Fracture Ballooning in Naturally Fractured Formations: Mechanism and Controlling Factors

Abstract

Abstract Fracture ballooning observed while drilling naturally fractured formations has often been mistakenly interpreted as influx of formation fluid or the loss of drilling fluids. This misinterpretation leads to costly well control procedures that may make the situation even worse. The main mechanisms and factors controlling the ballooning phenomenon must be well understood to avoid confusing this phenomenon with conventional losses or formation kick. Amongst several mechanisms that are quoted for borehole ballooning, the opening/ closing of natural fractures plays a major role in naturally fractured formations. In this work, a mathematical model describing the fracture ballooning process is developed and solved numerically using finite difference approximation. The governing equation is derived using principles of conservation of mass and linear momentum for transient radial flow in a single fracture. The effects of fracture parameters (aperture, extension and deformability) have been studied as well as fluid properties and operational conditions. Describing drilling fluid rheology with Yield-Power-Law (Herschel-Bulkley/YPL) allows for the investigation of the effect of drilling fluid rheology on borehole ballooning. Results show how the rheological properties of drilling fluid such as yield stress and shear-thinning/thickening effect, influence ballooning or mud losses in fractured formations. We conclude that the fluid loss in the fractures could be stopped either because of high yield stress of drilling fluid or limited extension of the fracture. The proposed model is also helpful for detecting and treating ballooning as well as evaluating fracture characteristics. The field potential application of the model is described. Introduction Fracture deformation, or ballooning observed while drilling fractured formations is the result of loss/gain due to fractures being opened and closed. Fracture opening/closing is caused by the annular pressure fluctuation at the wellbore resulting from the change in circulation rate. Mud losses take place when drilling fluid at the well flows into the fracture because the fracture is pressurized and opened. However, when the pumps are turned off such as during a connection the pressure at the well will fall and the mud in the fracture will return to the well due to fracture closing. Usually any flow during drilling is interpreted as an influx of the formation fluid and the common cure is to increase the mud weight and ensure an adequate overbalance. But if the mud weight is increased and the influx is only mud return, the situation will get progressively worse with a rise in equivalent circulating density (ECD). Mud losses will continue and the fracture propagation pressure may even be exceeded, resulting in total losses. Therefore, it is very crucial to understand the major mechanisms and factors controlling the ballooning phenomenon to avoid confusion with conventional losses or formation kick. Ward et al.1 suggested that diagnosis of downhole pressure response with a real-time PWD tool is very helpful to distinguish an influx from mud return. Several examples of ballooning modeling are found in the literature. Lavrov and Tronvoll 2–5 considered mud loss into a deformable fracture of finite length. Two different flow geometries, linear and radial flow were modeled separately. Possible leakoff through the fracture wall caused by the pressure difference between the incoming mud and the formation fluid was accounted for in the linear system. They also justified and used a linear fracture deformation. According to the theory of Lavrov and Tronvoll, the eventual stop of mud losses is due to finite fracture extension. They later studied the incorporation of non-Newtonian mud rheology, ballooning and associated mud loss into a single deformable, horizontal and circular fracture intercepted by a borehole at its center. The effect of different types of fluid rheology, i.e. Newtonian, Power-Law and bi-viscous fluid and various formation properties and operational conditions e.g., formation pressure, borehole pressure and fracture dimensions, were discussed.