Modeling of Water Production in Late Life of a Depletion Drive High Temperature Gas Reservoir
Authors: Ejaz Muhtashim , Uz Zafar Syed Saud , Mehmood Saad , Azeem Abdul , Noman Muhammad , Ahmed Abrar 
Published: May 2024

The field under study is a high temperature volumetric dry gas field, located in the Middle Indus basin of Pakistan. The reservoir has a temperature of ~ 350 °F and initial pressure of ~ 5400 psia, slightly over-pressured with hydrostatic gradient of ~0.51 psi/ft. The field is currently at mature stage with current reservoir pressure of ~ 250-300 psia. Due to high temperature and significant reservoir depletion, unusually high WGRs of up to 150 bbl/MMscf are encountered in wells which severely impact well hydraulics. The objective of this study is to properly model/history match non-aquifer water production, generate reliable forecasts capturing well hydraulics/load-up issues and devise optimum production strategy that improves overall field recovery.

The unusually high WGR for the producing wells in such a field can be generally related to two mechanisms: Significant vaporization of connate water with isothermal pressure depletion: As reservoir pressure declines with production, the equilibrium water vapor content of reservoir gas increases causing connate water within reservoir to vaporize. As this vaporized water is produced along with the gas to surface, it condenses resulting in high water production and correspondingly high WGR for the producers.Secondary imbibition caused by pore volume compaction occurring during depletion: The evolving water saturation during depletion process results in connate water mobilization as it exceeds critical value.

These concepts were incorporated in the dynamic model and successfully helped in modelling the connate water expansion and production with adequate history match of all producers.

The above-mentioned physical processes were modeled through conventional simulation formulations resulting in reliable estimation of field gas initial in-place. In addition, it also helped in generating realistic production forecast for all producers capturing proper well hydraulics with increasing WGR and hence providing accurate ultimate recovery under prevailing conditions. It also assisted in identifying wells which are susceptible to early shut-in due to load-up tendency. Consequently, the history matched model was used to evaluate different production optimization actions including low pressure wellhead compression and de-liquification techniques by identifying associated reserves under different operating conditions.

The paper highlights the significance of modelling water production in high temperature depletion drive gas reservoir and its implications on Gas Initial In Place (GIIP) & Expected Ultimate Recovery (EUR) estimation. Water gas ratio (WGR) matching was critical due to its severe impact on well performance. Production forecast generated with this approach provided realistic recoveries which might have been overestimated with other modelling techniques. In addition, it also helped to identify critical wells prone to well load-up and enable proper planning of optimization actions for such producers.