Water Softening of High TDS Produced Water

Abstract

Generating steam with an oil field water containing high concentration of total dissolved solids (TDS as sum of total ions) and high silica was tested at 1200 psi in the Wilmington Field in Wilmington, California. This work was during 1990 and 1991. Waters with total hardness of 1000-2300 ppm as CaCO3 with 200–300 ppm silica and TDS of 10,000 and 28,000 were treated with conventional strong acid and weak acid softeners. The steam was generated in a small 1 MM Btu/hr steam generator at 75% and 70% steam quality respectively. Besides obtaining operating costs to compare with previous laboratory and field data, this pilot was to confirm on a larger scale, laboratory data that high TDS, high silica waters could be used in steam generation without silicate problems if the hardness and the iron level were controlled to low levels. Our previous experience had indicated that a low iron level was not controlled in most steam generation using recycle produced waters which resulted in silicate problems, sodium iron silicate or acmite. Other softener pretreat systems such as hot lime, caustic, and steam stripping were considered or tested and would need to be considered for a large installation with water of this high hardness. A patent on using steam stripping as a pretreatment has just been issued as the result of associated pilot testing. In water softening, TDS is usually the sum of the cations or anions as CaCO3 but in this paper TDS is the sum of all the ions except sulfides unless otherwise noted. Introduction During the time of water shortage in Southern California, Union Pacific Resources, Inc. had an 80 MW cogeneration unit which used fresh water from L.A. Metropolitan Water District. The steam from this cogeneration was injected into a 2600 foot steam drive at 1200 psi. The formation had previously been waterflood and the reservoir water was about 28,000 TDS. The return steamflood produced water dropped to about 10,000 TDS in about three months and remained relative constant throughout the steam flood. Several options were considered, including tertiary treated sewage water, fresh water from sea water, use of underlying fresh water sands which were downdip of the injection barrier of L.A. Metropolitan Water District, purchasing unused water from other water districts, and recycle of the produced water. All were pursued simultaneous as the economics clearly favored tiny of the fresh water choices. It was estimated that if 10,000 TDS water were injected, the return water TDS would increase, but still be an acceptable alternate. Recycle of produced water has been done by operators in the Bakersfield, Taft and Coalinga area as well as Canada for years. The main Bakersfield area recycle water is fairly fresh water, 2000 TDS, although it can contain high silica. Hagist etc. and Hagist provide good general summary papers on treatment of water for oil field use. In 1967 Hagist reported the injection of 300 ppm silica into a steam generator water and indicated no deposition. There has been numerous cases of silicate deposits in the various recycled waters but most of these can be traced to hardness upsets or high iron levels in the feedwater. Some recycle has also been overseas in steam floods. For example, we participated in the 1986 field tests in Oman, (Rice), where steam was injected at 2200 psi from waters between 5000–14,000 TDS and 1500 ppm hardness as CaCO3. Silica content was low in these waters but soluble iron was high and resulted in some sodium iron silicate evaporator section problems prior to installing iron removal equipment. This water was conditioned by the use of strong and weak acids softeners. Preliminary Resin Tests The resin capacities of both strong acid and weak acid for the various TDS waters are known with in various ranges based on our own previous experience plus the large amount of work published concerning recycle of steamflood and caustic flooding. P. 143^