Cyclic Injection of Rich Gas Into Producing Wells To Increase Rates From Viscous-Oil Reservoirs
Authors: Shelton Jack L. , Morris Earl E. 
Published: December 2007

A huff-and-puff process using rich gas can increase production rates in viscous-oil reservoirs by lowering the oil viscosity and by providing additional expulsive energy. For satisfactory results, good contacting efficiency between the injected gas and the reservoir is necessary. The energy generated by the gas is only briefly important, although it can be the principal effect if gas injection periods are short.

Introduction

Reservoirs containing viscous, dead oil present production problems because the high oil viscosity and production problems because the high oil viscosity and the low reservoir energy result in low recovery rates and poor recovery efficiency. Hydrocarbon deposits and emulsions in or near the wellbore sometimes cause additional problems. To improve recovery, methods are needed that increase the reservoir energy and reduce the crude viscosity and that clean up the well area. Techniques that involve both injection and production through the producing wells are particularly production through the producing wells are particularly useful because one or more of the desired benefits is achieved in much less time than it would take with a well-to-well fluid injection program. A familiar example of an injection-production method is the steam buff-and-puff treatment to heat up the reservoir around a producing well and decrease the crude viscosity. Some benefit is also derived from the energy of the steam and its ability to disperse solids in the wellbore. Stimulation of deep or thin formations is difficult, however, owing to high heat losses. Solvents or chemical agents can be injected to clean up solids and emulsions in and near producing wells. These materials can also reduce crude viscosity, but their cost is usually too high to justify extensive treatments. A more recent method of recovering viscous oil was reported by Clark et al. In the operations they described, combustion exhaust genes were injected. Increased oil rates were attributed to decreased oil viscosity from carbon dioxide absorption in the crude and to increased reservoir energy from the injected gases. Other gases that can reduce crude viscosity, such as a rich hydrocarbon gas, can also be used. Our purpose here is to show the separate effects of the viscosity reduction and the energy increase mechanisms in achieving oil rate increases from the gas injection-production method. We include data from field tests using natural gas enriched with propane.

Important Mechanisms

The injection of gas into a producing well may increase flow rates by the following mechanisms:The injected gas dissolves in the crude oil and reduces its viscosity. Flow resistance in the critical region near the well is reduced.The injected gas increases the reservoir energy.Some cleanup of hydrocarbon solids in the wellbore or adjacent portions of the reservoir can also occur.

The oil flow rate increases resulting from these mechanisms could be somewhat reduced by increases in the gas saturation, which would reduce the oil relative permeability. As it turns out, the relative contributions to the oil rate increase by the first two mechanisms depend upon specific factors in the injection and production operations.

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