Underbalanced Drilling Borehole Wall Mechanical Stability Studies

The problem on the borehole stability, in the drilling project, has been so complex. However, the problem of the borehole stability will be exacerbated under the underbalanced drilling, because the fluid column pressure in the process of the underbalanced drilling is lower than the conventional drilling. In order to make this technology better develop, this crucial difficult problem must be urgently solved. The solution of the stress distribution has been obtained again on the basis of the conventional mechanics model, regarding the rock as the porous medium, considering pore pressure of the fluid and the flow - solid coupling action and the the friction when the fluid gets through the porous medium, then the caving pressure has been analysed by the failure criteria. The conclusion makes sense to the actual process and construction work.First of all, the mechanics model is established and the solution of the stress distribution is obtained on the basis of the conventional mechanics model, regarding the rock as the porous medium, considering the pore pressure and the flow - solid coupling action. Besides, under the process of the underbalanced drilling, therefore the friction will be considered when the fluid gets through the porous medium. Through the instance, it is proved that the result conforms to the actual situation.Second, the traditional method has been used to confirm the parameters of rock mechanics by the empirical formula and the stress coefficient of the formation by the curve of the hydraulic fracturing experiments and calculate the value of the horizontal stress by the model of the bedding earth stress. Through the instrance of the D40, it is proved that these methods are reasonable and feasible.In view of the fact that the old methods can not forecast the undrilled formation pore pressure which is beneath the bit with effectiveness, accurateness and conveniency. This article used the grey theory to establish the grey model to forecast the pore pressure. Through the instance, it is proved: This method can precisely calculate the undrilled formation pore pressure.Finally, based on the above study, the bore stress and the pore pressure and the caving pressure and the factors which impact the caving pressure have been calculated and analysed by the program. There is a conclusion that during the underbalanced drilling, the situation of the borehole instability will happen easily considering the pore pressure and the flow - solid coupling action and the friction when the fluid gets through the porous medium, which is coincided with the true situation.