Abstract

The primary purpose of surfactants used in stimulating sandstone reservoirs is to reduce surface tension, contact angle and leakoff control. However, many of these chemicals adsorb rapidly within the first few inches of the sandstone formations, reducing their effectiveness in deeper penetration. This phenomenon causes surfactants to adsorb or plate-out reducing their effectiveness in post fracturing fluid recovery.

This study describes experimental and field case studies of various surfactants used in the oilfield. Several different surfactants including a nonionic ethoxylated linear alcohol, nonyl phenol ethoxylate, an amphoteric, a cationic and a microemulsion system were investigated to determine their adsorption properties when injected into a laboratory sand packed column. A laboratory simulated comparison study of commonly used surfactants and microemulsion was used to identify their leakoff and water recovery properties from gas wells.

Field data collected from Bradford, Balltown and Speechley sandstone formations confirmed experimental sand packed column and core flow investigations. Reservoirs treated with microemulsion fluids demonstrate exceptional water recoveries when compared with conventional surfactant treatments. Wellhead pressures, flowing pressures and production data were collected and evaluated using a production simulator to show effective fracture lengths, damage surrounding the fractures and drainage areas with various fluid systems. These investigations and presented case studies can be used to minimize formation damage.

Introduction

Surfactants or surface active agents are predominately used in hydraulic fracturing fluids to reduce emulsion tendencies between reservoir oil and treatment fluids. This problem is normally addressed by incorporating a non-emulsifier within fracturing fluids. However when gas wells are stimulated with water based fracturing fluids, fluid retention and a reduction in interfacial tension between the rock and the injected fluid are the key driving forces worthy of consideration for well cleanup. One of the continuing challenges in Appalachian Basin gas wells is post fracturing fluid recovery due to low pressure reservoirs. This could be due to the water based fluid creating fluid retention, or interfacial tension between the injected fluid and the reservoir rock, or capillary end effect on and around the vicinity of the face of the fractured rock1. To reduce these problems, commonly available surfactants are incorporated within the fracturing fluid to reduce surface tension. However, surfactants alone do not provide adequate water recovery properties for the reservoirs in the Appalachian Basin. Low temperature, pressure and permeability of these reservoirs may be some of the major reasons that less than 50% of the treatment fluids can be recovered from gas wells through conventional methods.[1] As illustrated in Figure 1, it is possible that large quantities of injected fracturing fluids could be trapped in the area near the fractured proppant pack.

When this condition occurs, higher water saturation adversely impacts the relative permeability resulting in gas being trapped, thus significantly impairing gas production into the fractured face and ultimately into the wellbore. This condition along with capillary end effects eventually leads to longer swabbing times, well cleanups, and poor well productivity.

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