Quantitative Analysis of Mud Losses in Naturally Fractured Reservoirs: The Effect of Rheology

Abstract

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.

Hydrodynamic modeling of mud loss controlled by the coupling of discrete fracture and matrix

“…In addition, the field data on the rheological parameter of drilling mud can be directly employed. Thus, the YPL is subsequently selected as the model to describe the fluid rheology (Majidi et al, 2008a(Majidi et al, , 2008b(Majidi et al, , 2008cPordel Shahri et al, 2011;Pordel Shahri and Mehrabi, 2012;Mehrabi et al, 2012;Livescu, 2012):…”

“…In the previous numerical model (Majidi et al, 2008a(Majidi et al, , 2008b(Majidi et al, , 2008cPordel Shahri et al, 2012), the mass balance equation for an incompressible fluid in the absence of a leak-off through the fracture wall is given by…”

“…(24) and Eq. (25) as investigated by Majidi et al (2008aMajidi et al ( , 2008bMajidi et al ( , 2008c and Pordel . Eq.…”

“…The fracture width can be inversely obtained by fitting type curves and actual data (Lietard et al, 1996). Type curve analysis took the center stage in mud loss studies for a long period of time, and it became the main tool for fracture aperture inversion despite of its low accuracy (Lietard et al, 1996;Verga et al, 2000;Majidi et al, 2008aMajidi et al, , 2008bMajidi et al, , 2008cHoffman and Narr, 2012). Lietard et al (1999Lietard et al ( , 2004 considered non-Newtonian incompressible mud with Bingham rheology to develop a model for the radial mud flow into a non-deformable fracture with constant aperture and infinite length based on Darcy's law.…”

“…However, the facture distribution they used failed to correspond to any practical formation, and the fluid flow between the fracture and the matrix was not considered in their model. Majidi et al (2008aMajidi et al ( , 2008bMajidi et al ( , 2008c proposed a model for simulating the mud loss of non-Newtonian drilling fluids in naturally fractured formations. The flow of yield-power-law (YPL) fluids was coupled with Newtonian reservoir fluid in a single fracture.…”