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Abstract Recently, the flow of fluids into a fracture from a point source has been the subject of several different papers. Cleary and Fonseca first suggested that convective transport should play a major role in the placement of proppant when the flow into a fracture was from a point source, Clark and Courington presented data showing that for non-viscosified fluids convection was the dominant mechanism of transport. However, they showed, that for uniform fractures, viscosifying the fluid made a large difference in the transport mechanism. In a later paper, Clark and Zhu presented data for non-uniform fractures and viscosified fluids weighted with either salt or silica flour that showed that the presence of minor non-uniformities serve to negate the effect of convection even more than viscosifying the fluids. In this work, we have extended the work presented in the previous two papers to high viscosity Newtonian fluids and crosslinked fluids. The experiments have all been done with various concentrations of silica flour to simulate added proppant Both changing the nature of the non-uniformities and crosslinking the polymer solution have a profound affect on the flow into the fracture and the convective process. Introduction In their 1992 paper, Cleary and Fonseca introduced the concept of convection as a mechanism for proppant transport. They based their conclusion on an "order of magnitude" analysis of settling velocity versus convective transport. This analysis yields estimates of convective settling that are 100 to 1000 times the Stokes type settling. It is not the purpose of this paper to critique this analysis, but a survey of the literature on fracture modeling indicates that there has been little or no evidence of convective settling being dominant 3 in any model study reported prior to 1994. Clark and Courington showed that, with simple fluids, the viscosity of the fluid in the slot made a big difference in the flow pattern in the slot. This was confirmed by data presented by Clark and Zhu. In this study, they presented data that showed that a thickened fluid in the slot lessens the tendency for fluids to flow convectively. Although, early in the experiment there is clear evidence of convective type flow, the convective flow is not sustained during the experiment. In the fall of 1994, Barree and Conway presented a paper that indicated that convection could dominate some flows but it is not entirely clear that boundary effects were eliminated. P. 571
- Research Report > New Finding (0.35)
- Research Report > Experimental Study (0.34)
- North America > United States > Texas > Permian Basin > Yeso Formation (0.99)
- North America > United States > Texas > Permian Basin > Yates Formation (0.99)
- North America > United States > Texas > Permian Basin > Wolfcamp Formation (0.99)
- (21 more...)
Yield Points in Oilfield Cements
Clark, P.E. (U. of Alabama) | Sundarem, L. (U. of Alabama) | Balakrishnan, M. (U. of Alabama)
Abstract The determination of yield points in cement slurries important in the complete description of slurry flow properties. Yield points affect both the start-up pressure properties. Yield points affect both the start-up pressure after a temporary shutdown and void filling properties cements. Values for the yield point are normally obtain by extrapolation of rotational or tube flow rheometer data. This technique is subject to both experimental and analytical errors. Recently, several papers have described a more direct method of measuring yield points in slurries. A rotational viscometer equipped with a vane rather than bob can be used to measure yield points accurately. The yield surface with the vane test fixture is in the fluid rather than at a fluid-solid interface as it is with a solid bob or a tube flow apparatus. Experiments with a vane fixture using a shear rate controlled rheometer can be subject to errors caused by windup of the torque spring. In this study a controlled stress rheometer was used to circumvent these problems. Yield points for various oilfield cement slurries are presented. Data reflect the differences in additives, aging presented. Data reflect the differences in additives, aging time and water content. The vane technique can be used to produce accurate and reproducible measurements of yield points in cements. Using a controlled stress instrument has several advantages in making these measurements. Introduction Yield stress (y) of a fluid is defined as the minimum stress required to deform the fluid. In this paper will also be referred to as the Bingham yield stress. Below the yield stress the fluid acts as an elastic solid and above the yield stress the fluid flows with a plastic viscosity (). is This is a statement of the von Mises criterion for fluids with a yield point. The equation (Bingham equation) which governs the flow of a fluid with a yield stress is given by ...............(1) where is the shear stress and is the shear rate. Unfortunately, for fluids equation 1 is an idealization of the actual flow. In controlled shear rate experiments the stress normally builds exponentially at small shear rates to yield data that is similar to the data points shown in Figure 1. Papanastasiou presents a constitutive equation (2) that accurately describes the behavior of a real system over a wide range of shear rates. ................(2) Where is the viscosity of the fluid after yielding, D is the rate of strain tensor, is the stress tensor, and is the second invariant of the rate of strain tensor. This rather imposing equation can be reduced to .................(3) for viscometric, one-dimensional flows. Equation 3 is similar to the Herschell-Bulkley equation with a term () that modifies the yield stress parameter. P. 537