Research on the Mechanism of Nano-Displacement Agent to Water Blocking Damage of Tight Sandstone Gas Reservoir
Authors: Zhang L. , Xu H. , Wang J. , Jiang H. 
Published: September 2023

ABSTRACT
Exploring ways to enhance the liquid-phase seepage ability and increase the flow-back rate of external fluids in the production process is an important way to solve the problem of water blocking in tight reservoirs. Based on the analysis of the characteristics of water phase retention damage caused by foreign fluid invasion during the development of tight sandstone gas reservoirs, a set of ether nano-displacement agent system was optimized. The mechanism of wetting modification is revealed from the perspective of the physical composition of nano-displacement agent, and the mechanism of action between nano-displacement agent and rock and different clay minerals is clarified. Laboratory experiments show that :(1) 0.5 wt% CNDAD1# nano-displacement agent fluid can change the wettability of sandstone to gas-phase wetting, "air-salt water-rock" three-phase static contact angle increased from 38.5° to 126°. (2) SEM and EDX results prove that the nano-displacement agent has good macroscopic static and dynamic adsorption performance on the sandstone surface. This paper provides a reference method for exploring the use of environmentally friendly chemical agents to remove water blocking damage in tight sandstone gas reservoirs.

INTRODUCTION
Tight sandstone gas reservoirs refer to sandstone gas reservoirs with overburden matrix permeability less than or equal to 0.1 mD. Single wells generally have no natural productivity or natural productivity is lower than the lower limit of industrial gas flow. Industrial natural gas production needs to be obtained through fracturing, horizontal wells, multi-branch wells, etc. (Wei G, Zhang F, Li J, et al., 2016; Jia C, Zheng M and Zhang Y, 2012; Zou N, Zhu R, Wu S, et al., 2012). According to the data, China contains a wealth of tight sandstone gas reservoirs, tight sandstone gas resources account for about 27.5 % of China ‘s natural gas resources, mainly distributed in different basins in China, such as Sichuan, Tarim Basin and other regions, and most of the reserves have not been effectively developed (Wang J, Hu Y, Liu Y, et al., 2019; Fuhai J, Tang D, Xu H, et al., 2012; Bybee K, 2007). Therefore, the development of tight sandstone gas reservoirs will become one of the key sources of future energy security in China, and it is also the focus of unconventional oil and gas development. However, most of the tight sandstone gas reservoirs have low natural productivity of single wells. The combination of horizontal well drilling and fracturing is one of the main ways to realize the commercial exploitation of tight sandstone gas reservoirs. However, during the fracturing process, the fracturing fluid flowback is not timely or the edge and bottom water invades the reservoir during the production process. The reservoir will produce a large amount of external water phase retention or edge and bottom water invasion water, forming a ‘ liquid phase trap ‘ and further tight sandstone gas well productivity (Dutta R, Lee C H, Odumabo S, et al., 2012; You Q, Wang H, Zhang Y, et al., 2018; Zhang L, Zhou F, Mou J Y, et al., 2018).