Experimental study of friction coefficient of rocks in high pressure and tight gas reservoirs in Sichuan

The calculation of torque and drag in horizontal wells and extended reach wells is an important element in drilling operations, but determining the value of the friction factor is difficult in the calculation of friction. Sichuan Basin constitutes a special gas reservoir system with high pressure and tight formations. They are favorable for gas production which can be accomplished economically only with horizontal drilling and hydraulic fracturing, and Xujiahe Formation is the focus of the exploration and development. In order to determine the reasonable friction factor for directional well and horizontal well in Xujiahe Formation and complete comprehensive evaluation method, both theoretical study on friction mechanism and experimental investigation on friction factor need to be carried out. A series of experiments are conducted for different lithology (sandstone, sand-mudstone, and mudstone), steel, and lubricants (mud systems). The test results are as follows: lubricants have the most obvious influence on the friction coefficient; the friction-reducing effect of the same lubricant on different friction pairs varies; and lubricants have very different effects on the same friction pair. Under the condition of lubrication with clear water, the friction coefficient of rock increases slightly as the normal load rises. Under lubrication with mud, the change range of the friction coefficient of rock with the increase of normal load is low. Rotation speed has limited impact on the friction coefficient. Under the same test condition, the friction coefficient of sandstone is a little higher in the three kinds of rocks. Through a large number of experimental studies, the value range of the friction coefficient of different kinds of rock in the fifth member in Xujiahe Formation is obtained, providing a scientific basis for the drilling design and the calculation of torque and drag in high pressure and tight formations in Sichuan.