Modelling Transient Production Forecasting of Horizontal Wells in Abnormal High-Pressure Tight Gas Reservoirs
Authors: Cao Lina , Wang Hehua , Jiang Liping , Zhang Bo , Ganzer Leonhard , Li Ying , Wang Xiaochao , Li Ke , Pang Siyu , Xiao Heng 
Published: November 2023

Abstract
Natural gas stands out as a cleaner source of energy due to its relatively lower carbon emissions, making it an appealing choice for numerous countries. Therefore, it is essential to gain a deeper understanding of natural gas reservoirs to ensure sustainable extraction and unlock their full potential. The behavior of fluid flow and production in complex gas reservoirs, especially those characterized by abnormal high-pressure and tight porous media, is not yet fully comprehended, necessitating further investigation.
Traditional Darcy's law is no longer applicable in tight porous media with high pressure. To overcome this challenge, a new composite seepage model was developed. The model incorporates stress-sensitive permeability and threshold pressure gradient. We employed perturbation theory for permeability modulus and the Green function method for the inhomogeneous special solution, effectively addressing its nonlinearity. Through integration transformation methods, a dimensionless rate solution under constant bottom-hole pressure was derived in the Laplace domain. Eventually, a new model involving multiple factors has been proposed for production prediction in such gas reservoirs.
Studying unsteady gas flow dynamics in tight formations provides valuable insights into flow patterns. To investigate transient flow characteristics in tight gas reservoirs, log-log figures were generated through Stehfest numerical inversion. Flow periods were classified based on standardized time stages for rate curves. A parametric study revealed that stress sensitivity damages permeability, causing a larger pressure drop in intermediate and late flow regimes. This effect is reflected in upward tendencies in rate derivative curves. A higher threshold pressure gradient indicates poorer reservoir properties, making fluid flow more difficult, as evidenced by steeper downwarping in production rate curves. The combined impact of stress-sensitivity and threshold pressure gradient accentuates the variation trend in these curves. Multi-stage hydraulic fracturing can effectively address the negative impacts of these two factors, which impede seepage. Enhancing fracture conductivity can decrease even eliminate the threshold pressure gradient, while increasing proppant strength can slow down elastic and plastic deformation of reservoir rock, thereby reducing the loss of permeability.
The transient seepage model developed in this paper serves not only for production prediction but also to explain the related formation and well parameters. It functions as a traditional well test interpretation tool, particularly remarkable as it relies solely on daily production data, enhancing workflow efficiency and reducing testing time. The interpreted parameters are valuable for designing hydraulic fracturing operations, evaluating the potential of tight gas reservoirs, and ultimately increasing gas production rates.