Strong Plugging-Inhibition and Low Interfacial Tension Drilling Fluid for UGS Rebuilt from Depleted Oil Reservoir

PG2 block of Jidong Oilfield is a UGS rebuilt from depleted oil reservoir, the Sha-1 section reservoir is characterized by medium porosity and permeability, medium-large pores and fine throats, severe pressure deficit. Mudstone clay minerals are high and micro-fractures are developed in the upper overburden east section. During the drilling process, there are some difficult problems, such as depleted reservoir leakage, reservoir damage and mudstone cover instability. Through the combination of three types of plugging agents, the selection of inhibitors and surfactants, a strong plugging-inhibition and low interfacial tension drilling fluid was optimized and constructed. The plugging rate of the system for pores -throats of sandstone and micro-fractures of mudstone reached over 72%, with a pressure-bearing capacity greater than 5 MPa. The first and second rolling recovery rates of mudstone reached 93.2% and 72.14% respectively, and the permeability recovery value of sandstone reached 87.9%.The on-site application demonstrated excellent leak prevention, wellbore stability and reservoir protection. No complex situations such as well leakage or wellbore collapse occurred in the application wells. The average wellbore enlargement rate was 7.1%, with regular wellbore diameters, providing favorable wellbore conditions for improving cementing quality. The proportion of high-quality cementing sections for production casing was 87.4%, and the continuous high-quality cementing section in the cover was 35.8 m. During the gas injection operation, the average gas injection rate per well reached 16.6 × 104 m3/d. Practice showed that the drilling fluid system provided a strong technical guarantee for improving the construction quality of injection-production well of PG2 UGS.