Generating steam with an oilfield water containing a high concentration of total dissolved solids (TDS) was tested in the North Midway Field, Kern County, California. Conventional zeolite and weak acid cation exchange softeners were used for feedwater treatment, and steam was generated in a small scale oilfield boiler. Feedwaters with TDS concentrations up to 22,500 parts per million were successfully softened to "zero" parts per million were successfully softened to "zero" hardness levels and converted to 70% quality steam at 2300 psig. Results show that it is not necessary to have relatively fresh water for use in conventional oilfield steam generators. Furthermore, water treatment costs are economic.
Steam is produced in most oilfield generators by force circulating the feedwater through a system of watertubes heated by hot flue gases and radiant heat emissions. Depending on operating parameters, this usually results in a mixture of 70% steam and 30% hot water (by weight) at the generator outlet and discharges directly into an injection well. There is no mechanical separation of the steam and hot water mixture nor is the remaining hot water recirculated through the heater tube to be eventually converted to steam. Because "wet" steam is produced, the water quality requirements are much less stringent than for a conventional boiler.
Highly soluble sodium salts constitute most of the dissolved matter found in high TDS waters produced in western Kern County. They are tolerable produced in western Kern County. They are tolerable as feedwater constituents and can be concentrated in the hot water portion during steam generation. The solubility of these salts is quite high and increases with temperature as shown in Table 1. They should not cause serious scaling or corrosion as long as free oxygen and hardness are not present.
Treating for free oxygen is relatively straight forward and will not be discussed. However, hardness removal by conventional cation exchange softening is adversely affected by high concentrations of sodium salts in the feedwater.
Unpublished laboratory results indicate hardness leakage from a softener begins at TDS levels above 5,000 parts per million (ppm). At approximately 20,000 ppm, no hardness removal occurs with conventional softening. Any detectable hardness is undesirable in boiler feedwaters. This is primarily why high TDS waters have not been used in oilfield steam generators and relatively fresh waters are used.
Laboratory experiments run prior to the subject field test indicated that high TDS oilfield brines can be softened to "zero" hardness levels by using weak acid softening resins. The waters tested were similar to those produced in most California heavy oil fields.
Beginning in January 1978, a test was initiated to observe weak acid resin performance in the field and establish any adverse effects in a steam generator supplied with a softened high TDS feedwater. The test site was located in the North Midway Field, California (Figure 1). Feedwaters containing TDS concentrations ranging from 7,000 to 22,500 ppm and steam pressures from 500 to 2,300 psig (3,447 to 19,958 KPa) were utilized.
Some of the data generated during the field test are presented in this paper. The procedures used to soften the high TDS steam generator feedwater are provided including the results of softener and steam generator performance tests. Analysis of the steam generator heater coils, verifying that scale deposition and corrosion had not occurred, are discussed. Estimated water treatment chemical costs showing the use of a high TDS feedwater to be economically viable are also presented.
Since adequate water softening is an important consideration in using high TDS steam generator feedwaters, let us briefly discuss the principles involved.