Aquifer Gas Injection Offers an Alternative for Handling Associated Gas Produced from Deepwater Fields
Authors: Qian Yanlin , Wattenbarger Robert A. , Stuart L. 
Published: June 2014

Abstract
The shift of hydrocarbon exploration and production to deepwater has required new approaches to field development. Floating production structures provided a method to produce oil and transport it to market via tanker. However, handling the associated gas represents a new challenge.
This paper considers the viability of storing produced associated gas in an aquifer for recovery at some future time. A simulation study was conducted with a hypothetical sandstone aquifer in an anticlinal trap. Five years of injection were simulated followed by five years of production (stored gas recovery). Particular attention was given to the role of relative permeability hysteresis in determining trapped gas saturation which tends to control the efficiency of the storage process. Various cases were run to observe the effect of location of the injection/production well.
The results indicate that it is technically feasible to recover gas injected into an aquifer, provided the aquifer exhibits the appropriate structure, size and permeability. However, more than half of the stored gas is lost due to trapped gas saturations and high water saturations with corresponding low gas relative permeabilities. During the production (recovery) phase, it can be expected that water-gas production ratios will be high. Well location has a role in recovery efficiency, but is not as important as the relative permeability effects. The economic limit of stored gas recovery will be greatly affected by producing water-gas ratios, especially at great depth. For a specific prospect for gas storage in an aquifer, special lab tests should be done to exam the imbibition process. An economic analysis should then be tied to an accurate simulation of the prospect.

Introduction
Development of deepwater oil reservoirs presents a number of challenges. The use of floating structures allows oil to be produced at extreme water depths event in remote locations where pipeline infrastructure is nonexistent. While oil can be tankered from these floating structures, the associated gas must be handled in some manner. A number of gas handling alternatives are being applied/developed around the world. These include:1,2,3,4Pipeline Transportation - conventional single-phase gas pipeline or a multiphase pipeline where the gas is combined with produced oil.Gas Injection - injection into the producing reservoir or injection into a nearby or uphole aquifer.Liquefied Natural Gas (LNG) and transportation to shore via tanker.Gas to Liquids (GTL) and transportation to shore via ship or tanker.Compressed Natural Gas (CNG) and transportation to shore via ship.Gas to Wire (GTW) - offshore generation of electricity for transmission to shore via high voltage subsea cables.Gas to Solids (GTS) - conversion to solid forms such as hydrates for transport to shore via ship.Lease Use - conversion to other forms of energy for use offshore in the operation of the producing field.
These technologies are at various levels of maturity; from conceptual to pilot tested to mature. With the recent approval of FPSO's for use in the U.S. Gulf of Mexico, interest in developing ways of handling associated gas has increased dramatically.