Preparation And Gelation Mechanism Study Of Polymer Gel Systems For Conformance Improvement In High-temperature Reservoirs

The in-situ cross-linked polymer gel is one of the leading technologies for production stimulation in China’s oil fields.It can manage excessive water production and increase recovery by improving the conformance.However,with the continuous exploration and production of oil and gas resources,oil companies are facing with the problem of diminishing oil and gas production year by year.Many oil companies have gradually changed their objectives from conventional reservoirs to deeper layers with high temperatures.For example,the reservoirs in the PB2 block in Jidong Oilfield and the Tahe Oilfield IV block have temperatures approaching 140-160°C in the middle of the reservoirs.Moreover,due to the serious heterogeneity of these reservoirs,they began to face the problems of excessive water production rapidly after waterflood development.Therefore,research and development of high-temperature resistant polymer gel system at150°C is needed.In this paper,the gelation time,gel strength,and long-term stability of the polymer gel system were evaluated using thick-walled ampoules,and gel strength,thermal stability,and microscopic network structure were measured using a rheometer,a differential scanning calorimeter/thermogravimeter（DSC/TG）,and a scanning electron microscope（SEM）,respectively.By screening four different types of polymers,the terpolymer ZP-4 polymerized from acrylamide（AM）,2-acrylamide-2-methylpropane sulfonic acid（ATBS）and N-vinyl pyrrolidone（NVP）shows a good temperature resistance,and can be prepared with hexamethylene tetramine（HMTA）and resorcinol to form in-situ cross-linked terpolymer/phenol resin gel system at 150°C.The composition of this gel system is 0.6 to 1.0%ZP-4,0.3 to 0.4%HMTA,0.3 to 0.4%resorcinol,and0.05%heat stabilizer.In addition,this high temperature polymer gel system has long-term stability.After 5 months of aging at 150°C,the dehydration degree of the corresponding gel system is still lower than 5%.The gelation time can be controlled within 4 to 14 hours,and the gel strength can be adjusted within the range of Sydansk code D to H.The gelation mechanism of different phenols and aldehydes and terpolymers under high temperature conditions was clarified.Compared with formaldehyde,HMTA can delay the release of formaldehyde at high temperatures,avoiding the instability of gel strength due to rapid cross-linking.However,the formaldehyde generation rate by pyrolysis of paraformaldehyde is very slow at 150°C,which can not form a stable gel because of untimely crosslinking with the terpolymer.Compared with phenol,the cross-linking sites of catechol,resorcinol and hydroquinone increased correspondingly,and a stronger gel system can be formed at 150°C.Moreover,resorcinol has the most active crosslinking point,which can form high-temperature polymer gel system with the largest storage modulus and best long-term stability.Polyethyleneimine has low biological toxicity and is an environmentally friendly cross-linker for food contact.The effects of molecular weights of different polyethylenimines on the gelation properties of terpolymer/polyethyleneimine gel systems under high-temperature（150°C）conditions were investigated.When the molecular weight of polyethyleneimine is higher than 10,000 Daltons,a low biotoxic in-situ crosslinked terpolymer/polyethyleneimine gel system can be formed at 150°C.The composition of this gel system is:0.6 to 1.2%ZP-4,0.3 to 1.5%PEI,0.1 to 0.5%sodium carbonate and 0.05%anti-oxidant.The initial gelation time is 2 hours to 7 days,and the gelation strength can be controllable at Sydansk code D^H.However,the long-term stability of this gel system is weaker than that of the previously described terpolymer/phenolic resin cross-linked gel system.The influencing factors of the gelation kinetics of the terpolymer/phenolic resin crosslinked gel system at 150°C were then investigated.Increasing the polymer concentration will enhance the gel strength of the polymer gel system but will also shorten the gelation time of the polymer gel system.In addition,an increase in the concentration of the cross-linking agent improves the gel strength of the polymer gel system,but when the concentration is too high（more than 0.6%）,the polymer gel system may suffer from poor long-term stability due to excessive cross-linking.Moreover,the concentration of divalent ions in the gelant has a significant effect on the gelling kinetics of the gel system.Mg²⁺will compress the diffusion double layer of the polymer molecule and thus shorten the gelation time.When the concentration of Ca²⁺is less than 0.001 mol/L,it will be complexed with part of the carboxyl groups to elongate the gelation time.After this critical concentration,the gelation time will be shortened.Last,heterogeneity physical models were used to evaluate the plugging performance and conformance improvement effect of the preferred terpolymer/phenolic crosslinked gel system.The system can effectively seal the fractures with the opening of 0.15⁰.30 mm and the high-permeability layers in the interlayer heterogeneity physical models under high temperature conditions.This high temperature terpolymer gel system also has a good water control and oil recovery performance in inter-layer heterogeneous models.After blocking the high-permeability（i.e.,excessive water production）layer,the overall water cut of the model dropped from 98.0%to 33.3 to 38.0%,and the chase fluid turned to the unswept low-permeability layer,and the oil recovery efficiency increased by 12.27 to 20.72%.The above research results have enhanced the understanding of the development of polymer gel systems under high temperature conditions,and clarified the mechanism of influence of polymers and different types of organic crosslinking systems on the gelation performance and long-term stability of the high-temperature polymer gel systems.In addition,it can deepen the theoretical knowledge of high-temperature polymer gels,improve the mechanism of chemical water enhancement,and guide the optimal design of high-temperature gel systems.