Wellbore Stability Analysis Model Considering Hydration Damage

During offshore drilling operation, mudstone formation is vulnerable to drilling fluid immersion, which would cause hydration swelling and imbibition diffusion, rock strength reduction, and wellbore instability. This paper takes #5 Well in BZ Block as an example and analyzes its wellbore stability by taking into account the influence of drilling fluid immersion on rock strength and introducing the hydration damage coefficient. The analysis results show that: (1) with the increase of formation exposure time, the drilling fluid invasion amount increases gradually, and Young’s modulus of rock can reduce by 32.55% at most; (2) the wellbore stability analysis model considering hydration damage accords with the actual application results better than traditional models; and (3) the wellbore instability is concentrated in the upper and middle formations, and the window of mud density in the formation above 1200m is too narrow to be controlled. Therefore, it is suggested that in the process of drilling at this point the drilling fluid density be raised moderately, but not exceeding the equivalent density of fracture pressure, preferably at about 1.4 g/cm3. This paper proposes a method of judging the degree of drilling fluid invasion and reveals the wellbore instability mechanism in this block to some extent, which is of guiding significance to the reduction of drilling risks in the block.