Filter Cake Formation During Extrusion Through Fractures
Also see SPE Production & Facilities, February 2003, pp. 65-72.

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4. New Filter Cake Model

Summary

A new model was developed to describe water leakoff from formed Cr(III)-acetate-HPAM gels during extrusion through fractures. This model is fundamentally different than the conventional filter cake model that was used during hydraulic fracturing. Even so, the model accurately predicted leakoff during extrusion of a guar-borate gel. Thus, the new model may be of interest in hydraulic fracturing. Contrary to the conventional model, the new model correctly predicted the occurrence of wormholes and stable pressure gradients during gel extrusion through fractures.

Introduction

This work was motivated by an attempt to understand propagation of formed gels as they extrude through fractures during water shutoff treatments. Formed gels lose water during extrusion through fractures. This water leaks off through the fracture faces and controls the rate of gel propagation. Leakoff also controls the rate of fracture growth during hydraulic fracturing and during produced water re-injection.

Table of Contents

1. Conventional Filter-Cake Model
2. Gel Behavior in Fractures
   • Pressure Gradient Required For Gel Extrusion Increases With Decreasing Channel Width
   • The Two Models Predict Similar Leakoff Rates
3. A New Model for Leakoff And Filter Cake Formation In Fractures
   • Water Can Leave The Gel And Leak Off Through The Fracture Faces, But Crosslinked Polymer Cannot
   • New Model Works Well For 0.25 ≤ N ≤ 4
   • During Extrusion Through Fractures, The Water Leakoff Rate From Immobile (dehydrated) Gel Is Negligible Compared To That From The Mobile (wormhole) Gel
   • Gel Permeability To Water
   • Predicting Gel Propagation Through Fractures
4. Model Differences
   • Comparison Of Predicted Shear Rates And Stress Levels
5. Potential Use In Hydraulic Fracturing