In the course of our laboratory investigation of HEC gel return permeability, with regard to formation damage (see also, SPE #27366) we found that the gel quality assessment, as related to the gel breakback is based on the ancient displacement studies related to reservoir engineering where flow through a Berea core is maintained at constant pressure and later the change in permeability is calculated as a percentage of the initial permeability. Furthermore, and to the detriment of some service industries, we found when these procedures were followed exactly as written by some well-intentioned spec writers, some of the tests qualified the HEC gel sold by one service company, while the same HEC gel sold by another company failed the same qualifying test badly. In fact, we witnessed HEC gel return permeabilities ranging from zero to about 13%. Appreciating the dilemma of the service industry within the context of "hard times", the authors, at their own initiatives, reconfirmed their initial findings: That is the main cause of damage could be only understood by a broad, interdisciplinary petroleum engineering approach, rather than through ordinary reservoir engineering practices. Our approach was to examine simultaneously the cellulose backbone chemistry of the HEC, the geochemical engineering aspect of the Berea core, the flow through it and interaction of gel breaker with HEC and the mineral contents of the Berea core.

On the basis of our findings, we concluded that damage to the Berea core resulted from: (a) the hydration and/or oxidation of iron minerals in the core, namely iron chlorite, ilmenite and iron (II) oxide, (b) most of the minerals reacted with the HEC gel breaker (in this case, sodium hypochlorite), (c) the transport and entrapment of hydrolyzed/oxidized particles of 0.4 to 1.0 micron by high viscosity gel, and (d) the appearance of newly formed clay particles (most probably kaolinite) at the pore throats. These new particles could have originated from the breakdown of chlorite clay after release of iron and/or from K-feldspars.

In order to aid the service industry, we reported that a serious procedural flaw in HEC gel quality control must be corrected before running a core flow test by following two simple steps: (1) If strong breakers are chosen for quick degradation of HEC gel, they must be compatible with mineral contents of the core, and (2) HEC gel/breaker/mineral contents of the core must be considered as one system. Following these simple corrective steps in our first test, we chose a "very" clean Berea core with extremely minute amount of hydratable iron sources which could react with sodium hypochlorite. In that experiment we qualified the HEC gel with nearly 90 - 95% return permeability. This was the same HEC that had failed the test previously.

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