Abstract

A new microemulsion additive has been developed that is effective in remediating damaged wells and is highly effective in fluid recovery and relative permeability enhancement when applied in drilling and stimulation treatments at dilute concentrations.The microemulsion (ME) is an optically clear, thermodynamically stable blend of biodegradable solvent, surfactant, co-solvent and water.The nanometer sized structures have extremely high contact efficiency at low concentrations (0.10–0.5%).

Lab data is presented that illustrates how the Microemulsion accelerates the cleanup of injected fluids in tight gas cores. The microemulsion additive results in lower pressures to displace injected fluids from low permeability core samples and proppant packs. The relative perm to gas is increased substantially as the water saturation is decreased. The enhanced relative permeability mechanism is the alteration of the rock-fluid interfacial tension or contact angle.It is demonstrated that this alteration effectively lowers the capillary pressure and capillary end effect associated with fractures in low perm reservoirs by as much as 50%, thus mitigating phase trapping and therefore permitting an increased flow area to the fracture and longer effective frac lengths.Examples of drilling fluids with and without ME are shown in which the relative perm to gas and oil are improved with the application of the microemulsion system.

Field examples are shown for fracture treatments of unconventional low permeability coals and shales, where water recovery and productivity are increased by 50% with the application of ME. A production analysis is presented for wells treated in the Barnett Shale that shows longer effective frac lengths and decreased damage surrounding the fracture.

Introduction

The use of surface active agents or surfactants in drilling and hydraulic fracturing has been commonplace since water became the preferred base fluid over oil.In oil reservoirs the use of surfactants is intended to prevent emulsification of the treating fluid with the produced oil. The treatment is generally optimized to balance the oil/water interfacial tension with non-emulsification (NE) additives. In gas reservoirs the use of water based fluid creates fluid retention problems.The primary function of surfactant addition has been to reduce surface tension between the treating fluid and the gas and thereby recover more of the treating fluid and restore the relative permeability to gas.The truth of the matter is that many drilling and fracturing treatments recover less than half of the injected fluid even with the application of surfactants to lower surface tension.

It must be assumed that these large quantities of fluid are trapped in the reservoir surrounding the wellbore and in the case of hydraulic fracturing the fluid is trapped in the area surrounding the fracture and within the fracture itself.This trapped fluid has a detrimental effect on the relative permeability, effective flow area and effective frac lengths and without question impairs well productivity.

There are more factors that influence the cleanup of injected fluids than simple air-water surface tension. An additional factor that is typically overlooked is the interfacial tension between the rock and the injected fluid which is of prime importance in dictating capillary pressure and capillary end effects in gas wells.In the case of oil there are the interfacial tensions between the rock and produced oil/condensate and injected fluids as well as the interfacial tension between the injected fluid and the oil/condensate.Little attention is paid to the rock fluid interfacial tension in either case except to insure that the treating fluid is water wetting.

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