Plugging Process For Protecting Fractured Gas Reservoirs In Workover Operations

In our country, fractured oil and gas resources are mainly contained in the Tarim Basin, Sichuan Basin, Ordos Basin, Bohai Bay Basin, the amount is rich. Natural gas occupies a large proportion. Fractured reservoirs are characterized with seriously heterogeneity. The fractures not only provide storage spaces for oil and gas, but as well provide good percolation paths between reservoirs and wells. However the fractures are always contaminated during the workover jobs, and the gas production of reservoir decreased. Therefore, it is of important significance to find a way to control the formation damage of fractured reservoirs.Taking the Shengshen and Xushen gas reservoirs of Daqing oilfield as the study object, the engineering geological characteristics of the study area and the leakage situation during the workover job are investigated, the potential reservoir damage factors was revealed, the combination of plugging materials is sifted, the plugging test is conducted, and the formulation and the technical idea of the fractured solidified water plugging system are proposed.The fractured formation in Shengshen and Xushen gas reservoirs is charactered of low porosity and low permeability, fracture developing and serious loss of workover fluid. The blocking and aqueous trapping caused by the invasion of solid and fluid are the main damage factors.According to the characters of the Shengshen and Xushen gas reservoirs, this paper requests thermostability, pressure bearing ability, granularity and rheological properties for the plugging material. After dozens of times testing, the ideal combination of pugging materials is ultimately confirmed. This paper borrowed ideas from the "shielding temporary plugging technique", proposed the "fractured solidified water plugging system", and the tests proved that the plugging effect is fairly good.The fractured solidified water plugging system is formed mainly by solid particles, solidified water particles and fiber nets materials. In lab, the system can effectively plug the artificial fractures wide 2mm and below under 140℃ and 5MPa. Plugging materials cumulated at the entrance of fractures in only a couple of seconds, becoming more compacted during the pressure acting. The formed temporary plugging layer can effectively prevent the solid and fluid entering the fractures, and prevent the contamination caused by the working fluid and the solid carried by it. It is easy to remove the plugging layer after the operation, the flowing back pressure is low than IMPa, the recovery rate of permeability is 100%.The research result had been used in Shengshen and Xushen gas field. The field work indicated that the fractured solidified water system can effectively plug the fractures in the reservoir, greatly reduce the loss of working fluid, the production recovers well after operating.