On the Effect of Mobile Oil Saturation On Fracturing Fluid Leakoff Characteristics In High-permeability Reservoirs
Authors: Gadiyar B. , Gupta A. 
Published: March 2010

Abstract
This paper presents the results of experiments and simulations conducted to investigate the effect of mobile oil saturation on the fracturing fluid leakoff characteristics under dynamic conditions in high permeability reservoir rocks. The fluid leakoff behaviour is examined for samples of varying permeability. In addition, the effectiveness of fluid loss additive in the presence of mobile oil saturation is investigated.
A conceptually consistent model to predict the leakoff rate in the presence of mobile oil saturation at the fracture face has been developed. The model is validated using data obtained from dynamic leakoff tests. The results demonstrate that the presence of mobile oil saturation at the fracture face causes the leakoff characteristics to be very different from the tests for which the fracture face is completely saturated with brine. The leakoff characteristics are also affected by the fracture face permeability.

Introduction
In recent years, increasingly higher permeability formations are being hydraulically fractured (frac-pack) to overcome formation damage and/or prevent sand production problems(1–3). It is usually assumed that during initial stages of frac-pack treatments, a large amount of spurt loss occurs, which contributes to low fluid efficiency and high pumping cost(4). Spurt loss should be controlled in order to reduce the pumping cost and improve the economics of frac-pack treatments. Fluid loss additives are expected to reduce spurt loss by blocking the pore throats of the reservoir rock in the vicinity of the fracture face. In the design of fracturing fluid, the presence of oil in the reservoir which can significantly affect spurt loss and the performance of fluid loss additives, is usually ignored.
Very few studies have been published on dynamic fracturing fluid leakoff behaviour in high permeability formation core samples. Further, the core samples in the available studies(4–6) were completely saturated with brine, in contrast to the in situ reservoir samples that have significant oil saturation. The effect of mobile oil 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 investigate the leakoff behaviour of fracturing fluids in the presence of mobile oil saturation. This objective is accomplished by conducting dynamic filtration experiments on core samples containing mobile oil saturation. A rigorous interpretation of these experiments involving cake build-up and multiphase flow is accomplished by using a mathematical model that incorporates the current understanding of the flow of non-Newtonian fluids, filtration and cake build-up, and multiphase flow in porous media(7).

Experimental Setup and Procedures
An experimental setup was designed and assembled for studying dynamic leakoff of fracturing fluids in the presence of mobile oil saturation. This setup is similar to the one used in an earlier study(7) except that the core holder is mounted horizontally.

Experimental Setup
Figure 1 illustrates the schematic of the experimental setup. A detailed description of the setup is presented by Gadiyar et al.(7) The major components of the flow system are: dynamic leakoff core holder, fluid displacement and pumping unit, pressure transducer manifold, and data acquisition system.