HIGH VELOCITY FLOW IN FRACTURES - Høyhastighet strømning i sprekker
Tonje Bentzen (1996)

ABSTRACT

High velocity flow measurements are done in the laboratory on fractured cores to simulate gas flow in hydraulic fractures. The purpose of the measurements was to find how parameters (inertial resistance, permeability and Klinkenberg factor) changed as function of fracture properties. Our fractured cores were created as reasonable models for fresh fractured rocks. The fracture surface in the cores were much more realistic than a saw cut fracture surface. Models for coupled and uncoupled wall-slip and high velocity flow were used for  curve fitting of the data. From these curve fittings it is clear that for longitudinal fractured cylindrical cores with impermeable end faces of the matrix, high velocity flow is not described very well by the Forchheimer flow model. Fracture spacers were induced, but did not seem to have a systematic effect on fracture width.  


INTRODUCTION


Low permeability hydrocarbon reservoirs have become an important source for the supply of oil and gas (Holditch and Blakley, 1989). For most of these reservoirs to be developed at commercial flow rates, hydraulic fracturing treatment is necessary. Hydraulic fracturing is the process of injecting high pressure fluid into a well to create tensile stresses in the formation exposed to the fluid pressure. If the stresses become large enough they will break down the formation and fractures are initiated. The fracturing fluid must contain proppants so that the crack will remain open beyond the period of pumping and crack propagation (Schechter, 1992). There are also other fracture systems which may exist in hydrocarbon reservoirs; 
natural fractures and fractures close to the well due to stress fields.

 When fluid or gas is flowing through a porous medium there are four different flow regimes that may exist (Darcy flow, Weak Inertia flow, non-Darcy flow or turbulent flow). The pressure loss in the different flow regimes are described by different equations, there are  reasons to believe that these equations also are valid for flow in fractured media.  Previously several studies have been done to simulate flow in fractures. The interesting part about this project, is that the measurements are obtained on flow through a very close to "natural fracture, with a rough surface.
   
This report includes laboratory studies done on four fractured cores, with well sorted spacers, in a high velocity flow set up. By collecting the results, calibrating and feeding them into a curve fitting program we got results which can tell us more about how the parameters b, ( and k  varies with different fracture properties.


CONCLUSIONS

1.  From the laboratory measurements done in this study there are no indications of a systematic effect of the spacers upon the parameters b, ( and k.

2.  For longitudinally fractured cylindrical cores with impermeable end faces, high velocity flow is not described by the Forchheimer flow model, due to downward dip for low pressures and   massfluxes. This dip was not only due to wall-slip flow since neither a classical nor a new wall-slip-high velocity model fits the whole range of data. This may be due to dual porosity, combined line source/sink and the inflow/outflow geometry.

3.  From these studies  we have been able to find a very interesting phenomenon, which is yet not well known in this field of petroleum science; although we  use spacers, there still exists an inertial resistance, due to the roughness of the fracture surface.


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