Feasibility study of improved unconventional reservoir performance with carbonated water and surfactant

Highlights

• Active carbonated water injection has potential application for tight oil EOR.

• ACWI followed by WAG can further improve oil recovery for tight oil reservoirs.

• The critical injected active carbonated water volume is about 0.8 PV during ACWI+WAG.

• The cubic equation of state coupled with Henry's law can simulate CWI more accurately.

Abstract

For unconventional reservoirs, water flooding performs poorly because of low displacement efficiency; gas flooding shows limited enhanced oil recovery (EOR) capability due to gas breakthrough. Carbonated water injection (CWI) and active CWI (ACWI) are promising EOR methods which combine advantages of water and gas flooding. This paper provides experimental and numerical studies of carbonated water and surfactant injection based on a case study in Changqing Oilfield, China, which is the first time to investigate the feasibility of CWI and ACWI for tight oil reservoirs. This study compares performances of active water injection (AWI), CWI, Water altering gas (WAG), and ACWI. Experimental results reveal that the oil recovery of CWI is 2.7% more than WAG. ACWI achieves the highest incremental oil recovery (9.43%) among four methods. The sensitivity analyses of ACWI + WAG is further implemented experimentally, which demonstrates that ACW as a pre-flood improves 7% of oil recovery during WAG process. For Changqing tight reservoir cores, the optimal injection volume of ACW is 0.8 pore volume. Numerical simulations are conducted to validate the capability of cubic Equation-of-State coupled with Henry's law (EOS/H model) for CWI in tight oil reservoirs, indicating EOS/H model is applicable to correlate phase behavior of carbonated water and oil system. This paper, for the first time, investigates the EOR performance of ACWI in tight oil reservoirs. These results explore the feasible application of using CWI/ACWI in tight oil reservoir development.

Introduction

Tight oil reservoirs have emerged as an important recourse with the application of multi-stages hydraulic fracturing [1]. In the oil industry, CO₂ enhanced oil recovery (EOR) methods are environmental-friendly and have huge EOR potential [[2], [3], [4]]. In tight oil reservoirs, however, CO₂ flooding usually shows serious gas breakthrough in a very short time, due to the difference of oil/gas viscosity and the existence of natural fractures in tight formation, resulting in a limited EOR effect [[4], [5], [6], [7], [8], [9]]. Take a typical Chinese tight oil reservoir, Changqing Oilfield, as an example, the final oil recovery is still no more than 8% of the original oil in place by CO₂ flooding [10,11]. Meanwhile, no sufficient CO₂ sources become another limitation of large-scale CO₂ application [12]. Water flooding is another common method for pressure maintenance in Changqing Oilfield, but it has limited displacement efficiency and injectivity [13]. So, the new method, diminishing the adverse effect of poor water displacement efficiency and gas breakthrough, is needed to be proposed.

Carbonated water injection (CWI), where CO₂ is dissolved in the brine prior to injection, is a potential technique to resolve Changqing's difficulties. During this process, CO₂ gradually transfers from water to oil due to its higher solubility in oil, resulting in higher oil recovery [14]. Compared with CO₂ and water flooding, CWI can reduce the gas breakthrough and improve the displacement efficiency respectively, which is an alternative EOR method for the tight formation where is lack of sufficient CO₂ supply.

Former researchers have examined the performance of CWI in sand-packs or reservoir cores under different pressures and temperatures conditions, the experimental results indicate that CWI had a good EOR potential in conventional porous media, with an incremental oil recovery between 12 and 40% compared to conventional water flooding process [[15], [16], [17], [18], [19], [20]]. The main mechanisms during CWI are introduced as oil swelling, viscosity reduction and coalescence of isolated oil ganglia as a result of CO₂ diffusion [[21], [22], [23], [24], [25], [26], [27]]. Sohrabi et al. [20,21], Riazi et al. [22,23] and Seyyedi et al. [24] conduct a serious of visualization flow experiments, based on the observation and analysis of experimental results, indicating that light oil vaporization and wettability alteration influence the CWI performance. Meanwhile, the reduction in the interfacial tension (IFT) of crude oil/aqueous phase is considered as another important mechanism [25,29,30]. Yang et al. [31] reported that dissolving CO₂ in water reduces the IFT of oil/water by 20%. However, Lashkarbolooki et al. [20] claimed that they found CWI increased IFT, because CO₂ diffusion impedes the transport of natural surface-active agents and their orientation at the interface of fluid/fluid.

