Experimental study of the impact of frac fluid invasion and mitigation of formation damage using surfactants in conventional and tight formations

Date

2019-12

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Abstract

Hydraulic fracturing technique has been the most widely accepted and profitable method used to produce oil from tight formations in U.S, that has been more prevalent since the advent of horizontal drilling of wells. Extremely high pressures are used to inject fracture fluid into the formation to prop open the fractures followed by a subsequent resting period of this fluid in the fractures before re-completing the well for production. Due to this process, formation damage in terms of invasion of fracture fluid into the matrix results which impacts the relative permeability of oil to flow into the fractures from the formation matrix surrounding it. Studies directed to understanding the detailed factors which could mitigate such formation damage have been minimal in the past.

Some researchers proposed and studied the impact of shut-in operation either before production or post-production to regain the lost permeability to oil caused by the invasion of fracture fluid. But these could either be operationally uneconomical or sometimes need undesired repetitive shut-in procedures. Surfactants have been conventionally used in the fracture fluid as flowback aids or emulsions breakers but have not been looked upon as EOR aids for hydraulic fracturing operation. Henceforth, this research study is formulated to highlight all the factors that impact the mitigation of invasion-induced formation damage in the matrix of conventional and tight formations.

The factors of initial wettability, depth of invasion of aqueous fracture fluid into the matrix and the presence of IFT-reducing surfactant in the fracture fluid are investigated to understand their impact on oil production rates and flowback efficiencies of invaded fluids, using a novel technique called microfluidics. This technique provides an easier and very reliable mode of quantification of saturation distributions inside the matrix. Wettability-altering surfactant is also investigated to understand its significance in not just the enhanced oil recovery from soaking operation but also from the production operation after the invasion process. Through well-designed and successfully executed microfluidic and core flooding experiments, it is found out that a high flowback efficiency does not always translate to a high oil production rate, as in the case of initial water-wetting formation. The strength of water-block in the water-wet matrix only present at the interface of fracture, influences the characteristic of flowback efficiency, but such a presence of water-block is not observed in oil-wet matrix. Addition of moderate-IFT reducing surfactant into a frac fluid benefits water-wet rock for all invading depths of frac fluid, but benefits oil-wet matrix only at a deeper depth of invasion. Moreover, it is also found out that a wettability-altering surfactant from oil-wet to water wet with moderate IFT-reduction exhibits a favorable oil recovery during the soaking process irrespective of permeability but does not lead to optimal oil production after the flowback process. With increased tightness of the rock, the invasion of surfactant into the matrix led to more ineffective reduction in invasion-induced formation damage. Based on this observation in tight rocks, it is suggested to not allow for any leak-off of surfactant fluid into the matrix through fractures in the case of shales.

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Keywords

Fracturing, Surfactant, Invasion, Leakoff, Flowback, Conventional, Tight, Microchip, Microfluidics, Chipflood, Coreflood, Imbibition

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