Reservoir Type Identification And Productivity Evaluation Of Carbonate Gas Reservoirs

Well deliverability and production’s uncertainty were definitely increased by the reservoir types of the single individual well caused by well-developed pore, fracture and cave in carbonate gas reservoirs with high heterogeneous, which brings huge challenges for the efficient development of gas fields. In this paper, identification of carbonate reservoir types and deliverability evaluation were studied in order to solve this problem. Following works were specifically completed.(1) Classification of well test curves was proposed based on dynamic well analysis theories. The complex flowing processes in carbonate reservoir were simplified to three basic patterns:homogeneous, multilayer channeling and radial composite. Therefore, the log-log curve of well test can be much adaptable to the identification of reservoir; further research was also conducted which consequently proved that pressure history curve and pseudo-gas productivity index can serve as assistant roles on reservoir identification.(2) With the combination of both dynamic and static research result, a comprehensive identification method for carbonate reservoirs was established. Considering the actual field situation, the identification reference charts consisting of four basic reservoir types (namely, the pore-type, the vuggy-type, the fracture-pore-type and the fracture-type) were also given to facilitate the application of the method.(3) Based on literature research and fluid properties analysis, the principal origin of the hydrogen sulfide in sour gas reservoir was observed to be the thermochemical sulfate reduction (TSR), which causes the hydrogen sulfide content to be interdependently correlated with both the gas composition and the reservoir physical property. With this theory, a qualitative understanding can be concluded as follows:higher hydrogen sulfide content was often accompanied by higher condensate oil content and gas dry coefficient, reservoirs of this kind were more likely to be the pore-type with high porosity; oppositely, reservoir with lower hydrogen sulfide content often implies a development of fractures. The findings above allow fluid properties to act as extra information in identifying reservoir types.(4) The "improved isochronal well test method without shut-in" was proposed for the carbonate gas reservoir of low matrix permeability, and its deliverability equations were also derived based on unstable pressure drop equation and superposition principle. Therefore, the time interval to obtain the deliverability can be significantly reduced, yet guarantees the accuracy; with this novel method.(5) Fracture-vuggy gas reservoirs were simplified to two primal composite types: dual-porosity type and radical-composite type according to the flowing modes, and deliverability equations were derived for both types. The sensitivity analysis showed that the accuracy of permeability plays an important role in the prediction of deliverability.To assure and verify its availability in the practical carbonate gas reservoir field condition, every single theoretical method mentioned above was verified by adequate available field data, results showed that the problem of identification and deliverability of carbonate gas reservoir can be solved by the these favorable methods, which make it possible to exploit the gas field efficiently.