A major problem frequently encountered in the petroleum industry is the discrepancy between the designed and calculated fracture length and conductivity. The two parameters are usually calculated using 'Well testing techniques for buildup and/or draw down tests. The design is usually achieved using 2-D or 3-D models. This paper presents an overview of the problem as it is currently perceived.
Subject paper details some of the factors that may contribute to the miscalculation of fracture parameters. These factors are: effect of clean-up of fracturing fluid, tailing. Changing conductivity inside the fracture as function of position and time, fracture height, and low fracture conductivity. Effect of producing time is also discussed.
Numerous field examples are cited, presenting both designed and calculated fracture parameters in some, and only calculated parameters in the remainder of the examples.
Two conclusions of this paper are that designed conductivity is higher than the calculated one, and that the difference cannot be accounted for by the expected margin of error. It has also been found that the dis-crepancy in fracture length is not as severe as was once believed.
Some wells cannot economically produce before hey are hydraulically fractured. Usually these wells are producing from low permeability formations, and hydraulic fracturing may be performed to boost well productivity. When a fracture is created, its length and conductivity are the primary parameters determining the well performance, It is usually recommended to have a dimensionless fracture conductivity, FCD, between 30 and 100. IE the formation permeability is very low (< 0.01 md) then it is fairly easy to obtain a high dimensionless fracture conductivity. Thus the critical step in the enhancement of an uneconomical well is creation of a fairly long fracture (in excess of 500 ft). A long fracture creates a means of transporting reservoir fluid into the wellbore. If formation permeability is 0.01 md, and fracture length is 1000 ft, then to achieve. a dimensionless fracture conductivity, FCD, of 50, fracture conductivity of only 500 md-ft is needed. However, 1f the permeability is 1 md, then a fracture conductivity of 50,000 md-ft is required. Obviously such high conductivity is unattainable, consequently, either the designed fracture length should be reduced or one should be satisfied with a much lower dimensionless conductivity.
Conductivity is of course, a function of type and Concentration of proppant at a specific position inside the fracture. Correlations have been developed for the various types of proppants. These correlations were developed under certain laboratory conditions that may or may not be a good representation of actual conditions inside a fracture.
Fracture length is usually designed using a 2-D or a 3-D program. In these programs, fracture propagation is assumed to follow a certain geometry (as will be discussed later).
There has been a discrepancy between the designed and calculated fracture parameters. In many cases very short fractures have been reported.