Cementing Steam Injection Wells in California

Abstract The results of steam injection for production stimulation have created an incentive among oil producers to increase their drilling activity into heavy oil deposits which generally are found at rather shallow depths. Introduction The use of heat in some form has been tried for many years to stimulate oil production. In situ combustion, downhole heaters, hot fluid injection and now steam stimulation are all being used. The ability to drastically lower oil viscosity in place using one of these methods, thereby increasing production rates and recovering additional oil, has brought about salvation for many low gravity crude producers in many fields. Heat is energy and many of the low gravity producers require all the energy which can be put into them to yield the heavy oil. Steam stimulation now appears to be the most promising and is being used by many producers in the California oil fields. Steam injection, by either of two methods, has become a complex problem. It appears that the displacement or flood technique is less troublesome once the field has been prepared for it, but the necessary flow lines and steam generating equipment makes the initial expense very high. Intermittent steam injection using the huff and puff technique may not be as costly, but it certainly has many trouble spots. It no longer appears that all that is required is a steam generator, it suitable water supply and any old well. As long as steam injection temperatures remain below 400F problems seem to be at a minimum. But as injection pressures increase with their corresponding temperature increase, unforeseen problems with the cement, cement to pipe and/or cement to formation bond and casing have arisen. This paper discusses the problems involved in obtaining a sound cementing job and suggests methods of allowing the cement to do what it is designed for. Description and Definition of Materials Used Much has been written on the effects of heat on cementing compositions. Laboratory investigations disclosed that above approximately 230F there was a pronounced loss of compressive strength and an increase in permeability of many compositions. Any compositions containing additives which are not chemically reactive with the cement and which cause a high water-cement ratio create poor temperature stability. Bentonite is probably the worst offender and should not be used in any composition in excess of 4 per cent by weight of the cement. We will not go into the limitations of Portland cement at elevated temperatures. It is significant, however, to stress the advantages of silica flour as a stabilizing additive at these elevated temperatures. This admixture has been evaluated in many laboratories using a wide variety of cementing compositions and found to be very beneficial. Results of the tests indicate that a minimum of 30 per cent silica flour by weight of the cement was required to obtain temperature stability with a maximum of 60 per cent by weight of cement. The more common quantity being used at the present time is 40 per cent. Table 1 presents slurry properties of compositions having application in thermal projects, while Table 2 indicates the effects of temperature upon the compressive strength. General Properties of Materials Cement slurries, when cured in a moist atmosphere, exhibit some slight expansion upon setting. This aids in development of bonding strength both to the pipe and to the formation. JPT P. 431ˆ