A comprehensive review of wellbore breathing
Wellbore breathing is a common occurrence during drilling operations, but the downhole mechanism and how it manifests itself in surface and subsurface processes, is not well understood. Wellbore breathing events often result in a drilling fluid gain at surface and are misidentified as a kick, resulting in unneeded shut in periods and associated non-productive time (NPT). Further, misidentification of wellbore breathing as underbalance often results in increases in mud weight, which only exacerbates the problem and may cause lost circulation. This work focuses on characterization of the wellbore breathing phenomenon in practical contexts of the pressure, volume, temperature, and time behavior of the components involved in the surface and subsurface system. This was accomplished through the examination of case studies published in literature to develop a comprehensive review of common experiences and incidents, as well as the operational responses to these. Methods for identification, differentiation from kicks and underbalance, mitigation and prevention of wellbore breathing are proposed, in addition to operational procedures for safe continuance of drilling operations. The coupled nature of pressure, temperate, and vii volume, as they pertain to wellbore breathing, are analyzed in detail in order to quantify their effects and how they can inhibit identification of wellbore breathing. This research also proposes adaptation of the hydraulic fracturing pump-in flowback test interpretation developed by Plahn, Nolte, and Miska (1997) for the interpretation of wellbore breathing events and estimation of the minimum horizontal stress. This work presents estimates of the minimum horizontal stress for fractured zones in five wells using PWD data recorded during wellbore breathing events, which was obtained from literature. These estimates were verified qualitatively, when possible, by comparing across multiple connections and with available LOT data.