Gas spillage and matrix compaction effect on shale gas production evaluation and hydraulic fracturing design

Fossil fuel is still the largest energy source globally with unstoppable high demand. Because of the continuous depletion of oil wells and reservoirs, more advanced production techniques such as hydraulic fracturing and horizontal drilling have been developed to access unconventional resources such as shale gas. However, due to the current complex nature of the low-permeability reservoirs, knowledge about the geological aspects, rock properties, and various design parameters as well as their relationships is of great significance in reservoir development and exploration. In particular, permeability directly influences hydrocarbon fluid flow and is controlled by both the rock permeability system and the natural fracture permeability system, which are both depended on the effective stress.

Up to date, considerable work has been reported on the effect of fracture permeability on the ultimate recovery and production of oil and gas. Despite being relatively lower than natural fracture permeability, the matrix compaction effect, otherwise referred to us matrix permeability reduction as a function of pressure, is still an important element in shale recovery. Unfortunately, no thorough investigations have been done on the effect of matrix component on hydraulic fracture design and hydrocarbon production. Besides, the effects of the gas spillage especially in the small pore in shale gas reservoirs are relatively large and thus cannot be ignored. Up to now, different correction models i.e. Klinkenberg’s and Fathi’s models have been proposed to account for the gas spillage effects in shale gas flow. However, the gas spillage effects on fracture design and production prediction especially for shale rocks have not been fully explored.

To this note, West Virginia University (WVU) researchers: Courtney Rubin (student), Dr. Mehrdad Zamirian, Dr. Ali Takbiri-Borujeni, and Dr. Ming Gu (corresponding author) from the Department of Petroleum and Natural Gas Engineering (PNGE) developed an approach, combining core-lab test and reservoir production modeling, for accurate characterization of the two main permeability effects and evaluation of their impact of hydraulic fracturing design and well production evaluation. The core-lab test was conducted in the Precision Petrophysical Analysis Laboratory (PPAL) built in WVU PNGE Department. The research work is currently published in the journal, Fuel.

Briefly, the research team initiated their studies first by developing empirical permeability correlations from core-lab tests under varying pressures and confining stresses. Next, the obtained data was used to run reservoir simulations using two scenarios one taking into consideration the effects of gas spillage and matrix compaction on gas production while the other approach not taking the aforementioned effects into account. Eventually, the two simulation approaches were compared in terms of gas production, cost, efficiency, and critical conductivity to determine the most appropriate approach for understanding the effects of gas spillage and matrix compaction.

The gas spillage effects exhibited significant effects at pore pressures closer to BHP levels, occurring at the wellbore vicinity and late production period, even though it had low effects on the gas production predictions for most case scenarios observed in the field. On the other hand, the matrix compaction effects were observed at all the pore-pressure levels throughout the entire production period. For Marcellus Shale, ignoring the two permeability effects resulted in an overestimation of the gas production by 11% in six months, over-designing of proppant pumping amount thus leading to high costs of fracturing of a single horizontal well. Therefore, the effects of gas spillage and matrix compaction in rock permeability are crucial for hydrocarbon production evaluation and design of hydraulic fracturing in shale gas reservoirs.