Development of a Fully Coupled Wellbore-Reservoir Model for Transient Temperature Profiling and Multiphase Phase Behaviour Prediction During Well Testing in Deep High-Pressure High-Temperature Wells, Offshore Nigeria

Transient temperature profiles in deep HPHT wells during production and build-up test sequences are governed by the coupled effects of wellbore heat exchange with the formation, multiphase Joule-Thomson cooling through perforations and choke systems, and evolving gas liberation from oil as pressure draws down, creating a thermodynamic complexity that existing wellbore simulation models do not adequately represent for Nigerian deepwater HPHT conditions. This study develops a fully coupled wellbore-reservoir model that simultaneously resolves transient temperature profiles, multiphase phase behaviour, and pressure transient responses during well testing in deep HPHT wells offshore Nigeria. The theoretical model couples a multi-segment wellbore simulator incorporating energy balance, momentum balance, and species transport with a radial compositional reservoir simulator through a fully implicit time integration scheme. An original formulation for the wellbore-formation heat exchange coefficient under transient multi-phase flow conditions is derived using a modified Nusselt number correlation for two-phase annular flow developed from high-pressure laboratory heat transfer measurements. The coupled model is validated against distributed temperature sensing measurements from two deepwater Nigerian well test datasets where temperature profiles were acquired during drawdown and build-up. Mean absolute temperature deviation of 3.8 degrees Celsius over the full wellbore length confirms the model's predictive capability. Applications to production test design for HPHT deepwater wells are demonstrated, including identification of minimum test duration required to achieve radial flow. Keywords: coupled wellbore-reservoir model, HPHT, transient temperature, well testing, deepwater Nigeria.