Physical And Chemical Properties Of Cement Exposed To Geothermal Dry Steam
Authors: Gallus J.P. 
Published: December 2007

Cements of various compositions were exposed to produced dry geothermal steam in The Big Geysers geothermal field in northern California. The cement specimens were exposed to the steam for varying periods of time up to two years. Steam temperature was maintained at a constant 240 degrees C (464 degrees F). The purpose of the experiments was to obtain a data base from which to predict the useful life span of selected cements under actual geothermal conditions.

Examination of cement compressive strength and permeability properties as well as study of the chemical composition changes at various time intervals by X-ray diffraction analysis established when a cement composition reached chemical equilibrium or "stability". With the chemical stability determined, a reasonable prediction was considered possible regarding durability of the cement for possible regarding durability of the cement for the expected service life of an average geothermal well.

A single cementing formulation was found to be sufficiently resistant to geothermal dry steam for application in the Big Geysers steam field. Two or three other compositions were found to be possibly adequate but additional testing is indicated possibly adequate but additional testing is indicated as necessary.

Introduction

The work reported in this paper was undertaken to determine:Whether or not any existing cement(s) may be satisfactorily durable for application in geothermal dry steam wells for an anticipated well service life of approximately 25 years orif a satisfactory new cementing composition must be developed.

Extrapolation of limited data available at the time had shown the possibility of cement failure within ten years. In addition, no information was available regarding the progress of cement strength retrogression in geothermal dry steam environments (see data points, FIGURE 1). Other, rather sparse information, also indicated that existing cements may deteriorate under geothermal conditions in very short periods of time.

With the upswing of interest in geothermal energy, the need became critical for reliable information on the effects of geothermal temperatures and fluids upon cements. We opted to empirically test a variety of cementing formulations in actual geothermal produced dry steam because the required data could so be obtained in the shortest possible time. Cement samples could be exposed to the actual produced dry steam at essentially bottom hole temperature and pressure using surface equipment.

Early field experimentation with conventional laboratory prepared, precured 2-inch cement cubes in direct contact with the produced steam had resulted in unrealistically high sample failure rates and testing problems. The failures of the unconfined cured solid cubes were attributable to both thermal shock and to the direct contact with the flowing steam. The severity and persistence of these early difficulties therefore provided the impetus for the development of an improved, very representative thermal cement testing method that enabled us to generate reproducible and generally satisfactory results throughout this investigation.

We wish to direct here the attention of the reader to the fact that this report describes experiments with cements exposed to dry produced geothermal steam, only, i.e. to fluid which is free of liquid, salts or other dissolved solids. This paper is therefore to be distinguished from another publication which describes cements in contact with geothermal flashing brines of different salinities.

APPARTUS, PROCEDURE, AND CONDITIONS

Samples of the cementing formulations to be tested were blended at the well site - exactly in accordance with API RP-10 recommended procedures - using water normally being utilized in cementing in the Big Geysers field.