Fracture ballooning usually occurs in naturally fractured reservoirs and is often mistakenly regarded as an influx of formation fluid, which may lead to misdiagnosed results in costly operations. In order to treat this phenomenon and to distinguish it from conventional losses or kicks, several mechanisms and models have been developed. Among these mechanisms under which borehole ballooning in naturally fractured reservoirs take place, opening/closing of natural fractures plays a dominant role. In this study a mathematical model is developed for mud invasion through an arbitrarily inclined, deformable, rectangular fracture with a limited extension. A governing equation is derived based on equations of change and lubrication approximation theory (Reynolds’s Equation). The equation is then solved numerically using finite difference method. Considering an exponential pressure-aperture deformation law and a yield-power-law fluid rheology has made this model more general and much closer to the reality than the previous ones. Describing fluid rheology with yield-power-law model makes the governing equation a versatile model because it includes various types of drilling mud rheology, i.e., Newtonian fluids, Bingham-plastic fluids, power-law, and yield-power-law rheological models. Sensitivity analysis on some parameters related to the physical properties of the fracture shows how fracture extension, aspect ratio and length, and location of wellbore can influence fracture ballooning. The proposed model can also be useful for minimizing the amount of mud loss by understanding the effect of fracture mechanical parameters on the ballooning, and for predicting rate of mud loss at different formation pressures.
Most of the "easy?? oil in high permeability reservoirs has been explored and developed to a great extent. More and more "difficult?? oil has been discovered. There are many problems in developing the "difficult?? oil in low or extremely low permeability reservoirs. One of the problems is the pressure sensitivity of permeability which declines significantly as pore pressure decreases or net overburden pressure increases. There have been a few mathematical models to calculate oil or gas production by considering the pressure sensitivity of permeability. However most of the models have not been verified using field production data. In this study, a new production model has been derived theoretically with the pressure sensitivity of permeability considered. Using the production data from a low permeability (less than 1.0×10-3µm2) oil field (Yushulin, Daqing), the model has been tested and verified. The pressure sensitivity coefficient of permeability has been calculated by using the new model with the field data. The results calculated using the new model also showed that the permeability near the well bottom decreased significantly because of the drop in pressure in low permeability reservoirs. An obvious permeability decline funnel could be formed even the formation was homogeneous before development. It was found that the productivity index is no longer a constant in low permeability reservoirs with serious pressure sensitivity of permeability. According to this study, it is necessary to consider the pressure sensitivity of permeability when low permeability reservoirs are being developed. Otherwise, the production will be greatly overestimated.
This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 153944, "Pressure Distribution in Progressing-Cavity Pumps: Test Results and Implications for Performance and Run Life," by Evan Noble, SPE and Lonnie Dunn, SPE, Weatherford, prepared for SPE's Progressing Cavity Pumps 2011 Digital Edition.
Momoki, Tsutomu (Graduate school of Engineering, Osaka Prefecture University) | Fukasawa, Toichi (Graduate school of Engineering, Osaka Prefecture University) | Kaneko, Takeshi (Graduate school of Engineering, Osaka Prefecture University)