Research On Low Permeability Gas Reservoir Drilling Fluid

As the degree of petroleum geological exploration and the enhancement of oil and gas field development technology, the embedded depth of oil and gas field discovered gradually increases, and porosity and permeability of the reservoir becomes lower and lower. Low permeability gas reservoir generally has the characteristics of low porosity and permeability, flow channel of gas, water and little oil is narrow, the seepage resistance is very large, and the interaction force between liquid-solid interface and liquid-gas interface is very big, which make the water blocking effect and stress sensitivity enhanced obviously, and result in changes of oil, gas and water permeability rule.This paper investigated the basic characteristics of low permeability gas reservoir and its protection mechanisms and analyzed the No.3 structure reservoir features of Jidong Nanpu Oilfield, with using SEM and X-ray diffraction technology analyzed the clay types, content and their occurrence features. Through developing low permeability core water locking damage experiments, the influence rule of the water locking damage was obtained: permeability is internal factor of the water locking damage, and the lower the core permeability, the more severe of water locking damage. The differential temperature, soaking time, interfacial tension and other external factors would also affect the water locking damage, the greater of the pressure difference, the longer the soak time, and the lower of temperature and interfacial tension, the more serious of the water locking damage. 10 kinds of selected anti-water blocking agents were tested the wettability, surface tension, foaming ability and their compatibility with drilling fluid, then the anti-water blocking agent FSJ-5 was optimized. With 0.15% aqueous solution of FSJ-5 the surface tension can be low reached at 14.1mN/m, and its foaming ability was weak, reverse wetting effect was good, and the compatibility was good which showed excellent anti-water blocking function.Based on the experiments above and through experiments of optimizing tackifiers, fluid loss agents, inhibitors and anti-water blocking agents, and the formulation low permeability gas reservoir drilling fluid is determined as follows: water+0.8%ZNJ-1+0.2%XC+3%F850 +2%polyoses+1%AP-1+2%superfine calcium carbonate+5%FJ-A7+0.4%FSJ-5. The drilling fluid has good rheological performance before and after 150℃/16h’s aging, and surface tension of the system can be reduced to 29.6mN/m, and it has good lubricity with friction coefficient 0.25. Its ability against NaCl pollution can reach more than 10%, and against CaCl2 and MgCl2 pollution can reach more than 0.5%, and the against drilling cuttings contamination can reach more than 10%. It has excellent inhibition performance with once shale recovery 91.38%, and the coefficient of linear expansion of shale core is 11%, and its recovery value of core permeability is 91.5%.