking and casing collapse, were often encountered in these formations. Shahejie groups(S) of Eogene period in the northern part of Dongpu Depression of Zhongyuan Oilfield in China is a kind of salt lake deposition. In the deposition stage there formed several horizontal and inclined complex salt formations of salt, gypsum and clay, usually covering the pay zones in large area of the oilfield [Li 1988]. On contact with water in drilling, salt dissolution, gypsum and clay swelling and caving in often resulted in serious pipe sticking. In the recent years, water-in-oil emulsion mud system and saturated brine mud have been used in Zhongyuan Oilfield to overcome the drilling problems in the salt formations. But these mud systems can effective only when the mud possesses adequate density to contain the viscoelastic-plastic flow of the salt, and to control the borehole closure in salt formations. From 1976 to 1987 a total of 255 wells of drilled through S3–2 and S3–4 section of the salt formations in this area. Serious pipe sticking occurred in 15 wells before 1984,in 2 wells in 1986. Six wells were abandoned and seven wells were side tracked or completed with stuck pipe remained, because of the difficulty in getting out the stuck pipe in salt formations. Eight of 17 problem wells were found during drilling S3–2 salt formation in Wendong area. If mud density is too low, the rate of borehole closure maybe high. On the other hand, if mud density is too high, although no pipe sticking happens, but formation damage may arise. Therefore, it is urgent to find out the method for determining the proper mud density to control the borehole closure in drilling salt formations, which requires the knowledge of physico-mechanical and rheological properties of rock salt under simulated downhole stress and temperature conditions.
To calculate the proper mud density for controlling the rate of borehole closure, it is necessary to study the creep characteristics of rock salt to be drilled and to determine its rheological constitutive eguation.
