Production optimization of shale gas systems

Production optimization of shale gas systems

This is a page showing ongoing activity by the IO Center. It involves highlighted research activities which are currently being carried out, description of goals, activities and expected results

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Mature shale-gas systems are characterized by a large number of geographically spread wells and pads, where a substantial number of the wells are producing at low erratic rates due to reservoir pressure depletion and the very low formation permeability. This state of the wells, referred to as liquid loading, severly detoriorates the well performance. This project describes a novel shut-in based operational scheme for decreasing the impact of liquid loading in shale multi-well systems.
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Shale-gas is land-based, unconventional resources of natural gas stored in very tight formations. These formations may store enormous amounts of natural gas, and currently constitutes more than 20% of the US gas production. Wells are drilled with long laterals and stimulated with multiple hydraulic fracturing treatments to achieve profitable rates.

 

 

All dry and semi-dry tight formation wells, including shale-gas wells, will eventually reach the state of liqud loading, a condition where the pressure in the well is insufficient to lift co-produced water and condensates to the surface.The time when this first occurs depends, among other, on the well completion, the depth of the well and on the formation permeability. A shared property, however, is that liquid loading can be prevented by always keeping the production rate above a critical rate needed to ensure continous removal of liquids in the wellbore.

The current project describes a novel shut-in based scheme for preventing liquid loading in shale-gas wells. By performing a cyclic scheme of shut-ins with the gas critical rate as the lowest allowed production rate, we allow pressure to build-up in the near-well bore region during shut-ins, giving a boost in the production rate when re-opening the wells while avoiding the wells to be loaded. This scheme can be performed with minimal loss of cumulative production and will substantially enhance the productivity of late-life liquid loading shale-gas wells.

In shale-gas systems, typically encompassing tens to hundreds of wells and production pads, there are challenges with performing a cyclic shut-in scheme. These includes:

  • Wells may come off high after shut-ins. This may result in erratic and oscillating flow-rates and pressures in the surface lines.
  • Connecting gas compressors normally require some sort of stabilized inflow; high variations in inflow may be operational and economically disadvantageous. 
  • Many spread wells and pads: some sort of overall production plan and rate-control is desirable.

To address these challenges, we suggest to compute optimized time-varying shut-ins such that the total production from a large field is close to a specified reference rate based on sales contracts (see illustration). This way, the wells can be scheduled such that the operator (company) produces the actual volumes of gas they are set to sell through the contracts.

Shale-gas wells are inherently dynamic, and render a somewhat differently optimization of the upstream value chain compared to conventional hydrocarbon resources. Hence, to be able to optimize shut-ins times, we have developed dynamic shale well proxy models and integrated these models in a simplified description of the surface gathering systems. By using an advanced, decomposable optimization scheme with this simplied system descpription, we are able to find optimal shut-in times for large shale-gas systems (10-100 wells) over planning horizons on several weeks.

 

 

 

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