Alteration wettability of carbonate rocks by Nano fluids and comparison with PEG and SDS

This work was conducted to study limestone rock wettability alteration to enhance oil recovery by flooding using different Nano silica (NS) sizes suspended in saline water and compared with flooding solution of polyethylene glycol (PEG) polymer and sodium dodecyl sulfate (SDS) surfactant, the stability of nanofluids measured by zeta potential. In the flooding system, the secondary recovery by silica nanofluids (0.01 wt. % NS concentration) achieved an oil recovery of 35vol. % and 26.08 vol. % for 10, 52 nm after primary recovery respectively, while PEG polymer and SDS surfactant achieved oil recovery of 5 vol. % and 10 vol. % only respectively. The stability of pressure difference approved that silica nanoparticle never causes any plug or damage for the carbonate rocks. عم ةنراقملاو ةيونانلا لئاوسلا ةطساوب ةينوبراكلا روخصلا للب ةيلباق رييغت لوكيلاك نيليثا يلوبلا مويدوص تاتيربك ليسيدودو


Introduction:
One of the major problems faced in the petroleum industry is the depletion of the oil reservoir. After exhaustion of reservoir natural energy, a large part of industry technology was focused on recovering more oil from that remained in the reservoir layer. During the past five decades, EOR (enhanced oil recovery) methods have been progressed and applied to mature and mostly trapped oil reservoirs to increase the efficiency of oil production by retrieving a large part of crude from the reservoirs after primary and secondary recovery processes [1].
Reduction in oil production in carbonate reservoir caused due to reservoir heterogeneity (different permeability zones, channels, and fractures) [2] . Water production due to fingering deep in the reservoir during the sweep process caused due to density and Viscosity differences between injected and natural fluids [4]. Knowledge of the wettability of carbonate reservoir rock is of utmost importance to researchers, where it is fundamental to the understanding of fluid flow through porous media and can affect the production characteristics greatly during water flooding [3].
In Iraq, Mishrif Formation characterized by a high degree of heterogeneity (Porosity of the formation is >22%, and permeability (23 to 775 md). Originally described as organic detrital carbonate with beds of algal, rudist, and coral-reef carbonate, capped by limonitic freshwater limestone [4].
Though the amount of crude oil available has to meet the worldwide demands, crude oil is considered a limited resource which led to serious oil crises accompanied by a general increase in the oil price caused by international reduction of oil production. So, the oil industry was forced to recover oil from complicated areas in which the oil is less attainable, resulting in the development of techniques for enhancing oil recovery (EOR) constantly [5].
Chemical EOR method holds promise for future improvements in oil production, especially in mature and water flooded fields. Chemical EOR classified into a polymer, surfactant, and alkaline agents; in addition, combinations of the three categories such as alkali-polymer, No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS) E56 alkali-surfactant polymer, and surfactant-polymer [6]. Chemical injection is qualitative fluid injection which perfectly changes phase conduct properties to increase oil production.
Decreasing the interfacial tension of oil /water is the domain of chemical processes. The surfactant used to reduce interfacial tension and polymers are controlled by the mobility of the surfactant solution.
The polymer is used to minimize the relative permeability to water more than to oil. So, the water production controlled without influences on oil production rate. While in sandstone reservoirs polymers injection have been the most applied enhanced oil recovery chemical method [30]. The most important preconditions for using polymer flooding are the formation of water chemical properties and reservoir temperature where the concentration of polymer will lose most of its property at high temperature or /and high saline of formation water in the reservoir, in addition, the polymer cannot be kept stable [7].
Hydrophilic nature of polymer and tendency to flow through the pore wall this makes fingerings during the polymer flooding in an oil-wet medium [8].
Many researchers have attracted attention in Nanoparticles material application in the EOR field because of their capability to change the wettability of the carbonate rock and its ability to decrease the IFT between oil and injection water. Furthermore, increasing the mobility of the trapped oi [9].
Nano-particles has the ability to travel easily through a reservoir; push the residual crude oil in the small pores that unrecovered in polymer injection. Despite continuous fluid bulks, dispersed particles can hit the porous media wall and remove the oil on the wall. Due to NPs small size, they do not aggregate, agglomerate to larger structures or adsorb onto the rock surface compared with reservoir channel that in micrometers size [10].

