Modeling Mud Loss in Fractured Formations

Abstract

Abstract A theoretical framework is developed for the apparent borehole ballooning phenomenon in naturally-fractured formations caused by the loss of a power-law drilling fluid into an isolated non-propagating deformable circular fracture of finite dimensions. A borehole ballooning/mud loss equation is derived and solved numerically. Effects of formation properties (formation fluid pressure, fracture extension and aperture) and operational factors (mud weight, mud rheology, borehole diameter) on mud loss dynamics are studied. Application potentials of the developed modeling approach are explained. Introduction Mud losses being one of the most severe drilling problems can be caused by different reasons. Complete mud losses are known to occur in heavily fractured formations. Limited losses with subsequent mud gains when the pumps are shut down and the circulation is stopped, are possible when an isolated fracture of limited extension is encountered during drilling. In non-fractured shale formations, mud losses have been attributed to the borehole wall deformation.1 This paper deals with one particular mechanism of mud loss, namely mud loss into a single isolated fracture. Drilling in overbalance results in mud coming from the borehole into the fracture, which is caused by fracture deformation in the opening mode. If afterwards the circulation is stopped without the well being shut in, the mud absorbed into the fracture returns into the borehole. This is often misinterpreted as a kick, leading to inappropriate well control practices.2 Timely recognition of mud losses during drilling is thus a necessary prerequisite for an adequate application of well control operations. A solid understanding of mud loss and ballooning mechanics is needed in order to be able to recognise these phenomena from drilling data in real-time. This is the motivation for the work described herein. Another reason for a deeper investigation of mud loss is a possible use of this phenomenon for formation characterization purposes. Information on fracture location and aperture is important e.g. for a proper design of mud damage removal and for an appropriate location of perforation intervals. It has been claimed by several independent researchers that mud loss data measured during drilling in fractured reservoirs may be used to assess fracture position, aperture and conductivity.3–5 Interpretation of mud loss data with the aim of formation characterization requires a solid understanding of the underlying mud loss mechanisms. This is especially true when the entire mud loss curve is analyzed, rather than barely a few numbers, e.g. total volume of mud lost into the formation. The first step in improving our understanding of mud loss dynamics is to develop a theoretical framework suitable for different fracture types (e.g. connected/disconnected) and different mud rheologies. Several examples of mud loss modeling are found in the literature. Sanfillippo et al.3 considered compressible Newtonian mud propagating in a non-deformable fracture of constant aperture having impermeable walls. By applying the diffusivity equation with a constant pressure difference boundary condition, and interpolating a tabulated solution of the problem, an approximate analytical relation between time and volume of lost fluid was obtained. This relation was then used to process mud loss measurement data and to obtain fracture hydraulic aperture using mud loss measurements in Agip wells. According to Sanfillippo et al.3, eventual stop of mud losses is due to the fracture plugging by solid mud particles, although the plugging effects were not explicitly accounted for in the model.