A Novel Method of Non-Darcy Well Test Interpretation for Ultra-Low Permeability Reservoirs

Abstract

Abstract Conventional well test interpretation methods do not consider the special seepage characteristics of ultra-low permeability reservoirs, resulting in pessimistic well test interpretation of permeability and undervalued skin effect caused by reservoir damage. This study has established a well test interpretation model considering stress sensitivity/start-up pressure gradient and mud invasion for ultra-low permeability oil fields. The well test interpretation results have been applied to guide the formulation of overall development plan in South China Sea. The mathematical expressions of stress sensitivity and start-up pressure gradient were obtained by using real core experiments. The quantitative characterization of mud invasion time-varying skin was realized by using numerical core experiments. Based on the new characterization method, a seepage model considering three factors above was established. The real space solution was obtained by using Laplace transform and Stehfest numerical inversion methods, a series of new standard plates were drawn, and corresponding ultra-low permeability well test interpretation software was compiled. Based on the numerical simulator considering stress sensitivity and starting pressure gradient, an average permeability of 5md mechanism model with 0.6m near well bore damage zone was built. A well test design was then implemented to get pressure buildup data for well test interpretation. The authors compared the conventional well test methods and improved ultra-low permeability well testing method with same data base. A conclusion can be drawn that both permeability and skin have been seriously underestimated by using conventional well test methods. However, the permeability and additional pressure drop interpreted by improved ultra-low permeability well testing method is coincidence with mechanism model, which proved the validity of our new methods. Thus the well test permeability obtained by new methods can be used for productivity evaluation of low permeability oil field development and the skin factor can be used to guide mud system preparation and formation protection process design. The influence of stress sensitivity/start-up pressure gradient and mud invasion damage of ultra-low permeability reservoirs have been pioneering discovered, and well test interpretation plates and software considering three factors above have been implemented for the first time in the industry.