Improving Recovery of Liquids from Shales through Gas Recycling and Dry Gas Injection
Authors: Fragoso Alfonso , Wang Yi , Jing Guicheng , Aguilera Roberto 
Published: November 2015

The objective of this paper is to investigate the possibility of using gas injection to improve liquids recoveries from containers in shale condensate and shale oil reservoirs. Liquids recoveries from shales are known to be very low. A method is proposed to increase these recoveries through gas recycling and by using dry gas that is available within relatively short distances of the shale condensate and oil containers considered in this study. This dry gas is not being produced at this time due to current market conditions.

In practice, some shale reservoirs such as the Eagle Ford in the United States and the Duvernay in Canada present the challenge of unconventional fluids distribution: shallower in the structure there is black oil, deeper is condensate and even deeper is dry gas. So the fluids distribution is exactly the opposite of what occurs in conventional reservoirs. Differences in burial depth, temperature, and vitrinite reflectance are used to explain this unique distribution.

Ramirez and Aguilera (2014) have shown that fluids in shale reservoirs have remained with approximately the same original distribution (i.e. approximately the same dry gas-condensate contact and approximately the same condensate-oil contact) over geologic time. These fluids are the target of the research results presented in this paper. The investigation involves three basic cases, all of them with horizontal wells.

In the first case, a single porosity compositional simulation is used to investigate the possibility of improved liquid recovery from the condensate container by using dry gas injection obtained from the recycling process plus dry gas from the deeper part of the structure. Fluid properties are similar to those of the Duvernay shale.

In the second case, dual permeability compositional simulations are used to investigate practical aspects of the condensate container that can lead to improved recoveries in the Eagle Ford shale. Sensitivities are run that include bottomhole pressure (BHP), natural fracture permeability and spacing, hydraulic fracture length and spacing, and distance between parallel wells. Results from dual permeability simulations are compared with dual porosity behavior. Fluid properties are similar to those of the Eagle Ford shale.

In the third case, compositional single porosity, dual porosity and dual permeability simulations are used to study the possibility of injecting gas in the oil container. A cyclic huff and puff gas injection is also investigated. Fluids and rock properties are similar to those of the Eagle Ford shale.

The study leads to the conclusion that dry gas from deeper shales can be put to good use by injecting it into the middle and upper parts of the structure. In the middle part of the structure there is a container where gas condensate is predominant. In here, a re-cycling injection project allows to inject dry gas stripped from the condensate fluids. This is supplemented with dry gas produced from the deeper part of the structure.

In the upper part of the structure there is a container where oil is predominant. In here, injection is implemented using dry gas produced from the deeper part of the structure. Permeability plays a critical role in the case of single porosity simulations. Dual porosity and dual permeability simulations indicate that oil recovery can be enhanced significantly in naturally fractured shales. Diffusion plays a fundamental role on the performance of shale gas injection particularly in the case of naturally fractured shales. It is found that cyclic huff and puff gas injection can help increase oil recovery.

To the best of our knowledge, the idea developed in this paper that includes all fluids (oil, condensate and dry gas) present in the same shale structure within relatively short distances of each other has not been published previously in the literature.