Flow Index and Flow Type for Gas-Bearing Carbonate Reservoir Characterization

Abstract Gas can be produced naturally from carbonate reservoirs in many cases; however, hydraulic fracking may be required in order to produce the gas from low permeability carbonate reservoirs. The objective of this paper is to illustrate a new method to identify gas-bearing zones in carbonate reservoirs, and to address whether the gas-bearing zones can flow gas naturally or not. The proposed method integrates effective porosity (PHIE) from petrophysical evaluation with conditioned deep resistivity (LLD_c) from wireline logs to create a flow index (FI) curve first, and resistivity log conditioning is one of the key steps for this method since resistivity will increase when the porosity decreases. Consequently, the carbonate reservoir is classified into three different flow types (FT) based on the FI and PHIE values. The parameters for FT are derived from representative vertical wells with gas producing records from sidetracked underbalanced coiled tubing drilling (UBCTD) laterals from the same gas-bearing reservoirs. The new method derived a 1D FI curve and flow types (FT1, FT2 and FT3), which were verified by production logs (PLT) from dozens of gas-producing wells. The results showed that FT1 has a higher FI, making it suitable for UBCTD, while FT2 has a lower FI and requires hydraulic fracking to produce gas, and FT3 is very tight with very low FI values, indicating that it will not flow any gas. In addition, 3D flow FI and FT modeling and sweet spots mapping can greatly help gas field development in terms of well location optimization, horizontal laterals well planning assessment before drilling, well placement, and geosteering, especially for UBCTD laterals. This new method has been successfully tested and applied in a few gas-bearing carbonate reservoir characterization and development projects.