Integrated Approach Optimizes Underground Gas-Storage Process
Authors: Carpenter Chris 
Published: May 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 207941, “Underground Gas-Storage Process Optimization Using Integrated Subsurface Characterization, Dynamic Modeling, and Monitoring: A Case Study,” by Longxin Li, Yuan Zhou, and Limin Li, PetroChina Southwest Oil and Gasfield Company, et al. The paper has not been peer reviewed.

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The operator’s Xiangguosi (XGS) gasfield facility began underground gas storage (UGS) operations in a depleted gas field in southwest China in 2013. Following this initial period, the site was reassessed to increase deliverability safely during the winter months to meet future peak gas demand. The results of that analysis informed the resulting system design, in combination with fit-for-purpose reservoir-surveillance systems; caprock-seal recording pressure, rock deformation, and seismicity data in real time; and regular wellbore inspection.

Introduction
The XGS UGS facility currently operates in a previously depleted gas reservoir in Sichuan province. Its construction took place during the second half of 2011; cushion gas injection started in June 2013. Located in a high-tectonic-stress region, the geological setting of the XGS field is structurally complex and highly faulted and the targeted carbonate formation is heterogeneous and naturally fractured.
When the studies described in the complete paper commenced in November 2019, the field hosted 22.80×108 m3 of working gas along with 19.80×108 m3 of cushion gas. As of May 2020, the reservoir had completed eight injection and six withdrawal cycles and delivered a maximum withdrawal rate of 21.96×106 m3/d.
The UGS conversion plans required a revisit to analyze the possibility of increasing the withdrawal rate to 28.55×106 m3/d, thereby positioning better for future peak gas demand. The drilling of five new wells commenced in 2019.

Planned Technical Approach
The purpose of the study was to optimize the XGS UGS facility’s working gas capacity and improve operational performance throughout asset life.
It was proposed to achieve this by 4D geomechanical and 3D dynamic flow-simulation models. The geomechanical model would hinge on an integrated subsurface characterization work flow combined with an advanced monitoring network tightly connected with the surface facility operation to ensure maximum containment and optimal injection and withdrawal rates and volumes. This holistic approach would aid in ascertaining optimal drawdown pressures to minimize rock fatigue and ensure operation of the UGS facility within safe pressure limits.

Fully integrated Subsurface Characterization
A detailed review of the existing petrophysical interpretation from wells calibrated with core analysis data was undertaken. Petrophysical interpretations from 29 wells were used to construct a reservoir classification and petrophysical property models. This led to the understanding that depositional facies were not the overriding control on reservoir quality in this case because the reservoir is highly affected by karstification, diagenesis, and fracturing. Prestack migrated seismic data was improved with acquisition footprint, noise, and multiple removal and then interpreted to map subsurface structure.
The edited well logs were used to build synthetic seismograms. A complex structural model was created that included 22 stratigraphic zones and a detailed fault model within the main field structure, integrating stratigraphic data and log curves from all recently drilled wells.