Study on Wellbore Stability and Failure Regions of Shale considering the Anisotropy of Wellbore Seepage

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.

Sensitivity Analysis of Wellbore Mud Pressure towards Anisotropic Shale Properties, Pore Fluid Pressure and Far Field Stresses

“…Figure 12 shows the frequency distribution of the angles β w at which the peaks of the mud weight occur. The distribution of the angles β w was calculated at different far field stresses (R = 1-1.5), pore fluid pressures (8 MPa,11 MPa and 15 MPa) and inclination of the weakness planes Figure 11c shows the percentage change in SG with the variation of the far field stresses ΔR (at constant pore fluid pressure pf = 11 MPa). The change in R from 1.1 to 1.5 corresponds to ΔR = +36% and the change in R from 1.1 to 1.25 corresponds to ΔR = +14%.…”

“…In these depleted fields, new wellbores must be drilled for gas injection operations. Consequently, drilling operation in shale is still a matter of concern [8][9][10][11][12][13][14][15][16][17].…”

Reliability of chemical osmosis as a practical method to extract water out of shale and promote shale’s mechanical stability

“…Also stated that deviated wells may require side-tracks to mitigate instability which means higher well costs [20]. studied effects of deviation angle and pore pressures in wellbore instability and found that it is insufficient to rely only on pore pressure to design the stable mud weight window [21].…”

“…Proposed two models of collapse pressure calculation and well-cycle instability area. The condition of underbalanced seepage flow and the rock strength anisotropy were taken into consideration in both models [22]. Presented a non-linear model that is 'thermo-chemo-poroelastic' to study the effect of distinct types of gradients on the distribution of stress and pore pressure in the wellbore.…”

Experimental Study on Permeability and Deformation Characteristics of Bedding Shale under Triaxial Shear-Seepage Coupling

“…Frash et al [27] studied the apparent permeability characteristics induced by shear fractures of bedding shale under high triaxial stress and the relationship between frac-ture geometry and apparent permeability through shear tests. Other scholars [28][29][30] have studied the stability and collapse area of shale boreholes considering the mechanical properties of natural bedding geological bodies and weak structural planes, which provide guidance and recommendations for drilling techniques in permeable, brittle, and hard bedding shale. Zhang et al [31] analyzed the influence of water activity on shale and found that the effect of anisotropy on the compressive strength of shale decreases with the increase in water content, which is the main factor controlling the mechanical behavior.…”