TO DEVELOP CEMENTS FOR THE CASING OF GEOTHERMAL WELLS WHICH CAN RETAIN THEIR PROPERTIES WHEN EXPOSED TO TEMPERATURES IN THE RANGE 150-350 DEGREES CELSIUS, PRESSURES OF 50 MPA AND BRINE SALINITES UP TO 250 DEGREES GAY LUSSAC.
Cementing materials laid in a casing formation annulus must be stable and maintain the necessary physical characteristics during the entire life of the well. The purpose of this study was to select the best components for formulating cement relative to conditions at the bottom of the hole. The long term behaviour of materials already laid in wells, or likely to be laid in them, was determined under the combined effects of temperature, pressure and formation fluid type. Bottom hole conditions were simulated in a laboratory apparatus which was specially designed to withstand high pressures (up to 50 MPa) and high temperatures (up to 350 C) and which provided long term resistance to corrosive fluids.
Various materials have been studied including hydraulic cements standardised by the American Petroleum Institute, experimental hydraulic cements, which are easy to market, and thermosetting organic resins. A classification by quality has been drawn up, based on mechanical resistance levels, hardened material permeability and long term stability. Results depended on the bottom hole conditions. In certain cases (vapour at 200 C approximately) the phenolic furanic resin based compositions have proven to be more stable than those prepared with hydraulic binders. Even though development is limited, the application of physical techniques (X-ray tomography, scanning microscopy, analysis with a Castaing probe) to materials hardened with a hydraulic binder base has made it possible to establish a relationship between the change in characteristics and the change in structure.
The tightness of the casing formation annulus is a safety condition of prime importance for both oil and geothermal wells. It prevents formation fluid channelling from one formation to another or to the surface of the well.
After cementing materials have been pumped into the casing formation annulus, they must maintain suitable characteristics during the entire lifetime of the well. This study concerns the long term behaviour of some actual or potential cementing compositions after they have been cured and hardened under bottomhole conditions.
Bottomhole conditions were simulated by specially designed laboratory equipment, including high pressure (up to 50 MPa) and high temperature (150 to 350 C) cells capable of withstanding long term exposure to aggressive fluids. Several types of materials were considered, including American Petroleum Institute (API) well cements, experimental hydraulic cements with a specified composition, and an organic phenol-formol-furan resin. Behaviour during aging is characterized by compressive strength and water permeability. Scanner tomographic analyses were performed on some specimens, giving a cartographic representation of density and porosity.
Class G cement silica systems which are characterized by a 2:3 silica cement ratio still maintained suitable performances at 345 C, at least for short time aging. However, above 150 C, tomographic analyses revealed a heterogeneity of the specimens due to slurry particle settling and probably to differences in the local structure of the cement. Class J cement did not seem to have better performances than class G cement silica systems.
A steam environment was unfavourable for a class G cement silica system and for a class J cement. Indeed, permeability increased very fast between 3 and 6 months after cementing. Under the same conditions, cements with aluminous slag were less damaged and their water permeability was only slightly modified.
The first tests with the phenol-formol-furan r esin seem to be hopeful.
THE WORK, WHICH IS AN EXTENSION OF WORK PERFORMED UNDER CONTRACT NO.EG-B1-008-F WILL INVESTIGATE IN THE LABORATORY THE BEHAVIOUR OF DIFFERENT CEMENT FORMULATIONS UNDER THE COMBINED EFFECT OF HIGH TEMPERATURE AND HIGH FLUID SALINITY. THE CEMENTS TO BE TESTED WILL INCLUDE THOSE IDENTIFIED UNDER THE EARLIER CONTRACT AND OTHER CEMENT BASES WHICH ARE COMMERCIALLY AVAILABLE BUT WHICH HAVE NOT YET BEEN EVALUATED.