Borehole ballooning-breathing is the term used to describe reversible mud losses and gains during drilling operations. In naturally fractured reservoirs borehole ballooning-breathing caused by opening/closing of natural fractures is the major mechanism of this phenomenon.An accurate model of the fracture-induced ballooningbreathing phenomenon could provide an aid in mud optimization and improve the well control procedures while drilling in naturally fractured reservoirs. Using this model, the hydraulic aperture of conductive fractures can also be obtained by continuous monitoring of mud losses and gains. Different models have been developed recently discussing this phenomenon. Each of these models considered some special conditions and investigated the effect of different parameters. This paper first discusses the previous models of fracture ballooning in detail. Then a new model is developed for radial flow of mud with Yield-Power-Law (HerschelBulkley) rheology in a single isolated deformable horizontal circular fracture. In this model exponential deformation law is used for fracture deformation, which is more realistic than the simplified linear deformation law. Also, this model is developed for both fracture ballooning and breathing phenomena. From the developed model it is concluded that cumulative loss in the case of exponential deformations less than that of linear deformation during fracture ballooning. This difference is due to pressure distribution in both cases. The pressure builds up faster for exponential deformation due to higher transmibility of the fracture. Different conditions are discussed in detail which could help in -breathing during drilling operation. hortcomings of the developed modeling approach are outlined. o . In non-fractured better understanding of the factors controlling borehole ballooning S
Fracture ballooning usually occurs in naturally fractured reservoirs and is often mistakenly regarded as an influx offormation fluid, which may result in misdiagnosed costly operations. Several models have been developed to treat this phenomenon and distinguish it from conventional losses or kicks. Among these borehole ballooning models and mechanisms, opening/closing of natural fractures is considered to have the main role in naturally fractured reservoirs. In this study a mathematical model is developed for mud invasion through a disk-shaped and deformable fracture with two impermeable walls and a limited extension. A governing equation is derived based on the lubrication approximation theory (Reynolds's Equation) for radial flow in a single fracture. Considering an exponential deformation law to describe the pressure-aperture relationship, and a yield-power-law model to describe mud rheology, makes this model more general and much closer to the reality than the previous ones. Describing the fluid rheology with yield-power-law model turns the governing equation into a versatile model as it includes various types of drilling mud rheology. The governing equation is solved numerically using finite difference method. Results show how different parameters can affect fracture ballooning and volume and rate of mud loss/gain. The effects of several parameters related to the mechanical properties of the fracture are analyzed. Shortcomings of the proposed model are outlined.
The main objective of net pay determination, as an important step of any reservoir study, is to exclude non-reservoir intervals so that better results are obtained from reservoir characterization, hydrocarbon in-place calculations, and dynamic flow simulation of the reservoir. This study is a comprehensive presentation of the most applicable methods available for net pay determination, highlighting their strengths, limitations, and their input data, and presenting a new procedure to prepare the input data, determine the reservoir net pay, and validate the final results. These methods include conventional best-fit line and quadrant methods in a porosity–permeability cross-plot, Worthington method, rules of thumb, cumulative hydrocarbon column plot, and production constraints. This study, unlike previous ones, presents a stepwise methodology to reach the correct answer considering both rock and fluid properties. The necessity for the definition of net pay is discussed in the current study in the first step. Determination of net pay and the net-to-gross ratio is done by definition of some cut-off values for petrophysical properties such as porosity, water saturation, and shale volume. The new procedure presented in this study as a flowchart to determine pay zone uses different methods to determine cut-off values. The sequential and systematic use of all these methods gives a consistent and more reliable answer. The key steps to determine net pay is to find the porosity cut-off based on a porosity–permeability cross-plot and a pre-defined limiting value for permeability and then to use this value to find the shale volume and water saturation cut-offs using their cross-plots versus porosity. To take into account the fluid properties effect, a mobility cut-off is used as the starting point instead of permeability. Cumulative hydrocarbon column plots are used as a sensitivity tool to determine what percentage of the hydrocarbons will be discarded by any cut-off value. Finally, the determined net pay should be validated using the results of production logging and wireline formation tests. The proposed methodology was applied to a real field to determine its net pay. Porosity and water saturation cut-offs were calculated to be 2% and 55%, respectively, and due to the clean nature of the reservoir, a shale volume cut-off was not necessary. Simultaneous application of porosity and water saturation cut-offs discarded 6.3% of the hydrocarbon column for the field example.
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