Flow Diagnostic Surveillance in a Mature Borderline Gas Reservoir
Authors: Zett Adrian , Merletti German , Abbas Alaa , Han Xiaogang , Liu Yang , Al Saadi Sawsan , Al Hajri Salim 
Published: April 2024

The Barik tight-gas sandstone in Khazzan and Ghazeer fields (Sultanate of Oman) is at the borderline between conventional and unconventional gas field development. Elements of complexity include efficiency of massive hydraulic fractures relative to rock types, commingled reservoirs production, differential depletion, condensate, and water presence. Managing the depletion plan requires a robust surveillance plan; a customized flow diagnostic program was designed to monitor the wells and reservoirs in the fields. The objective of this work is to describe the implementation and results of an alternative surveillance workflow that combines multiple vendors flow diagnostic instrumentations of production logs (PL), multi-detector pulsed neutron (MDPN) and surface flow meters in a science well.

Traditional methods such as conventional production logs failed to deliver tangible results in the two tight reservoirs. The incumbent sensors can be used for gross flow profiling and zonal split but are limited in identifying the fluid phases separately. A reassessment of conveyance indicated that surface readout can broaden the sensors usage and provide a better option for data quality control and optimize the stationary data acquisition. Alternative flow diagnostic technologies identified for further logoff using a "science" well where a MDPN instrumentation was deployed to complement the data gathered using two independent flow diagnostic toolstrings.

The manuscript describes the benefits and limitations of various sensors for multiple well conditions and reservoir properties. We combine multi-vendor flow diagnostic tools with MDPN-derived nuclear attributes (NA) to independently assess downhole condensate liquid yield. Observations indicate that fluid capacitance, acoustic and radioactive fluid density devices have almost no sensitivity to liquid phase in flowing passes; shut-in data provide very limited information of liquids which is too uncertain to be quantified. The most robust measurement for fluid phase determination consists of an array of local probes in the flow diagnostic tool; good fluid sensitivity was picked in both flowing and shut-in passes. This was complemented with the extraction of a new MDPN nuclear attribute responding to condensate in borehole. Additional MDPN nuclear attributes were used to assess the change in saturation components, including condensate.

The bespoke workflow provides an efficient way to evaluate sensitivity of various sensors to pick up responses of multiple fluid phases flowing through perforations. Understanding fluids behaviour is a critical step in sensors selection, data processing and integration. Fluid holdups along with MDPN derived saturation are integrated to open-hole petrophysical rock quality interpretation to have a proper static to dynamic reservoir characterization. The condensate yield from wireline data is in close agreement with the estimation of volume fraction of vapor phase and liquid phase at borehole conditions, based on the equation of state. The workflow can be replicated in analogue environments to interpret multi-phase flow and to assess viability of comingled production with a secondary producing target.