Significant number of gas and gas-condensate fields in Ukraine are characterized by significant level of depletion. For example, 80% of the production of PJSC “Ukrgasvydobuvannia” are coming from the fields depleted up to 75%. Gas-condensate fields are very particular since reduction of reservoir pressure leads to condensation of liquid hydrocarbons with formation of separate hydrocarbon phase that reaches critical saturations and becomes mobile only in the vicinity to the wells. These make a negative impact on the well performance and field development in general. To prevent the in-situ condensation of hydrocarbons, such reservoirs must be developed with artificial pressure support. Unfortunately, only in 5 Ukrainian reservoirs gas cycling or inter-well gas bypass was implemented. Heterogeneous distribution of reservoir properties leads to selective drainage, and in case of water drive – selective and highly controllable water encroachment, that complicates field development planning, especially production improvement, hydraulic fracturing, side-track drilling and placement of the new wells.
Detailed review and critical analysis of the research on the problems of hydrocarbon recovery enhancement from the depleted gas-condensate reservoirs was performed.
One of the priority tasks for further field development and increase of internal production is creation of numerical 3D reservoir models to be used in technological decision making based on optimization results. Mathematical model and numerical solution methods were reviewed. Alternative mathematical model of multiphase gas-condensate mixture filtration was proposed.
Comparison and justification of the advantage of multi-variant stochastic approach for creation of geological models was given. The method for the selection of representative stochastic realization of gas reservoir based on streamline flow simulation results and comparison was proposed. Condition check for compliance satisfaction between calculated drained reserves and volumetric method for the cases with 50% probability with simultaneous correspondence to material balance (P/z=Qprod). Because of that, fast history matching is achieved. The method was tested and implemented for one of the fields in Dnieper-Donets Depression (DDD).
Calculation results comparison between compositional and simplified black-oil PVT model, that describes the change of PVT properties as a function of pressure in tabular form, was investigated. The possibility of possible application of simplified model for correct characterization of phase transitions during gas-condensate reservoir depletion for different potential yield between 50 and 500 g/m3 was proved.
According to the results of numerical simulation of the typical heterogeneous gas-condensate reservoir of DDD the integrated comparison of different enhanced hydrocarbon recovery methods for three different potential yields of C5+ (100, 300 and 500 g/m3) with 50 and 100% voidage replacement for four different initial depletion stages of 25, 50 and 75% from dew point pressure and pressure of maximum liquid condensation was performed. Based on technical efficiency (incremental condensate recovery versus base depletion case) the methods are ranked as follows: 1) СО2 100%; 2) C1 90%, C2 5%, C3 5%; 3) C1 98%, C2 1%, C3 1%; 4) C1 100%; 5) C1 50%, N2 50%; 6) N2 100%; 7) water flooding. Economic efficiency (maximum cumulative NPV) methods ranking: 1) C1 100%; 2) C1 98%, C2 1%, C3 1%; 3) waterflooding; 4) C1 50%, N2 50%; 5) C1 90%, C2 5%, C3 5%; 6) N2 100%; 7) СО2 100%.
Based on CO2 injectivity results into the depleted gas-condensate reservoir with potential condensate yield of 500 g/m3, the synergy effect from incremental gas and condensate recovery and geological storage of carbon dioxide was calculated. Slow injection rate (750K m3/d) is favorable, leading to incremental recovery of gas 5,50% and condensate 5,18%, with possibility to store up to 3,6 Mtons of CO2 during 20-30 years of injection. This amount is equal to 1,35% of annual 2020 emissions assuming implementation of energy efficiency measures.
Results analysis of the chemical flooding on the same geological realization showed, that polymers are the only ones generating positive economic efficiency (NPV). For the rest of the methods, relatively high agents’ costs and required concentrations make them inefficient. High gas recovery factors (about 92%) are related to depletion caused be dense well pattern and low reservoir pressure, at which all gas is dissolved in the condensed liquid phase. At the same time incremental condensate recovery is not greater than 0,33%, that in general makes application of chemical agents economically not efficient.
