Study was conducted to evaluate development of gas-bearing formations in the Azerbaijan sector of the Caspian Sea. Study considered subsea wellheads tied into subsea manifold, and that manifold tied to offshore facility. Flow Assurance required the calculation of subsea Flowing Wellhead Temperature (FWHT) and Pressures (FWHP). 242 subsurface scenarios were conducted with reservoir model. To accommodate all subsurface scenarios in flow assurance assessments, it was required to carry out FWHT/P calculations for all.

Reservoir model was equipped with vertical lift performance curves for pressure loss calculations in tubing and logic for pressure loss estimation in subsea system. If correctly calculated, [FWHP >= dP(subsea) + Pseparator] logic should have been satisfied. As the reservoir model was not set for FWHT calculations, an external tool was required to cope with that task. Both nodal analysis software and dynamic flow modeling were considered as appropriate tools. However, as nodal modelling allowed much more automation, it was decided to use nodal analysis over dynamic modelling.

To improve FWHP calculations:

  • the logic was built into the reservoir model to:

    •  estimate dP(subsea) from gas rate vs pressure drop curves

    •  confirm validity of [minFWHP(wells 1, 2…n) >= dP(subsea) + Pseparator] statement:

  • step was re-iterated until the statement was satisfied

To improve FWHT calculations:

  • Enthalpy Balance method was tested for gas wells with 1-2% error against actual data

  • Then, nodal analysis models with the same method were built for the project wells

  • Code was developed to calculate FWHT as part of the ensemble model predictions in following steps:

    •  Well properties of each prediction step were transferred to nodal analysis software.

    •  kH was varied until nodal analysis software calculated gas rate matched to ensemble model output within 1mmscf/d error

Summary:

  • Described methods allowed to significantly increase accuracy in FWHT and FWHP calculations and accommodate all possible subsurface scenarios in Flow Assurance evaluation

  • Integration of subsea and topside hydraulics in subsurface modelling is important to develop flow assured design for development

  • Enthalpy Balance temperature prediction method provides good match to actual data

  • Use of coding provides huge opportunities to automate data analysis

Paper will present different approach to calculation of FWHT and FWHP in subsurface modelling, integration of subsea and topside hydraulics in subsurface modelling via alternatives ways, use enthalpy balance temperature modelling, integration between nodal analysis and subsurface modelling and coding can prove analysis of large subsurface data set.

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