Biochemical biogas upgrading retaining more CO2 from biogas to form biomethane opens new avenues for sustainable biofuel production. For developing this technology, maintaining sustain pH for CO2 center dot H2O is fundamental. This study proposes an innovative control strategy for in-situ biogas upgrading retaining and converting the CO2 in the biogas into CH4, via hydrogenotrophic methanogenesis without external agent. The Biogas-pH strategy limited pH drop over 7.4 by stop feeding and maintained the methanogenesis activity by biogas flow rate over 98 ml.h(-1). Low pH (7.37-7.80) decrease CO2 center dot H2O as a substrate in stage-I, higher pH (7.40-8.41) enhances CO2 to CO2 center dot H2O transfer by 6.29 +/- 2.20% in stage-II. Because of that 95% CO2 center dot H2O converts to HCO3- and CO32- when pH > 7.9, higher pH > 7.9 did not further upgrading the biogas. The carbonate buffer system shown open and close system characteristics in gas and liquid phase. The biogas CH4 was upgraded from 67.27 +/- 5.21% to 73.56 +/- 5.01%.