Formation Damage in High Temperature Dense Glutenite Reservoirs by Salt Sensitivity and Salt Precipitation

Ultra-low permeability tight glutenite reservoirs are widely distributed in the Bohai Bay, China. Compared with conventional sandstone reservoirs, the tight glutenite reservoirs are rich in clay minerals, highly developed with dissolved pores and microfractures, and have strong heterogeneity. Invasion of injected fluids into the glutenite reservoirs easily induces formation damage and blocking of pore throats, thereby reducing the flow capacity and the production rates of oil and gas. Cores taken from the glutenite formation in the BZ19-6 block were evaluated for salt sensitivity damage according to the industrial standards and high temperature high back-pressure steady-state method. The types and in-situ characteristics of the sensitive minerals and salt minerals were analyzed using SEM. The experimental results show that the salt sensitivity of the Kongdian glutenite is moderately strong to strong. Since the lower the permeability of the core, the higher the damage to the porosity and permeability of the core by salt precipitation, it is thus recommended that for dense cores with permeability less than 0.1 mD and reservoirs temperature higher than 100 ℃, the high temperature high back pressure steady state evaluation method be used to evaluate the flow sensitivity of the reservoir rocks. The clay minerals in the Kongdian formation are mainly silky/filamentous illite, altered kalinite and mixed illite/montmorillonite which exist in the dissolution pores in the form of comb shell type, dispersed filling and bridging etc., and are the main causes of salt sensitivity of the reservoirs. High temperature accelerates the evaporation of formation water, causing salt precipitation near the wellbore. The precipitated salts are mainly potassium salt and halite which exist on the surfaces of the mineral particles and walls of the pores, and cause the pore throats to be easily clogged. The main mechanisms of dense glutenite formation damage by salt precipitation include static evaporation of formation water which leads to clogging of the dense pore throats, salt crystal migration during dynamic displacement which clogs the pore throats and the weakening of the rock strengths by salt precipitation which leads to particle migration. To minimize reservoir damage by salt sensitivity, KCl-salt saturated polymer sulfonate and zwitterionic polymer drilling fluids are recommended as the drill-in fluids, the filtration rates of the drilling fluids should be reduced, and the crystallization and nucleation of salts in the drilling fluids be inhibited.