Mechanism and Strategy to Deal with Borehole Instability of Hard and Brittle Shales in Halahatang Oilfield

Mechanism and strategy to deal with borehole instability of the Triassic System and Silurian System hard and brittle shale in Halahatang Oilfield were studied both from mechanics and physic-chemistry. The study shows that for the formations drilled, the magnitudes of the three principal stresses are in the following order: σ_Z > σ_H > σ_h, and shales in the Triassic System are more easily becoming mechanically instable than those in the Silurian System. Shales in the Triassic System have ion exchange capacity, montmorillonite content, colloidal content and percent rate of swelling of 25.4 mmol/100 g, 36.3%, 76.4% and 23.2%, respectively, meaning that the shales have great surface hydration capacity and are easy to become physicochemically instable. In field operations of Well I and Well II, by increasing mud density and enhancing the inhibitive capacity of mud in the Triassic System, and by improving the plugging capacity of mud in the Silurian System, borehole instability was almost eliminated.