Abstract
The hard and brittle shale formation is prone to collapse and instability, and the penetration of drilling fluid along the bedding reduces the mechanical properties of rock near the borehole wall, resulting in serious downhole accidents. Therefore, in this paper, the geomechanical parameters of the reservoir in the Longmaxi formation of Jiaoshiba were determined by field hydraulic fracturing and laboratory experiments. Then, the stress distribution model of borehole wall under the condition of underbalanced seepage flow is established based on the experimental results obtained by mechanical experiments on underground cores. The instability zone of borehole wall under the condition of underbalance is calculated and analyzed. The results show that the two-way horizontal ground stress of the Longmaxi formation is higher than 2.2 MPa/100 m, and the original ground stress is high. Moreover, the mechanical parameters of the stratified shale stratum matrix and weak surface are significantly different. The cohesion (4.7 MPa) and the angle of internal friction (26.9°) of bedding plane are significantly lower than that of the matrix (7.77 MPa) and the angle of internal friction (46.7°). Hard and brittle shale is easy to be destroyed along the stratification. Under the condition of underbalanced seepage, the mechanical properties of borehole shale can be stable. It is found that when the borehole axis is vertically stratified, the collapse pressure is the lowest, while in other drilling directions, the drilling fluid density needs to be increased by 0.5 g/cm3 to maintain the borehole stability. With the increase of the inclination angle of bedding plane, the wall failure area increases. The results of this study can provide guidance and suggestions for drilling in Jiaoshiba block and other permeable hard and brittle shale formations.
Highlights
Unconventional oil and gas resources account for about 80% of the world’s oil and gas resources, among which shale oil and gas reservoirs account for a large proportion [1]
In situ stress is very important for safe drilling [9], and the stress concentration is different from that of isotropic formations on the borehole wall of stratified shale [10]
The in situ stress state of the Longmaxi formation was determined by on-site hydraulic fracturing and laboratory experiment, and the downhole core was subjected to triaxial compression experiment to obtain the mechanical parameters of shale matrix
Summary
Unconventional oil and gas resources account for about 80% of the world’s oil and gas resources, among which shale oil and gas reservoirs account for a large proportion [1]. Hikweon [21] considered the anisotropic strength of shale, converted in situ stress to bedding plane, and the results showed that compared with isotropic formation, the critical collapse pressure of shale formation increased significantly. As the shale water activity is less than the drilling fluid, in addition to the predicted hydraulic gradient, additional fluids will flow into the shale formation due to the chemical imbalance applied [29] This creates abnormal pore pressure near the borehole, followed by additional fluid-induced stress, and prolonged exposure to drilling fluid in the wellbore results in reduced strength. The mechanical parameters of shale matrix and weak surface were obtained by combining the direct shear test and triaxial compression test Using these parameters, a wellbore stability model considering anisotropic seepage under the condition of underbalanced drilling is established. The collapse pressure and failure area are analyzed, and the slight failure pattern of wellbore is studied in the same horizon at different shale inclination and inclination angle
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