Gas permeability evolution with soaking time in ultra tight shales

The single phase matrix permeability to gas in ultra-tight rocks such as shales, are typically in the order of around 1-500nD. When these rocks are exposed to liquid invasion during hydraulic fracturing operations, gas permeability can become significantly lower, due to the formation of liquid blocks at the fracture-matrix interface. Post fracturing shut-in time or "soaking time" is thought to alleviate this permeability damage by allowing for liquid blocks to spontaneously dissipate into the reservoir, driven by the strong capillary forces present in tight rocks. However, this redistribution of fluid also results in the formation of a larger invaded zone which may further damage gas permeability. This paper presents a means of analyzing the change in permeability on shale cores with soaking time and subsequent production. Key observations from this work are that the introduction of even relatively small quantities of liquids into the rock matrix can lead to a significant reduction in average permeability. Soaking time further exacerbates the situation by allowing fluid to spontaneously propagate deeper into the matrix. Effects such as clay swelling lead to irreversible impairment of base matrix permeability, and a low threshold water saturation for gas to become immobile means that any spreading of fracturing fluid with soaking time could reverse benefits accrued from lowering the fluid saturation in liquid blocks. The experimental work done in the current study is on Marcellus and Haynesville shale samples. The results obtained on soaking time experiments are compared with similar experiments on conventional and tight sands. This comparative analysis indicates that soaking time is beneficial to rocks with relatively higher permeability and low clay content whereas it is detrimental to ultra-tight lithologies such as shales. These lab scale trends can help contextualize observed field scale production characteristics based on reservoir lithology, base matrix permeability, and duration of soaking time, and thereby, provide operators a scientific foundation for the selection of an optimum post stimulation gestation time before beginning production.