Microemulsion-Assisted Fluid Recovery and Improved Permeability to Gas in Shale Formations

Abstract

Hydraulic fracturing of shale formations presents a well-known challenge due to high entrapment of frac fluids. Typical levels of fluid recovery from shale formations are on the order of 30% or lower. Fluid loss into shale produces liquid blocks that interfere with the gas flow in already tight formations. Friction reducers or other additives present in fluids can cause further formation damage.  Search for technologies that would maximize gas flow rate and yield high recoveries is on-going. In recent years microemulsions have been successfully used for remediating damaged wells and enhancing production from tight gas formations. In the present work we have explored the use of such additives for improving fluid recoveries from columns packed with shale/sand mixtures and for improving permeability of split shale cores to gas.

Introduction

Shale reservoirs have a number of characteristic features associated with them, such as extremely low permeability of the rock, high organic content, high clay content, fine grain size and plate-like microporosity. At times of high natural gas prices an interest in producing from shale formations and the need for developing advanced technologies for drilling, fracturing, and completion in such formations is especially high. Chemical treatments constitute an inseparable part of shale gas production technologies. The role of chemical treatments in increasing natural gas production and load recoveries from shale formations has been documented in numerous publications [1-6]. These studies have illustrated that surfactant and microemulsion treatments can be used for enhancing gas flow rates and fluid flowback from shale formations. The benefits of surfactants and microemulsions are related to their surface activity, which is revealed as their ability to lower surface tension at a gas/water interface and influence contact angle at the gas/water/rock three-phase contact line.  Both surface tension and contact angle have a direct influence on capillary pressure, which is the main driving force for fluid penetration into the porous media. Capillary pressure is given by a well-known equation

Equation                                                                                       (1)

where ? is the surface tension, ? is contact angle, and r is the radius of pores.   For a liquid of density ??, capillary rise, h, determining the depth of liquid penetration, can be calculated as

Equation                                                                                       (2)

where g is the acceleration of gravity.  From Equations 1 and 2 it is immediately evident that fine porosity, common for shale formations, results in high capillary pressure and promotes formation damage due to fluid blockage. Indeed, it is well known that load recoveries from shale formations are very low, and without using stimulation chemicals often do not exceed 20-30% [6]. Consequently, improving the flowback of injected fluids or recovery of formation fluids is directly linked to enhancing permeability of shale formations to gas, thus making production from such formations economically viable.

The treatment of shale formations with microemulsions results in an improved gas production rates and increased fluid recovery levels, often exceeding those achieved with common surfactants, as has been exemplified in the studies describing fracturing jobs in Barnette and Marcellus shale formations [1,6]. Although these and other studies indicated good performance of microemulsions in these applications, there is still a rather limited number of experimental laboratory evaluations aimed at investigating the benefits of using micoremulsion technology for enhanced gas production and fluid recoveries from shale rocks. More such studies are especially desirable in view of recently reported success in horizontal drilling in shale formations [7,8].