Hydraulic fracturing is being employed on more complex and lower permeability fields, making the associated reservoir characterization increasingly complex and difficult. Various methods exist to determine reservoir transmissibility (kh/µ), each with their own strengths and limitations. Well testing, pre or post-frac, on offshore depleted oil reservoirs is often uneconomical and can be problematic, while quantifying the productive height in laminated oil reservoirs is equally difficult. This paper will present field case histories demonstrating the successful application of after-closure analysis in various reservoir types along with highlighting the benefits and limitations of the process.
The use of after-closure analysis during diagnostic fluid injection tests (DFIT) is a well known method of determining reservoir transmissibility. This method enables linear and radial flow regimes to be identified, thus determining reservoir transmissibility. To further strengthen the process, this paper proposes a multidisciplinary approach that combines the after-closure methodology with conventional well test diagnostics.
Our results showed that we could successfully quantify reservoir transmissibility using after-closure analysis and yield the same results as well test analysis. Furthermore, these results were confirmed with post-frac well test analyses. Moreover, quantifying reservoir pressure prior to the injection test was the key to an effective design in under-pressured reservoirs.
We have shown that our multidisciplinary approach to after-closure analysis yields consistent results in determining reservoir transmissibility in actual field cases. Some operational and technical constraints were identified; some have been resolved while others still remain. Notably, using oil as a slightly underbalanced fluid ensured a pressurized wellbore which sufficiently extended falloffs such that a proper analysis could be conducted. In conclusion, after-closure analysis provides a powerful tool for reservoir characterization in complex reservoirs.