With the increase of energu demand of oil and gas and the decrease of conventional oil and gas resources in China, the exploration and development of tight gas reservoirs has becomes the first priority to enlarge gas reserves. The middle-deep sandstone of the Huagang Formation in the Xihu Sag of the East China Sea belongs to the category of tight sandstone reservoirs and is a typical low-permeability, low-porosity reservoir[Xu,G et al. 2012], with strong water-blocking damage, and drill-stem test (DST) results from many wells were showed much lower production than what expected. Conventional productivity evaluation methods are also difficult to accurately evaluated productivity with water-blocking damage condition.
In this paper we integrated core experiment, water-drive gas experiment, gas-drive water experiment, wireline formation test (WFT) results, logging interpretation, formation evaluation results, and numerical simulation analysis and proposed a new production prediction solution for a tight gas formation with strong water-blocking damage. The workflow is following:
First, mobility as a dynamic measurement derived from WFT was studied, which clarified that the near-wellbore zone has experienced water-based mud invasion.
Second, based on WFT, core lab data, well logging interpretation results, and neural network rock typing method, the formation was characterized into different rock types. The characteristics of each rock type and its degree of water-blocking damage were researched.
Then based on dozens of core lab data, the relationship between maximum gas relative permeability (Krg) and absolute permeability was identified. Furthermore, based on the correlation among core lab data, well logging interpreted permeability curve and maximum Krg, the effective permeability profile can be deduced. This profile can help to estimate the approximate maximum productivity of a well, but not accurately enough without irreducible water saturation input.
Third, with calibrated permeability and saturation, a single well model was built with different rock typing results. Each rock type was assigned with the corresponding relative permeability lab data. By this, the process of productivity prediction from the different rock types and the effect of water-blocking damage with the changing of water saturation can be demonstrated.
This solution cannot only simulate gas production with water saturation changing at the initial stage of production, but also simulates the water-blocking damage caused by mud invasion into the formation. It reproduces the generation and release process of "water-blocking."
After verification with multiple wells, this solution can accurately provide production evaluation for a tight gas reservoir with water-blocking damage. It also confirmed the influence of water blocking on tight gas production.