Design Flaws in Hydraulic Fracturing

Abstract

Abstract There are many widely held and often repeated misconceptions in the design of hydraulic fracture treatment. These include the design for fracture conductivity, fracture length (for which some have used terms such as "effective" or "apparent"), time of injection, and proppant injection schedule. These issues are intimately connected with the selection of proppants, fracturing fluids, and of course, the hardware for field execution. For those practitioners who have adopted Unified Fracture Design (UFD), the desired fracture geometry is not a matter of speculation. For the given reservoir permeability, drainage, and, volume of specific proppant injected, the fracture geometry is set. A design that follows UFD will maximize the post treatment productivity index. Thus, statements like "maximizing conductivity" or "maximizing effective length" are not appropriate and in fact, they are meaningless. However, there is a lot of room for improvement in design. These include the selection of a large mass of proppant or changing the proppant or using non-damaging fluids which, in this context, means damaging in the proppant pack or damaging to the formation by leak-off. We present in this paper a wide range of designs for three oil and three gas reservoirs with permeability of 0.01, 1 and 100 md, respectively. Results show the impact on the productivity index by 1) deviating from UFD design, 2) the beneficial effects of lack thereof using more of better proppant, and 3) the effect of fluid selection and their leak-off, and the resulting different kind of damage. We show which kind of damage is important and which is unimportant. We also examine the impact of the proppant injection schedule for both unrestricted and tip screen out (TSO) treatments. In the low permeability reservoirs, the selection of fluids and proppant schedule become crucial in obtaining the required fracture length. Fast ramping up of proppant is never indicated. Furthermore, leak-off damage is not important. For the high permeability reservoirs, selection of better proppants and fast ramping up of their injection are keys to the design. For the high permeability gas well, it is necessary to remedy the turbulence effects by changing the design further.