Abstract

Hydraulic fracturing is a powerful technology being used for decades in the petroleum industry to recover and/or enhance the permeability. The fully 3-D numerical simulation of the hydraulic fracturing process is a great useful tool to analyze and optimize the designing stage of the stimulation, it, however, is a difficult challenge due to the strong nonlinearity of coupling between the viscous flow of fluid and fracture propagation. In this study, a coupled hydro-mechanical 3-D numerical model based on the finite-element method has been developed to simulate the hydraulic fracturing process in naturally fractured reservoirs (a common feature of Iranian hydrocarbon reservoirs). In addition, by taking advantage of this model, reservoirs with damaged-zone have been investigated. In this model, the fracturing process governed by a cohesive-zone-model within a poroelastic medium. Cohesive-zone-model enables us to assign individual criteria for fracture initiation and propagation for each simulation. The results revealed that hydraulic fracturing might not be always beneficial in case of reservoirs with strongly damaged-zone. This is caused due to the increase of pore pressure in damaged-zone, and the plastic flow of the fracture wall. Thus, it is recommended to improve the permeability of the damaged zone in prior to hydraulic fracturing operation with another method such as acidizing. Furthermore, the results showed that the fracture propagation stops once the fracture tip meets major natural preexisting fractures. Accordingly, the final induced-fracture length is strongly affected by the location of the natural-fracture network. Moreover, based on the results, the highest fracture-closure pressure occurs at the fracturing initiation point on the well-bore wall. This should be considered at the proppant embedment designing stage, since if the proppant resistance fails to bear this load, the reservoir connection with the well-bore through the fracture would be either poor, or lost. Hence, the hydraulic fracturing job would fail, or not be efficient.

1.
Introduction

Hydraulic fracturing is a process designed for stimulating and improving the function of oil, gas, and geothermal wells (Brown, Smith, and Potter 1972; Ernst 1977) to enhance their productions. This technology could also be applied to other type of wells such as injection or water disposal wells (Zimmermann and Reinicke 2010). Moreover, this technology is being used in nuclear industry to extract uranium sources. In technology, by injection of a high-pressure fluid (fracking fluid consists of a mixture of viscous hydraulic fluids and sorted sand, so-called proppant) into a wellbore the rock formation would be pressurized and finally fractured. The increasing pressure propagates the induced- crack whose length may exceed more than several hundred meters. Afterwards, when rock is fractured and the pressure of the injected fluid is removed the suspended solid phase, which remains inside the crack will keep the fracture open. Such a fracture provides a high permeable flow path through for the fluid to be either extracted, or injected. According to review studies hydraulic stimulation technique is the most common applied wellbore stimulation technique worldwide (Mack MG 2000; Breede et al. 2013).

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