CO2 Low-Salinity Water Alternating Gas: A Promising New Approach for EOR

Significant advantages have been seen from combining low-salinity waterflooding (LSW) with other enhanced-oil-recovery (EOR) techniques. This paper proposes a novel concept of low-salinity-water-alternating-gas (LSWAG) injection with CO₂ under CO₂-miscible-displacement conditions. While LSW is an emerging EOR method based on alteration of wettability from oil-wet to water-wet conditions, water-alternating-gas (WAG) injection is a proven method for improving gasflooding performance by controlling gas mobility. Therefore, LSWAG injection promotes a synergy of the mechanisms underlying these methods that enhances oil recovery further.

Introduction

LSW is receiving increasing attention in the oil industry and is currently indentified as an important EOR technique because it shows more advantages than conventional chemical EOR methods in terms of chemical costs, environmental impact, and field process implementation.

Although the benefits of LSW have been realized, the mechanism for incremental oil recovery by LSW is still a topic that is open for discussion. Among the proposed hypotheses, wettability alteration toward increased water-wetness during LSW is accepted widely as the cause for the EOR. It has been found experimentally that low-salinity brine has a significant effect on the shape and the endpoints of the relative permeability curves, resulting in a lower water relative permeability and higher oil relative permeability. The mechanisms of wettability alteration because of ion exchange and geochemical reactions have been implemented successfully in a compositional simulator for modeling LSW. Excellent agreements between simulation results and important measurements from coreflood experiments and pilot observations were obtained with this modeling approach.

LSW could also have great benefits when combined with the CO₂ LSWAG process. CO₂ LSWAG injection can be used in oil production in two strategies:

• As an effective EOR approach for green fields and brownfields by using the advantages between them to overcome the current challenges associated with LSW and CO₂ WAG injection.
• As an agent that improves conformance control by blocking off the high-conductivity zones and diverting the injected fluid into unswept layers.

Unfavorable mobility in pure gasflooding results in viscous fingering and reduced volumetric sweep efficiency, and WAG injection helps overcome this problem and reduces the large amount of gas required for EOR projects, especially in offshore oil fields. However, oil production response is usually delayed in the WAG process compared with single-slug CO₂ flooding. Although oil recovery is predicted to be higher in the WAG process, the economics may not be favorable because of the delayed production. LSW can accelerate the oil production in the early stage, whereas CO₂ WAG injection can help promote ion exchange and reservoir geochemical reactions, which are the favorable conditions for LSW itself.

The second strategy came from the idea that a dramatic decrease in salinity gradient with sufficient amounts of Ca⁺⁺ in the injected water can mobilize clay minerals, plug the porous media, and reduce the absolute permeability in the watered-out layers. The injected fluid is then diverted into low-permeability zones and provides additional oil recovery from these regions.

Up to now, there has been a lack of experimental evidence to conclude definitively that LSW induces water blockage. Thus, most projects have been focused on the first strategy. However, previous investigations were limited mainly to coreflood experiments at laboratory scale or simple 1D homogeneous simulations that were far from the reality of this hybrid process. This paper aims to overcome the gaps in the past evaluations of the CO₂ LSWAG process by use of an advanced and comprehensive simulation approach with a mechanistic LSW model in an equation-of-state (EOS) compositional simulator.

Modeling of CO2 LSWAG Process

The key features of modeling CO₂ flooding and LSW are

• Geochemical reactions are coupled fully to the multiphase multicomponent flow equations and the equations for EOS flash calculations.
• Ion exchange and wettability alteration during the course of LSW are considered to be the main mechanisms of the additional oil recovery.
• The multiple ion exchanges were modeled on the basis of chemical equilibrium between ions in the aqueous phase and clay minerals.
• Various intra-aqueous reactions involved in LSW and WAG processes can be modeled.
• Incorporation of various mineral-dissolution and precipitation reactions can affect the ion-exchange process.
• Multiple relative permeability sets can be used to model the alteration of wettability.
• The relative permeabilites of oil and water are altered by a scaled ion-exchange-equivalent fraction that represents the ion exchange and clay properties.

In the literature, LSW has been evaluated in both secondary and tertiary flooding modes; the CO₂ LSWAG process can be implemented after either waterflooding or LSW.

Case Study—1D Simulation

A 1D model was developed to simulate this process. In this model, the authors considered the reversible ion exchange between Ca⁺⁺ and Na+ as well as aqueous and mineral reactions.

