Gelled hydrocarbon has been used as a fracturing fluid from the early days of fracturing. Over the years, its use has diminished in the U.S. However, its use internationallycontinues to the present day in Canada, South America, Russia and East Asia. The paper will discuss the use of gelled oil fluids in Canada, particularly in low pressure tight gas wells, where the use of CO2-energized gelled hydrocarbon fluids has been very successful. The chemistry of these fluids and their application is discussed.The energized fluids have been used in a variety of formations and in wells of permeabilities from 0.1 mD to 10 Darcies, depths in excess of 3000 m and BHT from 10 to 110 ºC. Initial and longer-term production data from wells fractured with the energized fluid will be compared to wells fractured with conventional gelled hydrocarbon fluids to show the effectiveness of the system in tight gas applications.
The case study will include gas wells fractured and producing from the Rock Creek formation east of Edson, Alberta, Canada from 47–10W5 to 56–15W5 in the Western Canadian Sedimentary Basin (WCSB). Observations on proppant type and proppant amount are made in addition to comments on the fracturing fluid selection.
Natural gas accounts for 22% of world wide energy consumption today. As world-wide use of natural gasincreases to meet energy demand, there has been increased interest in unconventional gas sources. These includes coal-bed methane, tight sand gas, shale gas and gas hydrate wells. Commercial production of unconventional gas is in its infancy, with development of tight gas, coalbed methane and shale gas being performed mainly in North America. The resource assessment data of unconventional gas varies widely; however, it is universally accepted that the unconventional gas resource base, excluding gas hydrates, is more than twice that of conventional gas resources. Gas hydrates are excluded due to limited technology and success at developing the resource. In North America, gas resource from tight sands is estimated to be around 1371 Tcf (1). Production of unconventional gas was only 1 Tcf / year in the 1970s, but has increased to around 4 Tcf / year in 1997 when it accounted for a total of 20% of the total national gas consumption (2). Almost 70% of the unconventional gas production is from tight sands. In contrast, Latin American production of unconventional tight sands gas is in its infancy. The resource base of 1293 Tcf of tight gas is second only to North America.
The definition of tight gas is somewhat arbitrary, but is generally accepted to be gas produced from formations with an average air absolute permeability of less than 20 mD. In-situ permeabilities in these types of reservoirs are generally less than 1 mD and can be down into the micro-Darcy range (10[-6] D) in many cases.
By using appropriate drilling, completion and in some cases large-scale fracturing techniques, several operators have succeeded in obtaining economic production rates from formations exhibiting in-situ matrix permeabilities as low as 10[-6] D. In many cases, gas may exist in such low permeability formations, but, due to adverse capillary forces, high in-situ saturations of trapped water, and in some cases, liquid hydrocarbons are present. If these saturations are too high, economic production from the zone is difficult without appropriate fracturing techniques.