Tang Hongming

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

Abstract 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.

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.