Fundamental investigation of an environmentally-friendly surfactant agent for chemical enhanced oil recovery

Highlights

• Application of a new green surfactant is investigated for the first time regarding chemical EOR.

• Impact of the presented surfactant on IFT reduction and wettability alteration is examined.

• Effects of salt on IFT reduction and CMC values are demonstrated.

• Carbonate and sandstone rocks are utilized for the purpose of wettability alteration tests.

• Secondary and tertiary oil recovery schemes are implemented on carbonate rocks to quantitatively compare their efficiency.

Abstract

Surfactant injection is an important chemical enhanced oil recovery (EOR) technique with beneficial impacts for oil recovery from subsurface reservoirs due to interfacial tension (IFT) reduction and wettability alteration. However, most of the available or proposed synthetic surfactants have negative environmental impacts. Here, a novel synthesis procedure of an amino-acid based (non-toxic; easily biodegradable) surfactant is described and its application for chemical EOR is rigorously tested using IFT, wettability and coreflooding experimental tests. The effect of salinity on the IFT reduction highlights the potential impact of injection water salt concentration on its performance. Two sets of carbonate and sandstone rock samples are used for wettability alteration tests. Two displacement tests quantitatively assess the performance of the proposed surfactant during injection as part of secondary and tertiary recovery schemes. The synthesized amino-acid based surfactant demonstrates good synergy with appropriate injection water salt concentrations. The wettability test results suggest that both sandstone and carbonate reservoir rocks would potentially benefit when subjected to chemical EOR using this surfactant. Comparisons between secondary and tertiary surfactant flooding schemes suggest that the surfactant is potentially more effective during secondary injection.

Introduction

Sustainability of nonrenewable resources, such as oil, requires that every effort is made to enhance recovery of existing resources from sub-surface reservoirs using methods that are environmentally friendly as well as commercially viable [1]. Development of typical oil fields over time usually involves three distinct stages of development: primary recovery; secondary injection-supported processes; and enhanced oil recovery (EOR) or tertiary injection processes. Natural energy from within the reservoir is the key mechanism driving oil recovery in the primary processes, but this typically leaves between 75% and 95% of the original oil in-place (OOIP) within the reservoir pore space. Secondary processes such as water and gas flooding achieve further improvements to oil recovery, but still typically leave more than 50% OOIP trapped in oil reservoirs. It is the tertiary EOR techniques, such as non-hydrocarbon gas, thermal, and chemical flooding, that have the potential to further boost the oil recovery [2], [3], [4], [5], [6], [7], [8].

EOR focuses on pushing as much residual oil out of the reservoir pore space as possible, i.e., oil that cannot be readily recovered by primary and secondary processes due to disadvantageous reservoir or fluid characteristics, such as inappropriate mobility ratio, capillary pressure, high fluid viscosity and heterogeneity within the reservoir pore rock [1], [9], [10]. Amongst the EOR techniques, chemical injection, including surfactant, alkaline, and polymer flooding, has demonstrated positive oil recovery performance at both field and laboratory scales [11], [12], [13], [14], [15], [16].

Surfactant flooding can enhance the water injection sweep efficiency by reducing the interfacial tension of the water-oil system (i.e., reducing the capillary pressure) and changing the wettability state of the reservoir rock [17]. A dimensionless parameter called capillary number illustrates the effect of capillary forces in the fluid displacement process and is equal to the ratio of viscous forces over the capillary forces:

$$N_{\mathit{ca}}=\frac{\mathit{Viscous}forces}{\mathit{Capillary}forces}=\frac{V\mu }{\sigma cos\theta }$$

where

• V is the injection fluid’s Darcian velocity (m/s);

• μ is the injection fluid viscosity (Pa·s);

• σ is the interfacial tension (N/m);

• θ is the contact angle; and,

• N_ca is the capillary number (dimensionless).

It has been suggested that residual oil saturation (ROS) has an inverse proportionality with the capillary number, which is depicted in the desaturation curve [1], [2], [9], [18], [19]. This means that increasing the capillary number is a key objective when attempting to enhance displacement processes. This can be achieved by either letting the contact angle approach 90° (reflecting a fluid-rock system in a neutral wettability state), increasing the viscosity of injection fluid and/or reducing the water-oil system interfacial tension.

Surfactant molecules are amphiphilic organic substance, i.e., they are comprised of two basic structures, i.e. hydrophobic (water-hating) and hydrophilic (water-loving) components. Consequently, surfactants are soluble in both polar (such as water) and nonpolar (such as oil) compounds. Surfactants are categorized into anionics, cationics, nonionics and zwitterionics based on the charge of their polar head groups. In petroleum applications, however, anionic surfactants are the type most widely used at the field scale, because of their commercial viability, good retention properties in porous media and relative stability [2], [20]. These surface active additives can generally improve oil production via three main impacts: (1) reduction in interfacial tension (IFT) at the oil-water surface which helps to mobilize the trapped oil; (2) wettability alteration of the rock surface to enhance the imbibition rate; and (3) the spontaneous formation of microemulsions or emulsions [9], [21].

