Pushing the Limits in Hydraulic Fracture Design

Abstract

Abstract In a number of previous publications we have introduced the concept of Unified Fracture Design (UFD) with a central theme of maximizing the dimensionless productivity index (JD) following a hydraulic fracture treatment. We have shown that for a given mass of proppant to be injected in a well with an assigned drainage in a reservoir of a given permeability there exists a specific dimensionless fracture conductivity at which the JD becomes maximum. We called this the optimum conductivity. Smaller and larger values of this conductivity result in smaller JDs. All the important fracture and reservoir magnitudes of permeabilities and volumes are related through the Proppant Number. Once the optimum conductivity is determined (which for a large range of Proppant Numbers is equal to 1.6) then the ideal fracture dimensions (length and width) are de facto set and they should be considered as the desirable target. For each injected proppant mass there is a corresponding Proppant Number and at the optimum conductivity the dimensionless PI can be readily determined. Increasing the proppant mass or the proppant-pack permeability would result in an increase in the JD, which has a maximum limit of approximately 1.9. This value can never be accomplished in reality and there are three reasons preventing it, one economic and two physical. The economic reason is obvious. Increasing the job size would result in the flattening of benefits, not justifying the incremental costs. In the same vein, using a much better (and more expensive) proppant may not be justified by similarly flattening benefits. Of the physical problems, the first affects low-permeability reservoirs where the indicated fracture width may be too small; it cannot be less than three proppant diameters. For high-permeability reservoirs, indicated very large widths will certainly result in very large net pressures, exceeding operational limits. The latter may also lead to unacceptable fracture heights. We present here a series of parametric studies pushing the physical limits of fracturing in a wide range of reservoirs by injecting very large proppant volumes and experimenting with extraordinarily large proppant sizes (large proppant pack permeabilities) seeking the maximization of the JD within reasonable economic limits. This work appears to be particularly suited for high permeability formations and it shows that current industry practices are overly conservative and timid and a more bullish approach would lead to major production enhancement benefits. Introduction Valkó and Economides1,2 introduced a physical optimization technique to maximize the productivity index. The well performance depends on the x-direction penetration ratio, Ix:Equation 1 and the dimensionless fracture conductivity:Equation 2 Because the penetration and the dimensionless fracture conductivity, through width, compete for the same resource: the propped volume, the injected propped volume provides a constraint in the form:Equation 3 which, can be considered as the ratio of two cross-sectional areas: propped area to reservoir area - multiplied by the permeability ratio and by two. Multiplying both the numerator and denominator by the net pay thickness, hp leads to:Equation 4 where Nprop has been defined by Valkó and Economides as the dimensionless proppant number. Vp is the volume of the proppant in the pay. It is equal to the total volume times the ratio of the net height to the fracture height.