A method is proposed and applied to one of the fields in the South Caspian Basin that allows to reconstruct the initial state of a partially depleted gas-condensate field using a limited set of initial data and assess the prospects of its development.
The method is based on numerical PVT-simulation with sequential stepwise gas saturation with equilibrium condensate to the dew point pressure that corresponds with the pressure at the gas-oil contact (GOC). The obtained composition at the GOC is then used to reproduce the variation of gas composition and condensate content with height above GOC taking gravity into account. At the final stage the obtained results are verified against well production data.
PVT-modeling was based on laboratory studies of recombined gas samle. The residue C10+ was divided into two pseudo components C10+(1) and C10+(2). To adjust the model to the experimental results, the properties of C10+(1) and C10+(2), and the shift-parameter of methane (Sv) were modified.
PVT-modeling allowed the following gas compositions to be calculated:
at the sampling depth and time;
at the sampling depth in the initial state;
at GOC depth in the initial state.
The calculation of gas composition variation with height above GOC was performed. For each of the compositions obtained, the potential content of C5+ was calculated.
From the development point of view, not only the initial state is of interest, but also the change of gas-condensate ratio (CGR) with reservoir depletion. To model this process, a constant volume depletion (CVD) experiment is generally used. The obtained dependencies allowed us to compare the results of calculations with the historical data.
Actual values, on the whole, correlate well with PVT-modeling data, and values that do not fit into the general paradigm of the obtained regularities are a reason for more in-depth studies of the reasons that led to such a difference.
The approach used allows to:
partially fill the lack of research by taking into account the physical principles of reservoir formation;
build an initial equilibrium reservoir state taking gravity into account;
reduce the uncertainty of reservoir production data.