Zhang Liehui

Mechanism and control factors of water blocking in tight sandstone gas reservoir

The tight sandstone gas reservoirs play an important part in unconventional gas exploitation and development. Nevertheless, the reservoir is very susceptible to water blocking damage in drilling, completion, stimulation and production because of its characteristics. Scholar’s researches pay more attention to the change of permeability before and after the water blocking damage, but the microscopic mechanism of water invasion is little studied. With deepening the cognition of water blocking mechanism, based on the low-permeability tight sandstone reservoir cores of Bashijiqike formation in Keshen J block, the water blocking damage process is evaluated with optimized experiment (spontaneous imbibitions and water flowback) considering the actual reservoir characteristics, MRI and NMR T2 spectra technique is creative used to analysis the water phase invasion process, and the control factors about residual water is discussed. Some conclusions have drawn from that analysis. First of all, the water phase invasion with different initial water saturation is characterized by NMR T2, the smaller pore is occupied by initial water saturation, water phase invasion usually occur in bigger pore with T 2>100 ms and saturated at S w =45%–60%. Moreover, the water phase invasion process among different permeability cores and fractured core is comparison analyzed. The spontaneous imbibitions rate in the core ( K K >0.1 mD). And the water saturates the fracture fast, diffuses from fracture to matrix next. Finally, the rock physical property, fracture, initial water saturation, production pressure, the salinity of intrusive fluid and clay minerals are important factors control the water blocking damage. This paper deepening the cognition of water blocking mechanism in tight sandstone , also perfected the control factors of water blocking damage, and it will be of great importance to the exploration and development of tight sandstone gas reservoirs.

Theoretical study on unsteady testing permeability of fractured core in tight sandstone gas reservoirs

It is significant to accurately evaluate the fracture and matrix permeability of tight sandstone reservoirs. Based on the unsteady state test flow characteristics and physical model of tight sandstone reservoirs, unsteady flow mathematical model in fractured core has been proposed and solved by the Laplace transformation method and Stehfest numerical inversion method, which acquired pressure response mathematical model of upstream and downstream. The fluid cross flow characteristic curve for the fractured core in tight sandstone gas reservoirs has been obtained and studied in this paper. Base on the unsteady state test flow theoretical, the unsteady state test experimental of fractured core in tight sandstone gas reservoirs has been studied, which is used to analyses on property of fractured core. The results show that the process of the flow for fractured core in tight sandstone gas reservoirs can be divided into three sections, including the fracture seepage period for gas flowing crossed in fractured from upstream to downstream; the matrix seepage period for gas flowing crossed in matrix from upstream, downstream and fractured to matrix; and the system equilibrium period. During fracture seepage period, the higher fracture permeability is, the shorter duration of fracture seepage period is; however fracture porosity does not have influence on the fracture seepage period. In addition, the matrix seepage periods greatly affected by matrix permeability and matrix porosity. The lower matrix permeability is, the longer the time required to reach the equilibrium. The smaller the matrix porosity is, the greater the equilibrium system pressure is. The methods and results of this research contribute to reasonable analyses on property of fractured core in tight sandstone gas reservoirs.

A New Method for Stochastic Simulation of Macromolecule Conformation

The conformation statistics of single chain of polymer is a most basic and important problem in polymer science. Common polymer chain could do internal rotation around its chemical bond because thermal motion, this makes polymer chain’s form change ceaselessly. Because the molecular weight of macromolecule is very large, the number of polymer conformation is too large to be resolved using the common mathematical methods. This paper researches the free rotation chain conformation in non-lattice chain of high molecule. Through setting-up mathematical model, a new method is proposed to simulate the polymer chain. Simulation result shows the polymer conformation in microcosmic and reflects the characteristic “random coil” of polymer chain actually. These results provide an important basis for further quantitative analysis and optimal design.The conformation statistics of single chain of polymer is a most basic and important problem in polymer science. Common polymer chain could do internal rotation around its chemical bond because thermal motion, this makes polymer chain’s form change ceaselessly. Because the molecular weight of macromolecule is very large, the number of polymer conformation is too large to be resolved using the common mathematical methods. This paper researches the free rotation chain conformation in non-lattice chain of high molecule. Through setting-up mathematical model, a new method is proposed to simulate the polymer chain. Simulation result shows the polymer conformation in microcosmic and reflects the characteristic “random coil” of polymer chain actually. These results provide an important basis for further quantitative analysis and optimal design.