The characteristic point of any typical surfactant solution above which aggregates of surfactant monomers are formed (also referred to as micelles) is known as the critical micelle concentration (CMC). It is crucial to characterize the CMC of any surfactant, since in many surfactant-based petroleum applications, CMC is incorporated as the optimized concentration. For the purpose of determining CMC point of a specific surfactant, surfactant physiochemical properties such as surface tension, osmotic pressure, interfacial tension, electrical conductivity, etc. can be graphically compared with the surfactant concentration. Typically, it is the inflection point on such plots that is taken to be the surfactant CMC [9], [22], [23], [24].

In recent years, diverse applications for synthetic surfactants have been suggested. For example, Al-Sabagh [25] studied the interfacial tension behavior of eight nonionic surfactants for the purpose of predicting their efficiency in a typical micellar injection process. Chen et al. [26] synthesized a series of cationic gemini surfactants and further investigated their effects on the IFT reduction of both crude oil and hexadecane in the presence of saline aqueous solutions. These cationic gemini surfactants could efficiently lower the crude oil-water system’s IFT to ultra-low values. However, they were unable to do this for a hexadecane-water system. Wang et al. [27] examined a synthesized betaine amphoteric surfactant as a tertiary oil recovery agent by conducting IFT measurement, coreflooding and wettability alteration experiments. Their results illustrated that crude oil-water systems can achieve ultra-low IFT values when applying only ultra-low surfactant concentrations (10–3000 ppm). Also, coreflood tests revealed that ultimate oil recovery may be as much as 70% of OOIP or higher when a combined betaine amphoteric surfactant-polymer system is applied. Cui et al. [28] synthesized a large hydrophobic betaine surfactant and examined its effects on the IFT reduction and flexibility in a surfactant-polymer injection process. They indicated that ultra-low IFT values can be achievable in crude oil-water systems in the presence of the large hydrophobic betaine surfactant at 45 °C. Moreover, approximately 18% of additional OOIP was recovered when this surfactant was implemented in a tertiary scheme. Babu et al. [29] synthesized a polymeric surfactant and evaluated its potential applications for chemical EOR through IFT and wettability tests. Kumar et al. [30] prepared an anionic synthetic surfactant from non-edible vegetable oil and evaluated its potential for application in EOR in terms of IFT, dynamic viscosity, wettability, CMC, and coreflooding tests. Kumar and Mandal [31] proposed a synthetic approach of a zwitterion surfactant along with its efficiency in IFT reduction and wettability alteration on quartz.

Unfortunately, most of the synthetic surfactants evaluated in the studies mentioned have associated environmental and human health hazards. These surfactants can affect the growth of microorganisms and algae in water and hence, undermine the food chain of organism. Generally, there is a relationship between the toxicity of water and the chemical structure of these surfactants. The aquatic toxicity increases with the hydrophilic-lipophilic balance (HLB) value and decreases with the number of ethoxylate groups of the surfactants. Surfactants also kill different types of microorganisms in water and prevent the degradation of other toxic materials, which need the microorganisms to be degraded [32]. Surfactants can enter the human body, damage the activity of enzymes and disrupt the physiological function of body [33]. Cationic surfactants have the greatest toxicity, and the anionic surfactants have toxicity between that of nonionic and cationic surfactants. Sodium dodecyl benzene sulfonate (SDBS) is reported to enter the human body through the skin and damage the liver and cause chronic symptoms [34].

While conventional reservoirs have always been recognized as a practical target for surfactant flooding, unconventional liquid reservoirs (ULRs) have also been tested for applicability of surfactant injection. In fact, surfactants have the potential to enhance ULRs imbibition oil recovery factor by altering wettability. By doing this, surfactants alter the capillary forces by turning oil-wet and intermediate-wet unconventional reservoirs into water-wet conditions. In this way, Alvarez et al. introduced the concept of low-concentration surfactants to utilize for EOR in ULRs. Their analyses proved that anionic surfactants are more effective in wettability alteration, enhancing imbibition, and improving oil recovery factors than nonionic surfactants [35]. Alvarez and Schechter demonstrated the efficiency of several surfactants in improving oil recovery in siliceous ULRs at the experimental scale by carrying out IFT and wettability measurements, and spontaneous imbibition experiments [36].

Amino acids are organic compounds composed of two key functional groups including carboxylic acid (COOH) and amine (NH₂). These organic compounds are known to have a substantial biodegradability rate, biocompatibility, small irritation properties and toxicity, thus, making them environmentally-friendly materials [37], [38].

In previous work, we have described the preparation of an amino-acid based surfactant (namely, (S)-2-Dodecanamido-5-guanidinopentanoic acid or simply (S)-Dodecanoyl-L-arginine) using a new one-pot synthetic approach and introducing its first petroleum engineering application [9]. Here, the synthetic approach coupled with a mechanistic investigation is used to evaluate the performance of this synthesized surfactant as a chemical EOR agent. Initially, the IFT and wettability of the surfactant solution in the presence of the oil phase and representative rocks (both carbonate and sandstone) are established. The effect of salinity on the IFT is examined. Considering the results of that analysis, an optimized surfactant concentration based on the assessed CMC (in a typical salinity) is selected to perform dynamic secondary and tertiary coreflooding tests. A quantitative comparison is made of the test results of oil recovery with the optimized surfactant and with a typical surfactant-free brine injection process.