Natural gas usually contains a mixture of methane and heavier hydrocarbons, such as ethane, propane, butane and medium- to long-chain hydrocarbons. As long as pressures within the reservoir remain high around production wells, the hydrocarbons can be economically produced in a gas phase. However, when pressure within the reservoir and around production wells decreases as hydrocarbon is produced, a phenomenon commonly known as retrograde condensation occurs. The heavier hydrocarbons condense to a liquid phase. The presence of liquid hydrocarbons in the formation rock around a production well causes significant reductions in the effective permeability to gas in the near-wellbore region.
The gas pressure near a wellbore may decrease below the dewpoint pressure of the natural gas while the pressure within most of the reservoir remains higher than the dewpoint pressure. Moreover, the condensed hydrocarbon liquid accumulates into a condensate bank near the wellbore that dramatically reduces the effective permeability of the reservoir to gas and, thus, significantly impairs the recovery rate of hydrocarbons. As a result, the formation of retrograde condensate can effectively prevent the economic production of vast volumes of natural gas.
In addition, the presence within the formation of liquid water greatly exacerbates this problem. Liquid water combined with retrograde condensate formation introduces a third phase to the reservoir, whereby the multiphase effects further reduce the effective permeability of the reservoir to gas. Therefore, the recovery of hydrocarbons is further impaired.
Several methods have been used in an attempt to reduce the problems caused by retrograde condensate formation. One such method attempts to reduce the condensate saturation by utilization of a condensate removal agent. For example, large volumes of carbon dioxide and methanol, natural gas, or other suitable condensate removal agents are injected into the near-wellbore region to remove the condensed hydrocarbons that have accumulated due to the decrease in pressure. Studies have shown that this technique, sometimes referred to as the “Huff ‘n’ Puff” injection process, can reduce condensate buildup near the wellbore. (“Wellbore Liquid Blockage in Gas-Condensate Reservoirs”, SPE 51050, 1998). However, these processes generally result in merely a temporary reduction of the condensate saturation subsequently followed by the rapid re-formation of the condensate, and a corresponding reduction in the effective permeability of the reservoir to gas. As a result, this technique is not an effective method to reduce the problems caused by retrograde condensate formation.
Another method attempts to reduce retrograde condensate formation through the injection of various water-wetting surfactants or non-wetting surfactants into the subsurface formation. These techniques have been shown to have minimal if any effect on near-wellbore production capacity. The inventors believe that this failure is due to omission of the effects of the third phase. The combination of retrograde condensate with its inherent viscosity combined with movable water negates any positive effects from the surfactants. Consequently, the rapid reformation of condensate near the wellbore results in a rapid reduction in the effective permeability of the reservoir to gas.
Moderate success has been achieved in reducing retrograde condensate formation by the use of pressure maintenance in a reservoir. In general, pressure maintenance systems attempt to maintain the reservoir pressure above the dew point pressure of the gas by the re-injection of lean natural gas into the well. For example, a gas re-injection process has been utilized with some success in the Ekofisk project in the North Sea. However, the exorbitant costs associated with a typical gas re-injection scheme minimize the large-scale application of pressure maintenance systems. Moreover, most pressure maintenance systems are also not effective solutions because of compatibility problems or contamination of the in-situ gas by the injected gas. Even if pressure maintenance is applied to a reservoir, the drawdown in pressure near production wells may cause severe reduction in gas permeability and decreases in well productivity.
During production of crude oil from some reservoirs, the flowing bottomhole pressure in the reservoir is reduced such that the pressure of the crude oil in the reservoir rock around a well falls to below the bubblepoint pressure of the crude oil. This means that a gas phase forms in the rock around the well, and this gas-phase formation will cause a reduction in flow rate of crude oil into the well.
Consequently, there is a need for a process that can effectively reduce permeability restrictions near the wellbore caused by retrograde condensate or gas breakout from crude oil that allows for the effective and economically feasible recovery of hydrocarbons.