Naturally fractured-caved carbonate reservoirs in China have some distinctive characteristics: deeply buried, multi-scale fractures, vugs and caves developed, strong heterogeneity, poor reservoir connectivity and no oil in matrix. Hydraulic fracturing is commonly used in order to connect offset fracture-cave system with wellbore and improve well performance for this kind of reservoirs, while how to properly evaluate hydraulic fracturing efficiency is still a major challenge. This paper presents an integrated evaluation method to solve this problem.
The method mainly involves pressure transient analysis and rate transient analysis. Firstly, typical diagnostic curves of different fracture-cave patterns with and without hydraulic fracturing respectively are established through design analysis of numerical conceptual models. Then the curves are used for actual well stimulation and evaluation. Finally, reservoir properties of fracturing wells such as OOIP, hydraulic fracture properties, reservoir permeability, etc. are evaluated based on the integrated pressure transient analysis and rate transient analysis method.
This new method has been applied to Tazhong No.I fractured-caved carbonate gas condensate reservoir in China. The reservoir's mid-depth is around 6000m and is currently under primary depletion development with an average annual production declining rate higher than 25%. Based on the understanding of seismic interpretation and geological study, 3 major reservoir patterns and 7 sub-patterns are classified. Then corresponding conceptual numerical models of these patterns with and without hydraulic fracturing are established, and typical diagnostic curves of pressure transient analysis and rate transient analysis for identification of these patterns are generated. And then, the results are applied to the actual well fracturing evaluation in Tazhong NO. I carbonate gas condensate reservoir.
This systematic technique has been successfully applied to Tazhong No. I fractured-caved carbonate reservoir in Tarim Basin of China, which properly and correctly evaluates the properties of the hydraulic fracturing wells. It also provides a reliable method for decision making of hydraulic fracturing, which is helpful for effective and economic development of the reservoir.