Alkali Solubility of Micronized Cellulose Fiber and Evaluation Using Return Permeability Measurement
Authors: Griffin J.M. , Cowan J.C. 
Published: December 2007

Micronized cellulose fibers have been used successfully to control drilling fluid seepage losses (less than 20 bbl per hour). While the success of micronized cellulose fibers has been well documented when used alone or in combination with calcium carbonate, their application for controlling losses in productive zones has been viewed with skepticism due to their limited solubility (35% by weight) in HCl. Consequently, an obstacle to the expanding role of cellulose fibers has arisen on this utilitarian additive.

In this research, this restraint has been removed as micronized cellulose fibers can now be completely digested (97%) in a specially formulated alkali formulation (33% NaOCl and 15% KOH). Extensive testing which led to the successful formulation is reported.

Having established solubility in an alkali solution, return permeability tests were conducted. Permeability of Berea cores was established using 7% KCl. Cores were then sealed by filter cake deposition of micronized cellulose fibers in a 9.0 lb/gal NaCl polymer system. Filter cake deposition was followed by treatment and digestion in the alkali formulation, and compared with digestion in an acid formulation. Permeability testing was conducted using a HTHP (high temperature, high pressure) core holder fitted for 1 inch × 1 - 3/4 inch for screening formulations initially and a Hassler sleeve core holder for verification of the most beneficial formulations.

Introduction

The success of micronized cellulose fibers in controlling both seepage and larger fluid loss in depicted sands has been well documented. Success has also been reported when cellulose fibers are used to control seepage loss in vuggy carbonates. Micronized cellulose fibers were introduced as specialty seepage loss additives for drilling fluids in the late 1970's.

The seepage loss additives were designed specifically for small losses of under 20 bbl per hour. They were specifically aimed at reducing differential sticking problems and/or loss of expensive fluids in low-pressure depleted sands that are common in the U.S. Gulf Coast Area.

The original micronized cellulose fiber product was designed primarily for use in oil base drilling fluids and was tailored to work in weight ranges above 12 lb/gal. All of the extremely fine fibers and extremely coarse fibers were removed from the product and a selected cut was taken from the middle size range. Removal of the extremely fine fibers reduced potential and actual, undesirable increases in viscosity. Removal of the extremely coarse fraction was justified as this fraction contributed little to seepage control.

The benefit for drilling fluids, when this selected-size-range loss additive was added to fluids containing adequate colloidal solids, was that seepage loss control could be maintained across high permeability, low-pressure, depleted sands. This control allowed for the successful drilling through formations that were previously noted for severe, differential sticking.

Although originally developed for weighted oil base muds, it was discovered early in the development of micronized cellulose fibers that they could be used just as successfully in water base fluids. The early 1980's saw a wide expansion of the usage of micronized cellulose fibers in water base muds.

The early use of micronized cellulose fibers was almost exclusively confined to drilling operations. Since the early 1940's, there has been a major resistance to the application of cellulose fibers against producing formations. Early formation damage studies that tested guar products, i.e., pulverized crude guar bean, showed significant impairment. For this reason, there was a reluctance to consider micronized fibers on producing formations.

However, a change in practice occurred during the mid 1980's when a major operator in the North Sea explored the use of micronized cellulose fibers for preventing massive losses of workover fluids to producing formations.

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