Effect Of Silica Particle Size On Degradation Of Silica Stabilized Portland Cement
Authors: Eilers Louis H. , Nelson Erick B. 
Published: December 2007

Work performed under the auspices of U.S. Department of Energy, Contract No. EG-77-C-02-4190 and coordinated by Brookhaven National Laboratories. By acceptance of this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a non-exclusive royalty free license in and to any copyright covering this paper.

Abstract
Portland cements require the addition of fine silica (quartz) to maintain reasonable strength and permeability at elevated temperatures. The production permeability at elevated temperatures. The production of the desired high temperature stable silicate, xonotlite, is dependent on the calcium to silica composition of the cement, curing temperature, particle size of silica, particle size of cement, and fluid (fresh water, brine, etc.) contacted by the cement. Greater deterioration is generally observed when compounds having higher calcium to silica ratios than xonotlite are produced. The production of one of these compounds, kilchoanite, is favored by using a fine grind cement with a coarser silica at elevated temperatures in contact with a heavy brine. A relevant factor is that a coarse silica has a greatly reduced rate of solution in a heavy brine at elevated temperatures.
Sand fractions from approximately 175 micron (80 mesh) to 3.2 average micron range were tested and compared with commercial fine silica and silica flour as stabilizers for Portland cement. The finer fractions, including silica flour, produced lower permeabilities and usually higher strengths. Addition of permeabilities and usually higher strengths. Addition of 35 percent silica flour by weight of Portland cement is sufficient to stabilize the hydration products of Portland cement under usual geothermal conditions up Portland cement under usual geothermal conditions up to 325 degrees C (617 degrees F), the highest temperature in this study. Coarser silica may be used where temperatures are lower, steam or light brines are encountered, or coarser cements with extra silica are used to prepare the slurries.

Introduction
The research reported in this paper was performed in connection with a Department of Energy (DOE) contract to investigate current geothermal cementing technology and develop improved cementing materials for geothermal wells. The chemistry of silica stabilized Portland cements exposed to dense brines at geothermal temperatures is unusual, and we find that this aspect of cement chemistry has not been previously mentioned in literature. previously mentioned in literature. Fine silica has been used to prevent deterioration of Portland cement at temperatures above 120 degrees C (250 degrees F) for a number of years. Today the development of wells at higher temperatures for recovery of geothermal fluids, deep gas, geopressured fluids, and artificially heated steam or fire flood wells places increased stresses on the cements. The behavior of Portland cements under such conditions has been published recently. Included are the behavior of geothermal cements under simulated and actual well conditions. The reported temperatures have varied from 150 degrees C (300 degrees F) to over 425 degrees C (800 degrees F). The environments have varied from static fresh water at saturated steam pressures to geothermal fluids (steam to 10 percent brine) at flowing conditions.
An extensive series of potential geothermal cements has been evaluated in the present project. In a preliminary screening program, many cement systems failed to meet strength and permeability requirements and were dropped from further testing. This included cements not reaching a compressive strength of 3500 kPa (500 psi) in 24 hours or 7000 kPa (1,000 psi) after 7 days. Likewise, normal weight cements with permeabilities greater than 0.1 md and lightweight permeabilities greater than 0.1 md and lightweight systems greater than 0.25 md were eliminated. During this phase of the screening program it was noted that the behavior of silica stabilized Portland cements cured in heavy (25 percent solids) geothermal brines was dramatically affected by the particle size of the silica originally added to the cement.