Mineral filling mechanism in complex carbonate reservoir fracture system：Enlightenment from numerical simulation of water-rock interaction

Abstract

The Tahe oilfield, located in the northern Tarim Basin, is characterized by complex fracture-cavity system and various mineral fillings. To reveal the mineral filling mechanism in this complex fracture system, this manuscript conducted a numerical simulation of water–rock interaction on basis of outcrop observation, cast thin section, electron scanning imaging and geochemical testing. Considering the influence of fracture dip angle, aperture, combination pattern, depth and temperature, 12 geological models were constructed in PetraSim software. The simulated results showed as followings: (1) The larger the dip angle, the faster the precipitation rate of calcite in faults. A larger amount of precipitation accelerated the reduction rate of calcium ions. (2) Dissolution and precipitation occurred more easily in faults with larger aperture. The larger the aperture, the faster the water flow. It contributed to the injection and flow of CO2 and facilitated calcite precipitation. (3) Multiple sets of faults with numerous intersection areas resulted in more complex distributions of mineral filling and precipitation. The calcium ion loss and precipitation rates in fault intersection areas were faster than that in a single fault. (4) In the intersection areas of faults with different apertures, dissolution and precipitation were prone to develop in the fault with larger aperture. (5) As a consequence of the upwelling of hydrothermal fluids, calcite precipitates transferred from relatively shallow parts to deeper parts. The amount of calcite precipitation in faults was larger than that in strata. (6) As temperature increased, the precipitation rate of calcite in faults and adjacent areas increased. Deeper faults with larger apertures had more abundant calcite precipitation.