Damage evaluation for water-based underbalanced drilling in low-permeability and tight sandstone gas reservoirs

With the characteristics such as small pore throats, strong water wetting, and well-developed fractures, the low-permeability and tight sandstone gas reservoir is liable to be damaged during operation, even in the underbalanced condition. At water-based under-balanced drilling, water blocking and stress sensitivity are the main damages for low-permeability and tight sandstone gas reservoirs. Such damages are evaluated by using Fractures Visualization Test System and Capillary Flow Porometer. The results indicate that low-permeability and tight gas reservoirs still have very strong water blocking damage in the process of underbalanced drilling, and the lower the initial permeability, the bigger the water blocking damage. Given certain effective stress, the lower underbalanced pressure is accompanied with more serious water blocking damage, and over-low underbalanced pressure can not protect formations. Given certain underbalanced pressure, the higher effective stress brings about more serious water blocking damage. Now, the water blocking damage evaluation in underbalanced condition is just under study. Next work should focus on the simulation of real in-situ operations in reservoir and applying the laboratory evaluation results into onsite engineering design.