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