Effect Of Working Fluid Intrusion On Stress Sensitivity Of Shale

Shale gas has become a new bright spot in the global unconventional oil and gas resources exploration and development. Shale gas reservoir is characteristic of potential damage of stress sensitivity due to its fracture and rich clay minerals. Stress sensitivity severely restricts the efficiency of exploration and development of gas reservoirs. Leakage and low flowback rate of fracturing fluid are common problems existing in drilling.The retention of Large amount of fluid enhance stress sensitivity behavior, impede the effect of production capacity-increasing.This thesis has chosen the Longmaxi group and Niutitang group outcrop shale as object of study. On the basis of describing micro-cracks and macroscopic crack, influencing factors of shale stress-sensitive are analyzed. Effecting on shale stress sensitivity of fluid sensitivity, oil-based drill-in fluid and fracturing fluid are evaluated. Based on atomic emission spectroscopy, scanning electron microscopy and rock mechanics testing and other technical means, The mechanism for stress sensitivity of working fluid immersion are analyzed from the micro, meso-and macro-scale.It is clear that fracture developed in shale reservoir. SEM analysis showes that it exists a large number of small holes and chaotically distributed micro-cracks, Part of fracture has been closed. Shale reservoir is rich in segent fracture, bedding fracture, oblique fracture, high angle fracture, vertical fracture from the observision of Macro-scale and layers-scale.Stress sensitivity of dry sample permeability and crack width are studied. The stress sensitivity is from medium to serious level. When the effective stress is less than20MPa, permeability varies a lot. When the effective stress is more than20MPa, permeability gradually turns to flat. The fracture width visualization experiment shows that the regular pattern of permeability stress sensitivity is basically in accordance with crack width.Have experimental studied shale stress sensitivity mode in the background of fluid. Shale presents high water sensitivity, salt sensitivity and alkali-sensitive damage. Minerals could be eroded by fluids which makes microstructure of clay minerals change. The extent sequence of the damage stress sensitivity is as follows:alkali sensitivity> salt sensitivity> water sensitivity.The mechanism of oil-based drill-in fluid strengthening stress sensitivity is explained. Oil-based drill-in fluids at high pH is the underlying causes to strengthen stress sensitivity shale. Oil-based drill-in fluids make micro-crack expansion and microstructure of clay minerals change,which reduce mechanical strength and enhance stress sensitivity.Recognizing the stress sensitivity of different crack condition. Experimental results show that stress sensitivity of fracturing fluid immersion sample is most severe, and stress sensitivity of proppant support samples is weakest. It was considered by intergrated analysis that the physicochemical interaction of fracturing fluid and shale makes fractures easily compressed and fracture width decreased to strengthen shale stress sensitivity because of fracturing fluid immersion, and proppant weakens stress sensitivity. Controlling the pH of drill-in fluid and preventing leakage in drilling and completion process, reducing the amount of fracturing fluid and promoting filtrate flowback and reducing crack surface erosion in fracturing, designing reasonable production pressure in production, those which are beneficial for formation damage.