Stream Conversion Technology and Gas Condensate Field Development
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In the last few years, integrated modeling has become a preferred tool in the petroleum industry to manage the value chain of different assets. It is slowly replacing the traditional modeling approach that treats each petroleum asset model separately. Having different discipline models and applications in a single platform will ensure more consistency of the value chain from one asset to another. Integrated modeling also enables engineers to optimize assets, both locally and globally, using an automatic approach. Coupling of different petroleum assets entails transferring and combining petroleum streams from one asset to the others. Stream conversion is a key requirement in integrated modeling because petroleum assets usually have their own fluid model, and it is rare to have a single common fluid model in both the subsurface and surface simulation models. This thesis investigates different stream conversion methods and provides important technologies for integrating different petroleum assets into an integrated asset model. These stream conversions are expected to have highly accurate results and reduce the computational time. Reservoir engineers have utilized both compositional and black-oil reservoir simulations for many years. Due to the CPU-time consideration, the EOS model used in a compositional simulation is normally limited to 6-10 components, a so-called lumped EOS model. We propose a delumping method to generate detailed compositional streams from either black-oil or compositional (lumped-EOS) reservoir simulations, performed as a simple post-processing step. These methods are based on a set of phase-specific and pressure-dependent split factors. The reservoir simulation phase behavior can be approximated by a PVT depletion experiment, such as the CCE depletion experiment. We have used this approach to develop the blackoil and compositional delumping method applied to the reservoir simulation output. The split factors are generated from simulated depletion PVT experiments using a detailed-EOS model. Delumping is performed phase-wise at the well-connection level, for each time step of the reservoir simulator. For gas injection processes, the amount of injection gas is estimated from stream information and, accordingly, removed from the stream before applying the phase-specific pressure-dependent split factors. We propose another conversion procedure to convert one fluid model stream to another fluid model stream in which the heaviest fractions have different component grouping. This procedure is called the two-step gamma distribution conversion. It consists of two steps: (1) splitting the heavy component into single carbon number (SCN) characterization and (2) lumping from the SCN characterization model to the destination fluid model stream. SCN characterization has been proposed as a generic accounting characterization that contains a SCN component up to C80 for a normal reservoir fluid and C200 for a heavy reservoir fluid. To obtain the best result, it is recommended that this conversion be performed phase-wise at the well-connection level. The implementation of the three proposed conversion methods has been demonstrated using a hypothetical integrated petroleum asset model. We believe that the combination of our proposed methods will provide important advances in the integrated asset modeling. The second section of this thesis addresses some key reservoir and production issues related to gas and condensate recovery from Khuff reservoirs in the Middle East - namely Ghawar Khuff, North Field and South Pars. These fields represent somewhere between 1,000 and 2,000 Tcf initial gas in place, with 30 to 70 billion barrels of condensate in place. We apply engineering methods and reservoir simulation to quantify the expected performance of Khuff gas condensate fields for a realistic range of geologic description, petrophysical and fluid properties, and production facilities based on published information. We review key data for reservoir and production design, summarizing the impact of geologic zonation, areal and vertical communication, mean permeability and its variation, relative permeability, water encroachment, and fluid composition on field performance. Because most commercial development projects involving gas sales export are based on delivery contract quotas (DCQs) of 1-1.5 bcf/D for up to 25 years, well-average plateau length and rate-time is used as a primary measure of performance. We try to describe the interplay of reservoir and production-facilities performance on overall design of field deliverability and total well requirements. Other production issues not considered in our work but with significant impact on Khuff development strategy include gathering system design, rate metering, platform vs. onshore processing, and single-phase vs two-phase pipeline flow. Economics are not considered in our evaluation. We estimate deliverability impairment from condensate blockage using relative permeability models that reflect the impact of velocity (capillary number improvement and inertial effect). The velocity effect is particularly important in Khuff wells because of the high-k, low-h layers with unusually-high flow velocities and convergent flow. Layer vertical and areal connectivity can have a profound effect on water encroachment. When sufficient lateral continuity exists, even small aquifers can result in rapid water encroachment through thin, high-permeability zones. This has been studied and is shown to have a lesser effect in Khuff reservoirs.