Drilling Fluid Technology for Long Horizontal Shale Gas Well JY2-5HF in China

Well JY2-5HF, completed at depth of 5,965 m, is an appraisal well drilled in Fuling shale gas feld, Chongqing, by Sinopec. The length of the horizontal section of this well is 3,065 m. The second interval of this well penetrated shale formations in its middle and lower sections and is at risk of borehole wall sloughing, pipe sticking and mud losses. An anti-sloughing drilling fluid, BH-200, formulated with synergetic additives, such as shale inhibitor BT-200, K-HPAN, KCl, a wellbore stabilizer, an emulsifed asphalt, a non-penetration additive, polyglycol, a multi-function solid lubricants and QS-2, was used to drill the well. The horizontal section of the well required that the drilling fluid should have good physical and chemical wellbore stabilizing capacity, good lubricity and good suspending capacity. An oil base mud, with O/W of 90:10 (26% CaCl₂ solution) and electric stability of at least 900 V, was used to drill the horizontal section. Field operations showed that the drilling fluids used well satisfed the needs of downhole and drilling engineering. Rotary steering and oil base drilling fluid are suitable for long horizontal drilling in shale gas wells. Formation pressure coeffcient measured after fracturing was generally higher than the pressure coeffcient during drilling by 0.1-0.2, and it is thus advised that this value be deducted when designing the density of a drilling fluid.