Study On The Mechanism Of Intrusion Into The Formation Of Water-based Underbalanced Drilling Filtrate

On the condition of underbalanced drilling, when the underbalance pressure is small, the liquid phase could still invade into the formation under the effect of capillary force and the chemical potential energy, which overcomes the effect of the underbalanced pressure. For the formation of the shale, the invasion of the liquid will greatly reduce the rock strength, causing a series of down-hole complicated problems such as well-bore stability. For the reservoir, the invasion of the liquid can cause water lock effect and reservoir damage, which cannot realize the true sense of underbalanced drilling. Therefore, researching the mechanism of liquid invading into the formation on the condition of underbalanced drilling is of great significance, which not only provides the basis parameters to the shale formation about borehole stability analysis of underbalanced drilling, also provides theoretical basis for the reservoir of underbalanced drilling.This paper analyzes the acting forces of drilling fluid invading into formation on the condition of underbalanced drilling. By the means of fluid mechanics, the mud invasion models of underbalanced drilling suitable for the mud shale and sandstone reservoir are established, which only considers single phase water flow in the mud shale formation and oil-water two phase flow in the sandstone reservoir. The results of the research are:?On the condition of underbalanced drilling, the motive power is capillary force and chemical potential, and the resistance is underbalanced pressure of the drilling fluid invading into formation;?Considering single-phase water flow of mud shale formation, the fluid invasion model of underbalanced drilling (or overbalanced drilling) is established, which is applied to analyze the wellbore stability of mud shale in Daniudi. The results show that the water influx near the wall increases with the time increasing. The strength of mud shale decreases with the water content increasing, which is more likely to result in borehole instability, and even improving the density of drilling fluid can't effectively prevent the secondary instability of borehole wall;?The mud invasion model analysis of underbalanced drilling for sandstone reservoir shows that the quantity and depth of drilling fluid water intrusion increase with drilling time increasing, underbalanced pressure and initial water saturation decreasing under the effect of capillary force. Considering the chemical potential is bad for borehole stability, when the capillary force and chemical potential function together, the quantity and depth of drilling fluid water intrusion increase. When the capillary force is the same, the larger the chemical potential is, the greater the quantity and depth of drilling fluid water intrusion are.