Experimental Study on Shale Borehole Wall Stability of Tight Oil Wells in Daqing Oilfield

Borehole wall instability has long been a problem in drilling the Qingshankou Formation and the Quantou Formation in the tight oil blocks in Daqing. Common methods of evaluating the stability of shale formations presently in use are hot rolling test and linear expansion test. These methods however, have defects caused by those who do the experiments, and the experimental results are therefore misleading in the understanding of the activity of the swelling shales and the selection of compatible drilling fluid. Multistage triaxial stress experiment, pressure transfer test (PTT) and thick-walled cylinder test (TWC), which can simulate more closely the downhole conditions, were used to study the stability of shale formations in tight oil wells in Daqing. Using the multi-stage triaxial stress test, the Mohr-Coulomb failure envelop of the formation rocks was drawn to determine the mud density enough to stabilize the borehole wall. PTT was conducted to understand the anticipated invasion rate of pressure caused by drilling fluid column in the hole and the delay in pore pressure increase for a specific fluid system. TWC experiment was used to study the failure characteristics of core samples exposed to drilling fluid under overbalanced pressure. These three test methods simulate the effects of drilling fluid on the stresses of downhole shale formation, and are instructive to the study on the factors affecting borehole wall stabilization.