Advances in the Miscible Fracturing Process for Gas Wells
Authors: Anderson Hal A. 
Published: December 2007

Carbon dioxide has been used in oil reservoirs for multiple contact miscibility, with methane or nitrogen as the displacing fluid. Carbon dioxide has also been used as an additive for fracture fluids to promote flow back and even as a base fluid for fracturing. A miscible process has been developed for gas wells using a special hydrocarbon base fluid and CO2. Application in the field has resulted in high fracturing fluid recovery and increased productivity.

This paper describes application of the miscible fracturing process to a broader range of gas reservoirs than previously thought possible. Additional laboratory tests on long stacks of core have enhanced knowledge about the processes involved in miscible fracturing. Slim tube experiments and other miscibility testing have clearly defined the range of reservoir conditions that can benefit from the miscible process. Refinements to the design process including frac fluid composition, carbon dioxide ratios, and volumes are presented.

Over 15 new field examples highlight the various design and operational considerations. The treatments are superior to other treatment types for gas wells in fluid recovery, sand concentration and time for recovery. Reduction in well testing times has resulted in over all reduction in costs. More accurate test results are another benefit.

Introduction

Carbon dioxide has been used in enhanced oil recovery for more than 40 years. Of the two properties of CO2 - decreasing the hydrocarbon viscosity and lowering the hydrocarbon miscibility pressure the most useful for hydraulic fracturing of gas wells is the miscible process.

Up until now, applications of CO2 to well stimulation have been as an energizing component to assist in treating fluid cleanup, through CO2 gas drive in the reservoir, and continuous gas lift in the well. There have also been uses in CO2 foams to provide improved proppant carrying capacity and fluid loss control. Technology was developed that facilitated using liquid CO2 as a fracturing fluid. In these applications, the objective was to reduce the amount of fluid left in the formation. Liquid CO2 with low viscosity and limited sand carrying capabilities, and phase segregation of the CO2 foams limited the applications.

The enhancement of hydrocarbon fluid miscibility particularly those in the gasoline and gas oil range by CO2 was seen as the foundation of a new fracturing process. This process uses a carbonated hydrocarbon fluid to achieve miscibility between the reservoir gas and the fracturing fluid. At the high pressures necessary for fracturing, the hydrocarbon/CO2 mixture is injected into the formation in single-phase. As this mixture leaks off into the formation, the pressure drops below the bubble point and enriched CO2 evolves. As no surfactants are added to promote and stabilize emulsions, the CO2 enriched with C7 to C12 and aromatics forms a miscible bank that allows the reservoir gas to efficiently displace the fracture oil.

This is useful in under saturated and low permeability gas wells where any increase in saturations can lead to an decrease in permeability.

Relative Permeability Effects in Fracturing

If the fracture fluid is considered to flood the formation as in enhanced oil recovery and if the inefficiencies of fingering are neglected we find a different response than that suggested by the "modeling".

During leak off of a water based fluid into a water wet formation, the water as the wetting phase tends to move into the small and medium pores through imbibition. When the gas comes to displace the water after the frac there will be a hysteresis effect because these smaller pores tend to hang onto the water tighter.

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