Unified Fracture Design: A New Look Towards Hydraulic Fracturing Design
Unified Fracture Design (UFD) is a relatively new design methodology to optimize hydraulic fracturing job. Previous models generally optimize fracture length for frac design but UFD uses proppant mass as the critical parameter for optimization. This design methodology cuts across all petroleum reservoirs i.e. from low to high permeability reservoirs and from soft to hard rocks. A proper design and execution of a fracturing treatment involves several disciplines such as reservoir, production & completion and requires a background in rock mechanics and fluid dynamics, constrained by the physical limits of the materials and equipment used as well as by the operational issues, and last but not least must satisfy certain economic criteria.
The central idea behind the UFD technique is to select the optimum compromise between propped fracture length and width, for a given proppant volume (proppant mass). Economides and Valkó introduced the concept of the dimensionless Proppant Number (NP) which is the ratio of the fracture volume and the reservoir drainage volume, adjusted by the reservoir permeability and the proppant pack permeability, and independent of fracture geometry, and then they found that for a given value of NP there is an optimal Dimensionless Fracture Conductivity (CfD), at which the dimensionless productivity index JD is maximized. The optimized and specific dimensionless fracture conductivity leads to the unique optimal fracture geometry (width and length).
In UFD approach, two types of optimization is done, first one is physical and the second is economical. Generating the optimum fracture geometry will lead to the maximum well productivity, which is the physical optimization. But all of that can be done within an economic optimization where the NPV can be maximized and selecting the proppant mass as the crucial parameter for the sensitivity analysis.
The stages for optimum design are as follows;
Determine the amount of proppant reaching the target layer through NPV.
Determine the proppant number.
Determine the optimum dimensionless fracture conductivity from proppant number.
Determine optimum length and width from the optimum dimensionless fracture conductivity and the available proppant.
Determine the injection time and proppant schedule realizing the optimum dimensions
Once the volume of proppant to be placed in one wing of the fracture, Vf, and the optimum dimensionless fracture conductivity, Cfd are known, the optimum fracture dimensions can easily be calculated.
Plot to estimate optimum productivity index from calculated value of optimum CfD. Taken from work of Economides & Valko.
This work is heavily augmented by the timeless work of Frac Champions - Micheal Economides & Peter Valko. Our industry can never repay these gentlemen for their contribution towards work on unconventional reservoir exploitation.
This blog is work in progress - we will outline the whole Unified Fracture Design process in simplest terms for the everyday user to take maximum benefit out of this novel technique. Please stay tuned!
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