BACKGROUND OF THE INVENTION

The invention relates to a method and system for enhancing natural gas production from an underground natural gas containing formation.

The formation may be a natural gas reservoir, which may be partially depleted, a tight gas reservoir in which natural gas is present in pores of a substantially impermeable formation, a water reservoir containing residual natural gas or a capped permeable formation into which natural gas is injected for storage and use during peak periods of natural gas consumption.

US patent 4765407 discloses a method for enhancing production of gas condensates from a gas condensate reservoir by injecting a mixture of carbon dioxide and nitrogen obtained from a Claus plant into the reservoir. A disadvantage of this known injection technology is that a mixture of carbon dioxide and nitrogen is corrosive and will induce corrosion of corrosion prone fluid injection and production facilities.

International patent application WO2012021282 discloses a method for enhancing recovery of hydrocarbons trapped in a hydrate containing formation by

intermittently injecting slugs of a carbon dioxide containing releasing agent and of a nitrogen containing reagent into the formation.

Canadian patent application CA2568358 discloses a method for fracturing a hydrate or shale oil containing formation by injecting liquid nitrogen into the

formation .

US patent 4434852 and International patent

application WO 02/10357 disclose enhance oil recovery methods employing nitrogen injection. US patent 7481275 discloses an enhance oil recovery method using nitrogen obtained from air and waste gas obtained from a methanol production plant.

US patent application US20050167103 discloses a method for enhancing natural gas production from a reservoir above a bitumen reserve by injecting waste gas containing carbon dioxide into the reservoir.

US patent 5,388,645 discloses a method for injecting an oxygen depleted airstream into a methane containing solid carbonaceous formation to release desorbed methane and enhance methane production.

US patent 4,393,936 discloses a method for enhancing natural gas production from a natural gas containing formation by displacing the natural gas by a less valuable gas, such as nitrogen.

A limitation of the known nitrogen injection technologies is that they are generally configured to enhance production from gas condensate, hydrate, Coal Bed Methane (CBM) and/or shale oil containing formations or from tight reservoirs by fracturing or desorption and that they are not configured to enhance natural gas production from a natural gas or residual gas reservoir by displacing gas below a Free Water Level (FWL) in the formation .

There is a need for a method and system for

enhancing natural gas production from a natural gas containing formation, which may be partly depleted and contain residual gas trapped below a Free Water

Level (FWL) in the formation.

Furthermore there is a need for a method and system for enhancing natural gas production from a natural gas containing formation in a cost effective and safe manner by using an inert drive medium that can be generated in large quantities at low cost. In addition there is a need for a method and system for enhancing natural gas production from a natural gas containing formation in such a manner that subsidence of the overburden is inhibited.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method for enhancing natural gas production from an underground natural gas containing formation, the method comprising injecting non-corrosive Nitrogen containing drive gas into the formation during a period of at least two months, during at least part of which period natural gas is produced from the formation, wherein the formation comprises residual natural gas trapped in pore water below a Free Water Level (FWL) and the Nitrogen

containing drive gas is injected into the pore water below the Free Water Level (FWL) in the formation..

The non-corrosive Nitrogen containing drive gas may be injected a) into a natural gas containing formation which is an at least partly depleted gas reservoir and which may have been invaded by water through the

expansion of a connected aquifer and thereby leaving residual gas in the original gas reservoir, b) into an aquifer, which may contain residual gas below the original Free Water Level (FWL) and which is connected to a gas reservoir and c) into an aquifer containing residual gas, which it is not connected to a gas

reservoir. N2 may be injected below or above a natural fracturing pressure gradient. Fracturing here is

temporary as a consequence of high injection pressure and fracturing will cease until the injection pressure is lowered or injection stops.

The permeable underground formation layer may be tilted and have an upper and a lower edge and the Nitrogen containing drive gas may be injected into the formation in the vicinity of the lower edge of the tilted permeable underground formation layer.

The natural gas containing formation may not contain a substantial amount of associated natural gas associated to crude oil, coal or another carbonaceous material, natural gas in a gas cap above an oil reservoir, crude oil and/or high amounts of retrograde condensates that drop out in the formation.

After injection of a slug of the Nitrogen containing drive gas a slug of corrosive drive gas comprising Carbon Dioxide and/or flue gases may be injected into the reservoir, wherein the slug of Nitrogen containing drive gas may have at an ambient pressure of 1 bar a volume of at least 100 million m ³ to provide in the reservoir a barrier between the slug of corrosive drive gas and the natural gas, which barrier inhibits mixing of the corrosive drive gas with the natural gas and inhibits the corrosive drive gas to reach natural gas production wells and other natural gas production facilities during the production of natural gas via these wells and facilities.

Optionally, the slug of non-corrosive Nitrogen containing drive gas has a volume of at least 10% of a swept pore volume at an ambient pore pressure in the underground natural gas containing formation.

After injection of a slug of the Nitrogen containing drive gas a slug of any type of other drive non-corrosive gas or gas mixture or water may be injected into the formation prior to injecting the corrosive drive gas .

In accordance with the invention there is

furthermore provided a system for enhancing natural gas production from an underground natural gas containing formation comprising residual natural gas trapped in pore water below a Free Water Level (FWL)in the formation, the system comprising at least one injection well through which non-corrosive Nitrogen containing drive gas is injected below the Free Water Level (FWL) into the formation during a period of at least two months and at least one production well through which natural gas is produced during at least part of said period.

