Case Histories: Damage Preventions by Leakoff Control of Fracturing Fluids in Appalachian Gas Reservoirs

Abstract

The primary purpose of surfactants used in stimulating Sandstone reservoirs is to reduce surface tension. Conventional surfactants adsorb rapidly within the first few inches of the sandstone formations, thus losing their effectiveness as the treating fluid leaks off. This results in trapped fluids and poor post fracturing fluid recovery. A surfactant solvent system in the form of a microemulsion can also be used to not only lower surface tension at very dilute concentrations, but alter contact angle and lower capillary pressure to further improve recovery. The unique microcells are also effective in controlling leakoff. 

This study describes the laboratory experiments and field case studies of various surfactants used in the oilfield. Several surfactants including an ethoxylated linear alcohol, a nonyl phenol ethoxylate and a microemulsion system were investigated to determine their adsorption properties when injected into laboratory sand packs. Laboratory studies were also conducted to compare the leakoff and water recovery properties from gas wells.

Field data collected from Bradford and Speechley sandstone formations confirms experimental sand pack 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 and drainage areas with various fluid systems. 

Lab and field data collected in these studies from Appalachian Basin reservoirs illustrates that the addition of a microemulsion to a fracturing fluid exhibits significant advantages over the conventional surfactant treatments when water recovery, increased effective fracture length and well productivity are of concern to the operator.

Introduction

The primary objectives of this study include:

1. Comparison studies of the microemulsion system with conventional surfactants commonly used in the oilfield to determine their adsorption properties into the proppant pack, leakoff, and water recovery.

2. Gather experimental data comparing the effectiveness of conventional surfactants versus a microemulsion in regained permeability and fracture clean up test.

3. Present case studies and production simulations where microemulsion treatments have improved water recoveries in treated gas wells in Bradford and Speechley formations.

One of the continuing challenges in Appalachian Basin gas wells is post fracturing fluid recovery due to low pressure low permeability reservoirs.  Most wells are stimulated with water based fracturing fluids and produce back less than half of the injected fluids even with the use of conventional surfactants that lower the air-water interfacial tension[1]. 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, effective fracture 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. This is even more important in low pressure areas where the available pressure may not overcome the capillary end effect leaving fluid trapped in the reservoir in a manner as illustrated in Figure 1[2].