Experimental Study and Modeling of Hydraulic Fracturing Fluid Leakoff in Presence of Gas Saturation
Authors: Gadiyar B.R. , Shivaswamy U. , Gupta A. 
Published: December 2007

Abstract
This paper presents the results of simulations and laboratory experiments conducted to investigate the effect of mobile gas saturation on the fracturing fluid leakoff characteristics under dynamic conditions. Fluid leakoff in low permeability gas reservoir rocks is examined under conditions of varying fluid composition and in-situ gas saturation.
A model to predict leakoff behavior in the presence of mobile gas saturation has been developed. The model has been validated using data obtained from multiphase flow dynamic leakoff tests on low permeability samples. The results show that the presence of gas saturation leads to a leakoff behavior which deviates from that obtained for formations which are completely saturated with brine.

Introduction
Hydraulic fracturing is an effective stimulation technique used to optimize the production of gas wells. The fractures and their flow characteristics are critical to the deliverability of many gas wells. Filtrate leakoff characteristics of a fracturing fluid have a significant effect on the gas flow characteristics of such hydraulic fractures and formation fracture interface. For a given reservoir system the leakoff of fracturing fluid into the reservoir should be minimized in order to achieve a cost effective fracture stimulation.
Several studies have been published related to the dynamic fracturing fluid leakoff behavior using formation core samples that are completely saturated with brine, in contrast to the in-situ reservoir samples that have significant gas saturation. The effect of mobile gas saturation in the reservoir on the leakoff characteristics and quality of filter cake has rarely been addressed in the literature.
The objective of the present study is to characterize the leakoff behavior of fracturing fluids in the presence of gas saturation in the reservoir. This objective has been accomplished by numerical simulations using a model that incorporates the current understanding of the flow of non-newtonian fluids, filtration and cake buildup, and multiphase flow in porous media. In order to ensure that all phenomena occurring during the leakoff process have been accounted for in the model, the simulator was validated using data from dynamic filtration experiments on core samples containing mobile gas saturation.

Model Formulation
In order to formulate a model to simulate dynamic filtration, the core sample is considered to be isotropic and incompressible. Gravity forces are aligned with the hydrodynamic forces due to the one dimensional vertical flow in our system. Further, the filtrate and gas are considered to be immiscible, and flow of the filtrate in the core sample is considered to be laminar, incompressible and isothermal. The porosity and permeability of the filter cake are assumed to be constant. The listed assumptions are reasonable for the laboratory filtration studies utilized for validation. For field applications the model can readily be modified to accommodate deviations from the assumptions. A schematic of the flow system being modeled in this study is shown in Fig. 1. The flow system involves interaction between various types of flow that are incorporated in the model as follows:transport of fluid phases in the reservoir rock,filtrate particle mass balance, andfilter cake model.
Transport of Fluid Phases: The continuity equation for the flow of the filtrate phase in the reservoir rock is given by:
(1)
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