Some researchers developed self-programming model to characterize the process of CWI, i.e., De Nevers [31] presented a one-dimensional CWI model, based on Buckley-Leverett flow, where CO₂ solubility in the aqueous phase was input as a function of reservoir pressure. Later, Ramesh and Dixon [32] proposed an improved three-phase black-oil model which could simulate the simultaneous flow of oil, water, and CO₂ in porous space. Alvarez et al. [33] built a one-dimensional model to evaluate the effect of CWI in low salinity reservoirs with Krichevsky-Ilinskaya extension of Henry's law to characterize CO₂ solubility in water. However, these models were designed to reveal CWI mechanisms and not suitable for CWI simulation in field-scale. CWI performance was also evaluated by commercial simulator. Kechut et al. [14] built a compositional model in core-scale to match their experiments by Eclipse E300. This simulator incorporates empirical correlations proposed by Chang et al. [34] for the solubility of CO₂ in the aqueous phase. They believe that E300 normally overestimate the performance of CWI.

In fact, the main challenge for the accurate CWI simulation is to characterize the CO₂ mass transfer from carbonated water to oil. Integrating a model of CO₂ solubility in water would have considerable influence on CWI simulation compared to the traditional compositional simulator where water is treated as an inert phase. Li and Nghiem [35] used Henry's Law to estimate CO₂ solubility in distilled water and used the scaled particle theory to consider the presence of salt in the aqueous phase. The fugacity coefficients of light components that are considered soluble in the aqueous phase (e.g. methane, ethane, propane and CO₂, etc.) can be derived from Henry's constant, and, for hydrocarbon phase, they are calculated by conventional cubic EOS with vdW mixing rule. Enick et al. [36] use Krichevsfcy-llinskaya equation to correlate the solubility of CO₂ in water and the decreased solubility of CO₂ in brine considered empirically by a single factor correlated to the weight percent of dissolved solids. Chang et al. [34] also proposed an empirical correlation as a function of temperature for the solubility of CO₂ in distilled water. The solubility in distilled water can be adjusted further to consider the effect of salinity to obtain the solubility of CO₂ in brine. In addition, Chang et al. [34] proposed an isothermal, three-dimensional, composition model with both fully implicit and implicit-pressure explicit-saturation formulations. The innovative point of their work is that CO₂ fugacity coefficients in the aqueous phase are computed internally from the correlation, and moreover the equal-fugacity constraint of CO₂ in the aqueous and hydrocarbon phase is introduced to solve the aqueous composition. Yan et al. [37,38] used Peng-Robinson EOS, modified by Søreide and Whitson, to describe the phase equilibrium in CO₂ and brine system. A one-dimensional slim tube simulator combined with a multiphase flash subroutine was proposed to model CO₂ flooding considering the influence of CO₂ solubility, where aqueous phase was treated as an inert phase or only dissolving CO₂ phase.

Active carbonated water injection (ACWI) refers to a method that adding surfactant in carbonated water as an injected fluid to improve oil recovery. This method combines the advantages of CWI and active water injection (AWI). Shu et al. [26] conducted core experiments to investigate the EOR performance of ACWI followed by CO₂ injection. Experimental results indicated that CO₂ performance during ACWI+CO₂ is significantly improved compared with individual CO₂ injection. The presence of surfactant can both improve core wettability and reduce IFT significantly, resulting in more CO₂ transfer from water into oil. Meanwhile, lower IFT can improve CO₂ injection after ACWI by eliminating water-block effect [23].

Until now, the researches of ACWI only focus on conventional reservoirs where the permeability is larger than 10 mD. For tight oil reservoirs, there is no relevant study in the literature, resulting in an unclear feasibility of ACWI for unconventional formations. In this paper, ACWI and ACWI followed by WAG are investigated, and sensibility analyses are conducted through tight oil reservoir cores with permeability lower than 0.3 mD. This paper, for the first time, explores the feasible application of using ACWI in tight oil reservoir development.