Abstract

Dynamic fluid loss measurements were conducted on core samples ranging in permeability between 0.02 to 140 md. These tests were run to measure the effect of several parameters on the foam fluid loss coefficients. The parameters tested were: core permeability, gel concentration in the liquid parameters tested were: core permeability, gel concentration in the liquid phase, foam quality, temperature, core length and differential test phase, foam quality, temperature, core length and differential test pressure. pressure. The type of foam that is used in most conventional fracturing treatments is a wall building fluid. Although this foam has excellent inherent fluid loss properties, the fluid loss values reported in this paper more closely resemble those of conventional fracturing fluids than paper more closely resemble those of conventional fracturing fluids than reported earlier. These values have been used in the successful design of field fracturing treatments.

These data support the mechanism of two phase flow in porous media suggested by Holm. The fluid passing through the cores was rich in liquid phase with composition proportional to the viscosity of the liquid phase. phase with composition proportional to the viscosity of the liquid phase. The broad range of fluid loss coefficients for foam calculated in these tests are intermediate in value to those reported in similar tests by Blauer and Kohlhaas, who obtained lower values, and King, who obtained higher values.

Introduction

Foam has been established as a successful fracturing fluid for several years. Claims as to its efficiency in fluid leak-off control have ranged from excellent to virtually no leak-off control at al,. Yet treatment experience has indicated that foam fracs do occasionally screen out. Since excessive fluid leak-off is one potential cause of a premature job termination, an adequate knowledge of fluid loss coefficients is essential for proper design of stimulation treatments.

Foam has previously been described as a non-wall building fluid. Such a fluid should have leak-off properties described by Howard and Fast by

where k is permeability in darcies, is pressure drop in psi across the matrix, is fractional porosity, and p is viscosity in centipoises. There is a problem with calculating for foam, however. Since foam is a two-phase structured fluid, and the bubbles may be in the same size range as the pores of the rock matrix, the rheology of foam in porous media is not well defined. The expansion of bubbles in the foam due to pressure drop can be significant.

A fluid loss coefficient can be determined empirically without knowledge of the fluid viscosity by using the equation from Howard and Fast

where m is the slope of an experimental plot of fluid volume versus the square root of time, and is the cross sectional area of the filter medium in square centimeters. is useful for wall building fluids, whereas is intended for non-wall building fluids. For non-wall building fluids the slope of the experimental plot of filtrate volume will be linear with time, rather than the square root of time.

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