What is natural gas from tight sands?

Natural gas from tight sands is conventional natural gas extracted from unconventional reservoirs.

Conventional reservoirs are composed of rock made up of clastic or carbonate grains arranged in such a manner that while they are in contact with each other, there is still a network of connected pore spaces between the grains. Important characteristics of such reservoirs are porosity - the volume of the pore spaces, and permeability - how connected the pore spaces are. Porosity is what stores the natural gas and permeability is what allows the natural gas to move. Natural gas from conventional reservoirs can be produced economically without the need for large-scale stimulation or sophisticated production equipment.

Tight reservoirs are those which have low permeability, often quantified as less than 0.1 millidarcies. However, a generally accepted industry definition is reservoirs that cannot be produced at economic flow rates or that do not produce economic volumes of natural gas without assistance from massive stimulation treatments or special recovery processes and technologies. Poor permeability is primarily due to fine-grained nature of the sediments, compaction, or infilling of pore spaces by carbonate or silicate cements precipitated from water within the reservoir.

Criteria were established in the United States for qualifying formations as tight which made them eligible for federal tax credits. The basic standard was an effective permeability to gas less than 0.1 millidarcie or a potential test rate below standards established by depth. In addition to the introduction of tax credits, the U.S. gas industry established a tariff on interstate gas volumes and established the Gas Research Institute (GRI) to fund and manage research in various gas topic areas, including unconventional gas and tight sands. The resulting research projects, combined with those of the U.S. Department of Energy led to substantial advances in technology which led to accelerated development of unconventional gas. The results of these programs led to most of the “routine” technology being employed in today's industry.

How is natural gas from tight sands formed?

Because natural gas from tight sands is identical to natural gas from conventional reservoirs, their genesis is the same.

There are two theories as to how natural gas is formed. The most widely accepted theory, the organic theory, maintains that natural gas formation begins with photosynthesis, where plants use energy from the sun to convert carbon dioxide and water into oxygen and carbohydrates. The remains of these plants and the animal forms that consume them are buried by sediment and as the sediment load increases, heat and pressure from burial converts the carbohydrates into hydrocarbons. Natural gas formation takes place in source rocks, usually fine-grained black shales. Continued pressure from burial forces the natural gas to migrate from source rocks into more porous and permeable rock such as sandstone and limestone, which, if overlain by impermeable strata such as shale, form reservoirs that contain the gas.

The other theory of natural gas formation is the inorganic theory - without biological origin. This theory speculates that hydrocarbons did not originate from buried plant and animal material, but instead were trapped inside the earth as it formed. Proponents of this theory contend that hydrocarbon sources can be found a few hundred miles below the surface and continue to this day to pump substantial amounts of petroleum and methane up through the Earth's deep cracks and pores to the shallow sedimentary levels.

How is natural gas from tight sands found?

Until recently, the discovery of tight sand reservoirs was the by-product of exploration for conventional reservoirs which, for an unexplored basin, usually includes:

• review of existing information
• aerial surveys to gather data regarding magnetic fields, gravity and radiation
• seismic surveys to locate and define subsurface structures capable of trapping natural gas
• exploration drilling to test subsurface structures for the presence of hydrocarbons
• logging the wells to determine porosity, permeability and fluid composition

Tight zones that are possibly gas-bearing are determined from wire-line logs, and those with the most potential are drillstem tested. Customarily, only those with well developed fracture systems are completed. Sometimes larger zones may be fracture stimulated.

Currently, exploration targets tight sands by focusing on basin centres where stacked sandstone sequences provide significant thicknesses of potential reservoir, and by seeking naturally fractured sandstones that can be directionally drilled at high angles to the fracture sets.