Chemical Characterization Problems of Water-Soluble Polymers

Water-soluble polymers are being used increasingly in oil, gas, and geothermal production. Applications include drilling, stimulation, workover and completion, and reservoir flooding fluids. The development of polymers and their application has been mostly empirical. Such a course of development was suitable in the past. However, empirical techniques do not satisfy present and future needs which include (1) the cost/performance relationship and (2) environmental effects associated with expanding polymer application. Therefore, a more thorough understanding of the polymer chemistry is required.The first step in doing this is to develop laboratory methods to characterize these complex materials and their degradation products. The problems are (1) understanding polymer chemistry under field conditions and (2) developing analytical procedures. These problems emerged dramatically during analysis of recent fracture stimulation of some geothermal wells. An involved study of the potential analytical methods was conducted. Polysaccharides were used for the actual field fracture jobs as well as for the analytical procedures. Correlations were made between the total organic content and carbohydrate content of the return waters as a function of residence time under simulated reservoir conditions. Preliminary indications are that more sophisticated information can be obtained by the use of emerging analytical techniques such as high pressure liquid chromatography (HPLC).Advantages gained from use of these methods and others are discussed. Introduction A variety of polymers are used in the petroleum industry for drilling, workover, and completion fluids. Many of these polymers can be used in the geothermal industry for similar applications. However, because the environment of a geothermal reservoir may be drastically different from that of a petroleum reservoir, it is critical that these polymers be investigated under conditions that simulate geothermal environments. In the past, physical property measurements of aqueous solutions of these polymers have been emphasized - particularly fluid rheology both for petroleum and, to a lesser extent, geothermal applications. These physical properties, which are valuable in selecting polymers or polymer blends for use, can be related to the chemical properties of the polymers. Properties such as molecular weight distribution and macrostructure, molecular conformation, side-chain structure, composition of the monomer units comprising the polymer backbone, chemical interactions in the make-up water, chemical and thermal stability, etc., play an important role in determining the ultimate physical properties of the polymer in solutions.Many of these chemical features have been overlooked, and the development of polymers for field applications has followed a strictly empirical course. This empiricism has led to a great deal of confusion when polymers must be selected for field use. A more serious drawback has been the lack of new polymer types - largely because little is known about how the physical properties desired can be related to polymer chemistry. This can be traced for the most part to the lack of chemical methods available in the past to characterize the polymers chemically in sufficient detail. The high-temperature requirements of geothermal applications impose severe limitation on the fracture polymers, particularly their performance and chemical stability under high-temperature conditions. SPEJ P. 721^