Integrating Pressure Transient Analysis and Production Analysis for Dynamic Reserve Evaluation in Tazhong 1 Ordovician Carbonate Gas Field, Tarim Basin

Abstract

Abstract Tazhong ?gas field is a typical vug-fractured marine carbonate gas condensate reservoir with characteristics of high heterogeneity and complex geological& dynamics, of which grand challenges face the development by using the classic evaluation method of dynamic reserves. On the basis of literature research, a formal definition about dynamic reserves is given. Besides, a workflow of dynamic reserve for carbonate gas condensate reservoirs is built relying on the long-term low accuracy data of daily production and short-term high-accuracy data of pressure build-up testing, which integrates full lifecycle short-term pressure transient analysis and long-term production analysis combined with the geological understanding. The factors affecting reserve estimate are analyzed based on a field application on Tazhong ?gas field, including initial formation pressure, PVT, the error of pt~pwf conversion, production time, producing water and routine of work. The simulation results show that the gas-liquid two-phase pseudo-pressure method is more appropriate than the pseudo-single phase method for calculating dynamic reserves for low gas-oil ratio gas condensate wells. The workflow can improve data utilization ratio, reduce the uncertainty of reserve estimation, avoid the development risk, optimize the development plan, and contribute the enhanced oil recovery in the late stage. It can be also used in other marine carbonate reservoirs. Recently, three thousand large-scale condensate gas fields have been discovered in the Ordovician carbonate reservoir of the Tazhong area, which is located in the middle part of the central uplift belt in the Tarim Basin?1-2?. However, the traditional material balance method for doing what is not applicable in this area due to many factors. First, the type of reservoirs is various, including cavity, fracture-vug, cleavage and matrix pore (dominant). Second, the flow mechanism is complicated, including seepage and conduit flow. Besides, fluid property is complex in nature, leading to low-, middle-, high condensate and volatile gas reservoirs. Third, controllability is low during the development, the production model involves non-constant pressure and non-constant rate. Fourth, it is difficult to calculate the bottom hole flowing pressure due to the small difference between the formation pressure and the dew point pressure, there is the retrograde condensation phenomenon in formations, and multi-phase flows happen in the wellbore. Fifth, there are few hydrostatic gradient data, and they do not decrease monotonically. Therefore, a great challenge facing developers is how to evaluate the dynamic production of complex carbonate gas condensate wells under the complex production status.