Significant mud losses often occur while drilling in fractured formations. Severe problems arise when drilling fluids invade high permeability conductive fractures that the well path intercepts. The industry has recently started to monitor mud loses in order to identify the fractures and characterize them. Methodologies are available to characterize the geometry and conductivity of the fractures by quantitative analysis of field measurements. These methodologies are mainly based on mathematical models that describe the physical phenomenon and the mechanism under which the flow within the fractures takes place. Once the models are provided one can look for causes of the problem and methods to minimize it.This paper presents a new model for mud loss of non-Newtonian drilling fluids into naturally fractured formations. Flow of Yield-Power-Law (Herschel-Bulkley) fluids has been coupled with Newtonian reservoir fluid in a single fracture. The governing equations are derived based on principles of conservation of mass and linear momentum for drilling fluid and pressure diffusion for reservoir fluid. Results are obtained based on semi-numerical solutions and plotted in terms of mud loss volumes versus time. The results demonstrate how rheology of the drilling fluid and formation fluid properties can influence mud losses. The relative contributions of both drilling and reservoir fluids is determined and compared.This model allows for predicting fluid losses for a given drilling fluid, formation fluid and operational conditions. Conversely, one can evaluate hydraulic aperture of the fractures by continuously monitoring mud losses and finding the best fit of field measurements of mud loss to the model. Field data are used to demonstrate the practical application of the proposed technique.The proposed model is valuable for drilling operations because it can help in minimizing the loss of expensive drilling fluids through optimization of drilling fluid rheological properties and selecting appropriate lost circulation materials. The model also benefits production operations by minimizing formation damage. In addition, well completion schemes can be optimized based on the improved knowledge of the near-wellbore fracture characteristics.

Summary Significant fluid loss while drilling through fractured formations is a major problem for drilling operations. From field experience, we know that the type and rheological parameters of the drilling fluid have a strong impact upon the rate and volume of losses. A mathematical model for Herschel-Bulkley [yield-power-law (YPL)] drilling-fluid losses in naturally fractured formations is presented. As a result, the effect of rheological properties of drilling fluid such as yield stress and shear-thinning/-thickening effect (flow-behavior index) on mud losses in fractured formations is investigated. We found that the yield stress can control the ultimate volume of losses while the shear-thinning effect can tremendously decrease the rate of losses. Therefore, mud losses in fractures can be minimized by optimizing the rheology of the drilling fluid properly. The model also allows for quantitative analysis of losses that take into account fluid rheology to characterize the fractures. Hydraulic aperture of conductive fractures can be obtained by continuously monitoring mud losses and fitting field records of mud losses to the model. The proposed model is very useful not only for drilling applications but also for well-completion design and fractured-reservoir-characterization purposes. To examine the validity of the model, a practical application of the proposed technique is demonstrated through a field example of mud-loss measurements in a fractured well in the Gulf of Mexico.

Significant fluid loss while drilling through fractured formations is a major problem for drilling operations. From field experience we know that the type and rheological parameters of the drilling fluid have a strong impact upon the rate and volume of losses. In this work, a mathematical model for flow of Yield-Power-Law (Herschel-Bulkley) fluids in fractures is presented. The governing equation is derived using the principles of conservation of mass and linear momentum for transient radial flow in a fracture. Results are obtained based on numerical solutions and plotted in terms of mud loss volumes versus time for a given drilling fluid under certain operational conditions. Results show how the rheological properties of drilling fluid such as yield stress and flow behavior index (shear-thinning/thickening effect), influence mud losses in fractured formations. According to this model the yield stress of drilling fluids tremendously decreases the potential for mud losses. The effect of yield stress on reducing mud losses is the same as the effect of overbalance pressure on increasing losses. The shear thinning effect of drilling fluids can also greatly increase the rate of losses. Therefore, mud losses in fractures can be minimized by properly optimizing the rheology of the drilling fluid. Using this model, quantitative analysis of losses that take into account fluid rheology in order to characterize the fracture can be achieved. One can obtain the hydraulic aperture of conductive fractures by continuously monitoring mud losses and fitting field records of mud losses to the model. For general applications, type-curves are provided that describe drilling fluid loss of Yield-Power-Law fluids into fractures. Newtonian, Bingham Plastic and Power Law fluids are special cases. The proposed model is very useful not only for drilling applications but also for well completion design and fractured reservoir characterization. Field data measurements are used to demonstrate the practical application of the proposed technique. Good agreement between the model and field data confirms the validity and applicability of the model. Introduction Drillers often encounter fractured formations which can either have positive or negative effects on the flow properties of the formations. But, large volumes of drilling fluid losses through these faults or fissures constitute severe problems due to improper functioning of the drilling fluids. Beside the problematic aspect of this issue and techniques to prevent losses such as under-balanced drilling or using the appropriate lost circulation materials, the industry has made several attempts to characterize the fractured formations by real-time monitoring of mud losses. Dyke et al.2 (1995) reported that losses through the matrix permeability or fractures can be distinguished by the characteristics of the losses. Losses through pores start slowly and gradually increase whereas losses into fractures are associated with a rapid initiation followed by gradual decline with time. Different sources of information such as image logs, core data and using more accurate flowmeters are suggested by several researchers to improve detection and characterization of the fractures 1, 10. However, quantitative analysis of mud losses is a more informative and reliable way of characterizing the fractures in terms of the flow properties of the fractures. Such an analysis should be based on mathematical model that describes the physical phenomenon and the mechanism under which flow within the fractures takes place.

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.

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