EFFECT OF FILTRATE CAKE QUALITY ON BOREHOLE STRESS AND HOLE STABILITY

The formation pore pressure is assumed as stable, while it changes in the surrounding formation of the hole during drilling, no matter it is over-balanced drilling or under-balanced drilling, thus deviations are produced on the borehole stress distribution and the safe range of drilling fluid density between the assumption and the real condition. As the quality of filtrate cake influences the change of formation pore pressure near the borehole, thus how the filtrate cake quality affect on the radial stress, axial stress, surrounding stress and shearing stress is analyzed quantitatively. The results show that with the improvement of filtrate cake quality, the supporting effect of drilling fluid column to borehole becomes more remarkable, and the anti-shearing and sloughing control capability also improve, thus the wellbore tends to be more stable. In addition, the contacting stress between formation fines increases also, and the tension fracturing resistance of rock becomes higher, with higher fracturing pressure detected; this also improves the stability of the borehole. The effect of filtrate cake quality on low permeable formation is especially remarkable.