Successful Management of Fracture Gradient and Mud Weight Issues to Avert Lost Circulation in a Brownfield Development Drilling Campaign: A Case Study of Well X

Abstract

Abstract Loss of drilling mud circulation presents one of the biggest threats to safe wells delivery, hence it is pertinent to understand fracture gradient and loss circulation mechanisms across the different lithologies in well drilling operations. The fracture gradient is an important parameter for drilling mud weight design. If the downhole mud weight is higher than the formation fracture gradient, then the wellbore will have tensile failures (i.e, the formation will be fractured), causing loss of drilling mud or even lost circulation (total losses of the mud). Therefore, fracture gradient prediction is directly related to drilling safety. The paper presents a case study of Well X situated in the coastal depobelt terrain of Niger Delta. The well is an oil development well targeting two (2) brown reservoirs. The top-hole section was drilled safely and subsequently cased-off. While drilling the next hole section with intervals of hard sandstone streaks, using a relatively high mud weight as predicted from a pre-drill borehole stability study, erratic torque was recorded and 18bbls/hr downhole mud losses occurred, in the Q reservoir – an intermediate water bearing reservoir. These mud losses were cured, and drilling resumed, though with persistent erratic torque and Rate of Penetration (ROP) as low as 4ft/hr. To improve the ROP, the Weight on Bit (WOB) was increased, which resulted in high Equivalent Circulating Density (ECD). Due to the high ECD, the hard sandstone streaks were fractured, total mud losses occurred (i.e., no mud returns to surface), the drill string stalled and got stuck in the freshwater-bearing Q reservoir. All effort to recover the stuck drill string and Bottom Hole Assembly (BHA), proved abortive. The drill pipe was severed, while the BHA was permanently abandoned with cement plugs and the main hole was lost. A cased hole sidetrack was initiated, and the well was thereafter successfully drilled to well Total Depth (TD). This paper therefore details the strategy adopted by the integrated subsurface and wells team in the planning and execution of the sidetrack for Well X, as well as technical considerations and operational decisions taken to avert a loss circulation.