The Nitrogen containing drive gas may be obtained from an Air Separation Unit (ASU) that separates the air into streams of nitrogen, oxygen and/or oxygen enriched air and that supplies the oxygen and/or oxygen enriched air to an industrial plant, which may be a power plant in which in a mixture of fuel and oxygen or oxygen enriched air is combusted to generate electrical energy and to generate flue gases which contain Carbon Dioxide that may be injected after a slug of Nitrogen containing drive gas into the formation in accordance with an optional embodiment of the method according to the invention.

In such case the injected Nitrogen containing drive gas provides a barrier that inhibits corrosive drive gas containing Carbon Dioxide or any other non-inert or hazardous drive gas to reach the natural gas production facilities .

These and other features, embodiments and advantages of the method and/or system according to the invention are described in the accompanying claims, abstract and the following detailed description of non-limiting embodiments depicted in the accompanying drawings, in which description reference numerals are used which refer to corresponding reference numerals that are depicted in the drawings.

Similar reference numerals in different figures denote the same or similar objects. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows how Nitrogen for use in the method according to the invention is separated from oxygen in an Air Separation Unit (ASU) ;

Figure 2A schematically shows how production of Natural Gas is enhanced by a Nitrogen Assisted Depletion Drive (NADD) method;

Figure 2B schematically shows how production of Natural Gas is enhanced by the Nitrogen Enhanced Residual Gas (NERG) method according to the invention;

Figure 2C schematically shows how production of Natural Gas is enhanced by another embodiment of the Nitrogen Enhanced Residual Gas (NERG) method according to the invention;

Figure 2D schematically shows how production of Natural Gas is enhanced by a further embodiment of the Nitrogen Enhanced Residual Gas (NERG) method according to the invention; and

Figure 3 schematically shows how production of

Natural Gas is enhanced in a tight gas reservoir by yet a further embodiment of the Nitrogen Enhanced Residual Gas (NERG) method according to the invention. DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

Figure 1 shows how Nitrogen (N ₂ ) is separated from oxygen (0 ₂ ) in an Air Separation Unit (ASU) (1) of a power plant (2) that generates electrical energy (3) by

combusting fuel using Oxygen (0 ₂ ) or an Oxygen enriched air mixture. The generated Nitrogen (N2) is subsequently pumped by a compressor (4) into a nitrogen supply conduit (5) that is connected to one or more Nitrogen injections wells 22, 31A-D as shown in Figures 2A-D and 3. Figures 2A-2D schematically show how Nitrogen (N ₂ ) that may be generated by the ASU (1) shown in Figure 1 is injected as an inert, non-corrosive drive gas into an underground gas reservoir formation 20 to perform a

Nitrogen Assisted Depletion Drive (NADD)or the Nitrogen Enhanced Residual Gas (NERG) process according to the invention .

The underground gas reservoir is located in a permeable gas bearing formation layer, also known as the reservoir formation 20, which is located underneath an impermeable cap layer 21 that traps the natural gas within the reservoir formation 20 and the Nitrogen (N ₂ ) is injected via a Nitrogen injection well 22 into a part of the reservoir formation 20 at a distance to the

production well at 20A of the reservoir formation 20 whilst natural gas (comprising CH ₄ and other

constituents ) is produced via a natural gas production well 23.

In the embodiment shown in Figure 2A the reservoir formation 20 only comprises a minor fraction of water which is dispersed in the pores of a tilted formation layer, so that there is no water accumulation in this part of the reservoir formation 20.

In the embodiment shown in Figure 2B there is significant accumulation of water (H ₂ 0) in the pores of near the lower edge 20A of the tilted reservoir layer, so that there is a water layer 24 having a upper water level 25, also identified as the Free Water Level (FWL) or IGWC, within the pores of the reservoir formation 20, but which water layer may comprise a substantial amount of natural gas .

In the embodiment shown in Figure 2B the Nitrogen (N2) is injected into the water layer 24 below the Free Water Level (FWL or IGWC) 25 to stimulate migration of natural gas (CH ₄ ) from the water layer 24 and to enhance flux of natural gas (CH ₄ ) through the reservoir formation 20 to the natural gas production well 23.

Figure 2C schematically shows how natural gas (CH ₄ ) has been partly separated from the pores near the lower edge 20A of the reservoir formation 20 and pore water reaches the production well 23 leaving trapped or residual gas behind.

Figure 2D schematically shows an embodiment where the pores of substantially the entire reservoir formation 20 are filled with a water-gas mixture comprising pore water and Natural Gas (CH ₄ ) , which mixture is stimulated to flow into the production well 23 by injecting Nitrogen into the injection well 22 near the lower edge of the reservoir formation 20.

Figure 3 schematically shows how production of Natural Gas (CH ₄ ) from a wet tight gas reservoir or wet residual gas formation 30 is enhanced by yet another embodiment of the Nitrogen Assisted Depletion Drive (NADD) or Nitrogen Enhanced Residual Gas (NERG) process according to the invention wherein Nitrogen is injected into one or more Nitrogen injection wells 31A-D below a Free Water Level (not shown) and natural gas (CH ₄ ) is produced via one or more production wells 32A-D

traversing the tight gas formation 30.