The UGS industry has borrowed much of its knowledge from oil and gas exploration and production practices. But also it has had to develop its own technology and methodologies due to repeated large pressure variations as well as other specific issues. The following is an overview of the technical considerations for designing and safely operating a UGS be it in a depleted reservoir or in an aquifer.

The primary technical consideration is the performance of the UGS system. Delta-pressuring, i.e. operating the storage at a maximum working pressure above the initial formation pressure, is an attractive option to enhance the performance of a UGS, especially in terms of working gas.

Currently, dozens of UGS are operated under delta-pressure conditions in the US, Canada and in Europe. However, delta-pressuring requires additional design and monitoring when compared to a conventionally operated facility.

Because the original formation pressure is exceeded during delta pressuring the sealing efficiency of the cap-rock formation must be evaluated. This sealing capacity of the intact rock is related to the threshold pressure, which can be defined as the minimum pressure required by gas to displace the water initially filling the pores. This is a key design parameter.

Typically, PTA involves a production period and a subsequent shut-in phase. It is used for the evaluation of the reservoir characteristics. In the UGS industry additional issues may arise due to the need for repeated assessment of well productivity and injectivity due to changing thermodynamic conditions in time and risk of sand production.

Geomechanical assessments of safety conditions call for lab tests on cores and in-situ measurements with modeling to characterise and describe the stress-strain behavior of the reservoir and cap-rock. In addition the overburden, confining rocks and discontinuities affected by the storage-induced pressure variations must be quantified. Hence there is the need for integrated dynamic and geomechanical modeling in UGS reservoir studies to reliably reproduce the combined effects of fluid flow and deformation processes under different operational strategies and repeated injection and withdrawal cycles, especially when over-pressure conditions are applied.

One of the most important aspects in the lifecycle of a delta-pressured UGS is monitoring. To this end, a suitable monitoring system is set to detect potential problems in terms of reservoir response, such as gas losses below potential spill points.

Due to the high production and injection rates, coupled with repeated pressure variations over long periods, there are significant stresses on completions. UGS wells require highly efficient completion and cementation between casing and wellbore. Hence the need for the correct assessment of cementation integrity from sonic and ultrasonic wireline logs.

Microseismic monitoring detects subsurface movements that could have been induced in a reservoir by the modification of pore pressures due to the reactivation or creation of small fractures. Despite the small magnitude of the induced events, microseismics can be helpful in tracking pressure changes, mapping fracture networks and hence identifying possible gas movements. The method’s main advantage is that it provides a continuous record in real time so that, once analysed, it offers an immediate insight into the physics of the subsurface.

Monitoring of surface movements is extremely useful to back-analyse the rock deformation properties, which are key input to the geomechanical model.