The ever-increasing international energy demands require exploration of new fossil energy resources. Unconventional oil and gas have received a great deal of attention in recent years as the technological advancements have made their production possible and more economical. Most of the shale developments took place in North America where the learning curve is being developed. Although shales still require lots of understanding and more advanced technologies, a substantial experience has been developed in North America. This paper presents an effort to summarize the current experience in shales of North America from different angles: rock mechanics, rock/fluids interaction, gas flow mechanisms through shale rocks, proppant embedment and water recovery after shale fracturing.
Three prospective areas for unconventional gas were found in the Kingdom of Saudi Arabia: in the Northwest, South Ghawar and condensate-rich shale gas in the Rub' Al-Khali area. The main targeted formations for unconventional natural gas are: the Ordovician Sarah, Silurian Qulibah, Qusaiba hot shale, Devonian Jauf and Permian Unayzah formations. The Qusaiba shale is located at depths of 7,500 to 20,000 ft throughout Saudi Arabia's basins. The Qusaiba Hot Shale in the Northwest area is relatively thick and it is considered to be the richest in all possible source rocks with a maximum total organic content of 6.15%.
Shales are composed of: kerogen, rock matrix and natural fractures. The mineralogy of shale varies from one field to another. Literature has confirmed that for Haynesville shale, the rock becomes more ductile with the increase in its clay content. Similar trends were seen for Lower Bakken shale. While other shale reservoirs, like Eagle Ford, Barnett and Middle Bakken are harder since they contain more quartz and calcite. The exposure of these clay-sensitive rocks to fracturing fluids does change their rock mechanical properties. This has been confirmed in literature where Middle Bakken shale lost 52% of its Young's modulus after exposure to 2 wt% KCl slickwater at 300°F for 48 hours.
The use of slickwater in fracturing represents a major challenge as it consumes huge volumes of this valuable resource. Recycling of produced water has been attempted in North America in Marcellus. An average amount of 3 to 8 million gallons of water are used in fracturing one well in Marcellus shale formation. In one application, re-use of the flowback water resulted in 25% reduction in the fresh water volumes and it reduced the cost of disposing produced water by 45 to 55%. The paper presents a summary of all of these findings from North America. A comprehensive understanding and analysis on unconventional reservoirs is required for the Middle Eastern reservoirs.