Soaking Flowback With Surfactant Improves Oil Recovery in Unconventional Reservoirs
Authors: Carpenter Chris 
Published: October 2018

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 187483, “Improving Oil Recovery in Unconventional Liquid Reservoirs by a Soaking-Flowback Production Schedule With Surfactant Additives,“ by J.O. Alvarez, F.D. Tovar, SPE, and D.S. Schechter, SPE, Texas A&M University, prepared for the 2017 SPE Liquids-Rich Basins Conference—North America, Midland, Texas, USA, 13–14 September. The paper has not been peer reviewed.

This paper evaluates the ability of different groups of surfactants to improve oil recovery in unconventional liquid reservoirs (ULRs) by experimentally simulating the fracture treatment to represent surfactant imbibition in a ULR core fracture during a soaking flowback. The effect of wettability and interfacial tension (IFT) alteration on the process also is analyzed. The results show that the addition of surfactants to completion fluids and the use of a soaking-flowback production scheme could improve oil recovery by wettability alteration and IFT reduction, maximizing well performance after stimulation.

Introduction

An understanding of rock/fluid and fluid/fluid interactions is vital in finding innovative ways to improve oil recovery in ULRs. Wettability controls flow behavior and distribution of the fluids in the reservoir. Many current wettability-determination methods have limited application because of the ultra low permeability of ULRs. For this paper, the authors chose the contact- angle (CA) method as the most viable way to determine wettability alteration in ULRs quantitatively. Unlike conventional reservoir rocks, shale chips provide ample and smooth surfaces to measure the CA of reservoir fluids directly at reservoir temperature. In addition, zeta-potential measurements are used in this paper. Zeta potential can be related to wettability by the stability of the double layer between oil and the shale rock surface. Stable solution film provides an indication of water-wet systems, whereas unstable solution film is considered oil- or intermediate-wet. Also, the strength and nature of the rock and the aqueous solution charge can be estimated by zeta-potential measurements.

In conventional reservoirs, altering the wettability of the rock to water-wet and reducing IFT to ultralow values help recover additional oil by mobilizing oil trapped in fine channels after primary recovery. However, these methods have limited application in ULRs. Therefore, the use of surfactant as well as recovery mechanisms may differ in conventional and unconventional reservoirs.

Methodology

This study investigates the interaction of completion fluids, with and without surfactants, and ULRs. In addition, it addresses the effect of wettability, IFT, and imbibition on recovering hydrocarbons from liquid-rich shale cores from the Wolfcamp. These objectives were achieved by performing CA, zeta-potential, and IFT measurements, as well as coreflooding methods, to simulate the fracture treatment at reservoir conditions and represent surfactant imbibition in a ULR core fracture during a soaking flowback. The complete paper describes a novel set of correlated experiments performed to evaluate and compare the efficiency of surfactants in altering wettability and recovering hydrocarbons from a ULR core.