The Experimental Study of Formation Damage during Underbalanced Drilling Caused by Spontaneous Imbibition in Fractured Reservoirs

In conventional drilling, high mud weight can create a large overbalance pressure between the wellbore and reservoir. This overbalance can result in the invasion of mud filtrate, drilled solids and foreign fluids into the formation. Subsequently, this overbalance causes significant formation damage, and reduced productivity, thus requiring further costly stimulation operation. Underbalanced drilling (UBD) is a drilling operation in which the circulating drilling fluid pressure is less than formation pore pressure. Underbalanced technology may be very successful in decreasing or eliminating formation damage if properly executed. However, the advantage of UBD can be lost in case of short pulse overbalanced conditions or in case of spontaneous imbibition. During UBD, due to existence of capillary forces and wettability characteristic, especially in low permeable zones, the drilling fluid imbibes into the reservoir rocks in opposite direction due to the reservoir fluids and consequently formation damage can occur. Several parameters, e.g. pressure difference or exposure time, can affect the severity and magnitude of this kind of damage. The main goal of this study is determining the saturation profile of imbibed drilling fluid and consequently evaluating the magnitude of formation damage which occurs in underbalanced drilling. Four sandstone plugs with a single longitudinal fracture with open ends were used in experiments. The plugs were exposed to different experimental conditions to measure the effect of various parameters, e.g. exposure time and overburden pressure, on the severity of the damage. In the next step, the actual saturation profile of imbibed water was determined by the X-ray computed tomography (CT scan) technique. It is shown that for lower underbalanced differential pressure, higher exposure time and lower drilling fluid viscosity, fluid invasion will increase.