Factors Affecting Dynamic Fluid Leakoff With Foam Fracturing Fluids
Authors: King George E. 
Published: December 2007

Abstract
Dynamic laboratory fluid loss testing of foam water base fluids on low-permeability, thin sandstone core wafers has established fluid loss coefficient values as well as defining some of the variables that control fluid leakoff from a foam. Liquid additives that can reduce fluid leakoff through the core wafers also have been developed. These additives reduce fluid leakoff by threefold or more in concentrations as low as 50 ppm of the water volume.

Introduction
Water and nitrogen foam for fracture stimulation has found acceptance due to the low water content of the treatment fluid and the excellent cleanup effects of nitrogen gas. Foam fracturing treatments have been used extensively in shallow, low-pressure gas producing formations and have in many cases producing formations and have in many cases outperformed gelled water fracture treatments in stimulating these formations.
Significant gas reserves are contained in low-permeability sandstone formations. Tests of foam fluid leakoff through low-permeability sandstone core wafers were necessary to furnish data for designing the optimum fracture treatment for these formations. This report presents the fluid loss coefficients for foam on low-permeability sandstones as well as discussing some of the factors that influence foam leakoff.
Until now, the addition of long chain polymers to the water phase of the foam has been the only method of reducing foam fluid leakoff. This paper discusses new liquid additives that have been tested and have shown the potential of controlling fluid leakoff without adding long chain polymers to the foam that may leave a residue in the formation.

Summary
Of the naturally occurring factors tested, only the permeability of the core wafer and the test pressure differential had any significant effect on pressure differential had any significant effect on the fluid leakoff of the foam. Foam quality and foamer concentration did not appear to have an effect on the fluid leakoff rate.
The addition of from 50 to 500 ppm of selected liquid leakoff additives will reduce the liquid and gas leakoff rates by a factor of two- to fivefold. These additives appear to act on the gas in the foam to alter bubble properties. The performance of these additives is approximately comparable to the leakoff control obtained with a 15-lb concentration of hydroxy-ethyl cellulose per 1,000 gal of water. The cost of these organic additives is about 1/50th of the cost of polymer addition.

RESULTS AND DISCUSSION

Apparatus and Test Procedure
The test apparatus used for all the foam tests is shown schematically in Fig. 1. A test stand supports all the cell-related equipment in close proximity to minimize friction or shear in the piping proximity to minimize friction or shear in the piping system. Glass-windowed viewing chambers are included to check the general foam condition before the fluid passes into the test cell. The visual inspection of passes into the test cell. The visual inspection of the foam determines if all liquid and gas are tied up in foam and gauges bubble size and shape, which are an indication of the condition of the foam. Normally, the most stable foams have reasonably uniform small bubbles with no large pockets of gas.
The basic design of the body of the test cell is similar to the API baroid high temperature, high pressure, fluid loss cell for measuring fluid loss on muds and fracture fluids. The test cell is constructed (Fig. 2) to allow flow of the foam over the face of the wafer and uniform contact of the foam on the surface of the wafer. The inverted funnel shown in Fig. 2 spreads the foam over the wafer to insure uniform contact of the surface with the foam, thus preventing a collection of water or gas on the surface of the wafer.