Various simulation scenarios were performed to compare the CO₂ LSWAG process with other recovery approaches, such as conventional high-salinity waterflooding (HSW), LSW, CO₂ high-salinity-water-alternating-gas (HSWAG) injection, and pure CO₂ flooding. Four cycles of CO₂ WAG injection were conducted after 0.4 injected pore volumes of HSW or LSW with a WAG ratio of 1:1.

First, the effect of the conventional HSW (Run A) on the oil recovery was considered compared with LSW (Run B). LSW has a great advantage on oil recovery. This benefit is because of ion exchange and mineral reactions. When the high-salinity brine was injected, no wettability alteration occurred because the injected-brine composition is similar to formation-water composition. On the contrary, the adsorption of Ca⁺⁺ during LSW altered the original mixed-wetness to preferential water-wetness, leading to a significant increase in oil recovery.

Although LSW has higher oil recovery than conventional HSW, a large amount of oil is still trapped in the reservoir. CO₂ HSWAG injection (Run C) was considered to increase oil recovery. In that run, approximately 0.4 and 0.6 pore volumes of high-salinity brine were injected before and after four cycles of the HSWAG process, respectively. Oil recovery by HSWAG injection increases by 25.3 and 19.6% of the original oil in place (OOIP) compared with the HSW and LSW, respectively. The additional oil recovery comes from the effects of CO₂ miscible flooding.

Fig. 1 (above) compares the oil recovery from four different recovery methods—HSW, LSW, the CO₂ HSWAG process, and pure CO₂ flooding. Although CO₂HSWAG injection has a higher ultimate oil-recovery factor than HSW and LSW and the final oil-recovery factors by CO₂ HSWAG injection and pure CO₂ flooding are relatively similar, the CO₂ HSWAG process experiences the problem of delayed production. This challenge can be overcome by using CO₂ LSWAG injection, in which the ultimate oil-recovery factor is maximized and oil is produced much faster compared with the CO₂ HSWAG process in the early stage of WAG cycles (Fig. 2).

The results confirm the advantage of the CO₂ LSWAG process in oil recovery. Generally, geochemical reactions play an important role in the CO₂ LSWAG process. The dissolution of calcite can promote the wettability alteration by supplying the Ca⁺⁺ source for the ion-exchange process. The dissolution of calcite increases the ion-exchange level.

A series of sensitivity-analysis runs was conducted, with the following observations:

• The oil-recovery factor tends to increase with an increase of the injected-Ca⁺⁺ concentration.
• Injected-Na⁺ concentration must be lowered compared with the formation water to promote ion exchange and mineral dissolution, resulting in a higher oil-recovery factor.
• HCO₃⁻ in the injected brine has detrimental effects on CO₂ LSWAG performance because it may lead to the precipitation of calcite and, consequently, a decrease of ion exchange and wettability alteration.
• An increase in the amount of calcite leads to an increase in the ultimate recovery factor by the CO₂ LSWAG process.

Conclusions

This paper presents a comprehensive evaluation of CO₂ LSWAG injection. It shows the CO₂ LSWAG process to be a promising EOR technique because it not only combines the benefits of gasflooding and LSW but also promotes a synergy between these processes through the interactions between geochemical reactions associated with CO₂ injection, the ion-exchange process, and wettability alteration. The CO₂ LSWAG process overcomes the late-production problem that is encountered frequently in conventional WAG injection. The CO₂ LSWAG process provides an incremental oil recovery of 4.5–9% of OOIP compared with CO₂ HSWAG injection. The success of the former depends on (1) the type and quantity of clay, (2) initial reservoir wettability condition, (3) reservoir heterogeneity, (4) the presence of reservoir minerals such as calcite and dolomite, (5) the composition of formation water and injected brine, (6) reservoir pressure and temperature for achieving CO₂-miscible conditions, and (7) WAG parameters.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 169071, “CO₂ Low-Salinity Water Alternating Gas: A New Promising Approach for Enhanced Oil Recovery,” by Cuong T.Q. Dang, SPE, University of Calgary; Long X. Nghiem, SPE, Computer Modelling Group; Zhangxin Chen and Ngoc T.B. Nguyen, SPE, University of Calgary; and Quoc P. Nguyen, SPE, The University of Texas at Austin, prepared for the 2014 SPE Improved Oil Recovery Symposium, Tulsa, 12–16 April. The paper has not been peer reviewed.