Mechanisms of Borehole Wall Instability of Terrestrial Tight Oil Wells in Songliao Basin and Drilling Fluid Countermeasures

Borehole wall instability has long been a problem in the development of the terrestrial tight oil in the Songliao Basin (Daqing Oilfield). To understand the mechanisms of the borehole wall instability, SEM and X-ray diffraction were used to analyze the microstructure of the core samples and the mineral composition of clays taken from the wells drilled in the basin. Laboratory studies were conducted on several aspects that are related to the stability of borehole walls, such as the hydration capacity of the core and clay samples, the impact of the mud cake quality, the physical-chemical parameters, the plugging capacity and inhibitive capacity of the drilling fluids used on the stability of the borehole walls etc. Based on these studies, a KCl oligomer polyamine drilling fluid was developed. Study on this drilling fluid showed that it functions normally at temperatures as high as 120 ℃, the extreme pressure friction coefficient of the drilling fluid is 0.12, percent recovery of shale cuttings in roller oven test with this drilling fluid is greater than 96%, and it is resistant to contamination by invasion of 10% bentonite. This drilling fluid has been used on 20 tight oil wells in Daqing, no borehole wall instability was encountered. The average percent hole enlargement of these wells was less than 6%, and 90% of the wells were cemented with excellent job quality. Tripping of drill string during drilling and casing running were all conducted smoothly. Field application has proved that this KCl oligomer polyamine drilling fluid is able to improve the borehole wall stability of the tight oil wells drilled in the Songliao Basin, and its use has provided technical support to the efficient development of the tight oil in Daqing Oilfield and to the mitigation of downhole troubles.