Fuzzy-ball workover fluid based gas stabilization and water control technology for the gas wells in the Lower Paleozoic carbonate reservoirs of the Ordos Basin

After the gas wells in the Lower Paleozoic carbonate reservoirs of the Ordos Basin are put into production, the formation pressure coefficient decreases. And in the process of well workover, reservoir gas channeling and wellbore fluid loss happen frequently. Therefore, it is necessary to plug the reservoirs, so as to ensure safe operation. In some reservoirs, gas production capacity is damaged by water production while water control is needed. Plugging fuzzy-ball fluid was adopted to carry the surfactant which can increase gas-water flow resistance difference into low-pressure gas reservoirs, to transform the traditional damage control into the gas stabilization and water control. In the experiment, a sand packed tube (diameter 38 mm and length 30-100 cm) was prepared to simulate the Lower Paleozoic carbonate reservoirs. The measurement results show that the displacement pressure is 4.59-16.18 MPa when the injected fuzzy-ball fluid flows to the outlet and it is increased by 7.04-18.22 MPa after repetitive fresh water injection, which is in proportion to the filling radius of fuzzy-ball fluid. Gel breaker is injected into sand packed tubes with four lengths to speed up the flowback. After nitrogen is injected under the same displacement pressure for 180 min, the permeability recovery ratio is 2.02%-6.07% higher than the formation water and the recovery ratio difference is in proportion to the filling radius. In four gas wells, the filling radius of fuzzy-ball fluid in the reservoirs increases from 2.12 m to 8.97 m, the formation bearing strength rises from 17 MPa to 21 MPa. The difference between daily water production decrease ratio and daily gas production decrease ratio rises from 18.68% 90 days before the operation to 62.90% 90 days after the operation. It is indicated that the temporary plugging strength and water control effect of fuzzy-ball fluid in the Lower Paleozoic carbonate reservoirs are both in proportion to the filling radius. Furthermore, fuzzy-ball fluid is conducive to the realization of temporary plugging and gas stabilization and water control in the process of well workover.