Rheological Property and Diversion Performance of Temperature-Control-Viscosity Acid Used for Heterogeneous Carbonate Reservoir Acid Stimulation

Abstract Reservoir heterogeneity usually causes trouble for uniform acid placement during carbonate acid stimulation. To minimize this negative effect, a novel polymer-based temperature-control-viscosity acid (TCA) is presented in this paper, which exhibits unique rheological property as temperature changes, together with obvious diversion performance. Experimental studies were conducted to determine the rheological properties of TCA affected by thickener concentration and crosslinker concentration utilizing the rheometer. After that, the appropriate formulation of acid system could be selected. Then, the influence of acid concentration and temperature on the rheological properties of TCA were examined, in order to understand the effects of external conditions. Meanwhile, acid coreflooding tests were conducted to visually demonstrate the diversion effect of TCA under different acid concentration and temperature. Finally, the relationship between rheological properties and diversion performance was comprehensively analyzed. Together with the rising thickener concentration, the viscosity of TCA grew. But the growth rate gradually got smaller. As the crosslinker concentration rose, the rheological properties of TCA became stronger. An over-crosslinking phenomenon could be observed, namely some water exuded form the acid gel, once the crosslinker concentration exceeded a certain value. The acid concentration had a significant effect on the viscosity of TCA, for a rapid viscosity growth since the acid concentration increased from 5wt% to 20wt%. The viscosity of TCA was nearly the same as gelled acid at atmospheric temperature. However, a clear divergence could be observed as the temperature increased. The viscosity of TCA improved sharply, as well as the acid gel strength, while the value kept almost constant for gelled acid. When the temperature reached 80 ℃ and higher, TCA gradually degraded and the rheological properties were weakened. Under proper temperature condition, the reinforced acid gel brought ideal diversion benefit and retarded reaction rate. Thus, a longer effective penetration distance and optimal acid stretching could be expected. Apparently, TCA has better applicability than conventionally used gelled acid in fracture-cave carbonate reservoirs for diversion and retarded reaction rate. Even compared with in-situ gelled acid depending on pH, TCA shows some advantages. TCA gets viscosified in live acid rather than in spent acid as in-situ gelled acid. Besides, TCA does not need extra breakers owing to its degradation under high temperature condition.