Flaws in Fracture Designs: A Continuing Problem

Abstract

Abstract Although hydraulic fracturing has become the second largest outlay of petroleum companies after drilling, design and execution issues are still heavily contaminated by lingering misconceptions and inadequate empirical "rules-of-thumb". Some of the most common problems include issues such as "maximizing" fracture conductivity or fracture length (sometime referred to as "effective" or "apparent"), or even more inadequate goals during execution such as cutting costs by shortening the injection time and using the cheapest possible proppants. Since the introduction of the Unified Fracture Design (UFD) approach, practitioners have a coherent method to design fracturing treatments aiming to maximize the post-treatment productivity index. The central idea of the UFD technique is to select an optimum compromise between propped fracture length and width, for a given proppant volume and depending on the properties of the reservoir and the selected proppant. We present in this paper a wide range of parametric studies for oil and gas reservoirs with permeability ranging from 0.01 md to 500 md, and we assess the impact of treatment size (mass of proppant injected) and proppant pack permeability, starting from the UFD-determined optimum designs and then departing towards suboptimal conditions. These include fracture length, the fracture conductivity and fracture height (by using the concept of fracture aspect ratio). We also apply the same approach to study gas wells, in which turbulence effects in moderate to high permeability reservoirs require the adjustment of optimum design geometry. Finally, we show how a proper candidate selection among a given portfolio for hydraulic fracturing treatments can easily provide a much higher incremental production from a much smaller number of treatments. We use both vertical and horizontal well fracturing. Our results show clearly that there is a lot of room for improvement in fracture designs for production enhancement, particularly dealing with larger treatments and the selection of better quality proppants (higher pack permeability), especially for gas wells. This leads to a systematic approach for design and a studious departure from optimum design conditions, when necessary, based on field constraints and reservoir and fluid characteristics.