Experiment Work:
The experiments of this research were carried out at the laboratories of the petroleum research and development center. The chemicals and the analytical test instruments are shown in Table1 and Table (

Preparation of Nano silica and Nano fluid:
The Nano silica was prepared from Iraqi local sand according to the procedure of previous work [11]. The average particle size and surface area of prepared silica were 52 nm and 474.9429 m²/g respectively. Nanofluid prepared in two steps. First, a certain amount of Nano silica added to achieve 0.01 wt. % to a base fluid prepared according to the Iraqi reservoir injection water specification as shown in Table 3 and kept under stirring. Second, suspended silica was subjected to sonication using an ultrasonic mixer for 15 min at high energy to avoid agglomeration of particles.

Polymer and surfactant solutions:
Polyethylene glycol and sodium dodecyl sulfate (SDS) solutions were prepared by dissolving under mixing condition in the same base fluid above to obtain a solution concentration from (0.01 wt. %) for polymer and respectively. The same procedure repeated for surfactant with concentration 0.01wt. %

Core flooding process:
Five of medium permeable core samples used with properties shown in Table ( (1) in petroleum research and development center PRDC that used for the core flooding. This piece of machinery can be used to study in a core flood oil recovery in a core sample.

Fig. (2) Scheme of Flooding Process
Operation procedure: Saline water (30cm 3 ) injected with flow rate 0.5ml/min as primary recovery. The pressure difference, injection pore volume, and water-oil recovered recorded with time for each run.
Then, the prepared nanofluid (30cm 3 ) injected as a secondary recovery at the same operating procedure of saline water. At the end of the run, distilled water was injected to clean pipes and core holder to prepare for the second run.

Stability of Nano fluid
The stability of Nanofluids was considered by measuring zeta potential. Figures (3-5) showed that the zeta potential values of NPs varied from -137.04, -137.32 and -141.8mV for 65nm, 52nm, 10nm respectively at pH 6.5. The high negative zeta potential value of NPs is Oil Recovery:

Nano fluid injection
A comparison between the recovery results in Figure (6) revealed that Nanofluid could increase an oil recovery after primary recovery processes to (35, and 26vol. %) for particles size 10, 52nm respectively, where the secondary injection submitted when the oil production during primary injection reached breakthrough point and the oil production stopped due to saline water cannot push more oil from small pores . This difference in recovery is due to the difference in surface area. High surface to volume ratio with high contact area enables a high diffusion rate, a mass transfer which can enhance the properties of the fluid [12].
The increase in incremental oil recovery with the decrease in particle size is in accordance with the adsorption of NPs that takes place because of the size of the particles could reduce the pressure through permeability reduction. Also, particles size could impact on dispersion ability, adsorption affinity, and catalytic activity of nanoparticles inside the medium [13].
Nano silica can change the wettability condition to water-wet and improves the oil recovery because one of the important mechanisms that have an effect on the oil recovery is wettability of the medium. In addition, reduction in the interfacial tension makes it easier for

No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS)
E63 oil droplets to move through pore throats by decreasing the work of deformation needed. A higher capillary number causes an increase in oil displacement efficiency. Furthermore, Nanofluid viscosity is higher than water. Therefore, the mobility of the injected fluid decreases and the sweep efficiency improves. [8].

Fig. (6) Oil Recovery Versus Injected Nano Fluid for Different Size of NPS
A comparison of results in the recent research was established with previous work. Where Table (5) showed that the oil recovery of recent research was more sufficient than other studies.  break down, and becoming ineffective. In addition, fractures and natural heterogeneity offer additional complexity to the process [15]. Nano-fluid pressure drop raised dramatically due to adsorption of Nano silica into the surface of the rock. According to recovery and pressure drop data, injection of 3 PV of Nano-fluid improves oil recovery without inducing any significant formation damage.

Fig. (9) Pressure Difference Versus Injected Fluid Volume for Different Fluid for Two
Step Injection

Conclusions:
Nano silica shows good stability in saline water that simulated to injection water of Iraqi reservoir. Core flood experiments reveal that Nano silica is good EOR agent and can produce a significant amount of oil above primary process 35vol. % and 26.08 vol. % for 10, 52nm Nano silica size respectively. While PEG polymer and SDS surfactant achieved oil recovery of 5 vol. % and 10 vol. % only respectively. The results of the pressure drop across the core sample show no plug or damage observed. This type of Nanoparticles has higher stability under dynamic condition, a low concentrations (0.01wt. %) of Nano silica fluids show a high ability to increase oil recovery from limestone cores through flooding system more than SDS and PEG in the same concentration.