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Technical Paper: Mechanically Induced Fracture Face Skin - Insights From Laboratory Testing And Numerical Modelling

Society: SPE
Paper Number: 144173
Presentation Date: 2011
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Abstract

In context of this work, a new formation damage mechanism is proposed: the mechanically induced fracture face skin (FFS). This new mechanism results from mechanical interactions between the proppants and the reservoir rock, due to the increasing stress on the rock-proppant system during production. Proppant embedment into the fracture face and proppant crushing leads to fines production and may impair the fracture performance. In order to achieve sustainable, long-term productivity from a reservoir, it is indispensable to understand the hydraulic and mechanical interactions in rock-proppant systems. Permeability measurements on sandstones with propped fractures under stress using different flow cells were performed, allowing localizing and quantifying the mechanical damage at the fracture face. The laboratory experiments identified a permeability reduction at the fracture face up to 90 %. The mechanical damage at the rock-proppant interface begins immediately with loading the rock-proppant system and for fracture closure stresses below 35 MPa; the damage is localized at the fracture face. Microstructure analysis identified quartz grain crushing, fines production and pore space blocking at the fracture face causing the observed mechanically induced FFS. At higher stresses, damage and embedment of the ceramic proppants further reduces the fracture permeability. Numerical modeling of the rock-proppant system identified highly inhomogeneous stress distributions in the granular system of grains and proppants. High tensile stress concentrations beneath the area of contact between quartz grains and proppants are observed even at small differential stress applied to the rock-proppant system. These high stress concentrations are responsible for the early onset of damage at the fracture face. Therefore, even low differential stresses, which are expected under in-situ conditions, may affect the productivity of a hydraulically fractured well.

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