Aphrons that are known as colloidal dispersed micro-bubbles (with size distribution between 10–100 microns), usually consist of a spherical core with an internal phase covered by a thin shell. If the core has a gas phase, the considered structure is known as colloidal gas aphron. In order to use aphrons as drilling fluids, they require a certain degree of stability. For this reason, the Aphron shell must have enough thickness (between 4 to 10 microns), otherwise it cannot bear the pressure, also it is better that protective layer has lower viscosity. Aphron stability is affected by the mass transfer rate between the viscous water shell and the bulk phase. This phenomenon is described as Marangoni effect (mass transfer along an interface between two fluids due to a gradient of the surface tension). As a result of this effect, water (or Aphron’s core gas) molecules tend to diffuse out from the Aphron shell into bulk liquid, which is lead to destabilize the Aphron [
1]. Generally, Aphron consists of gas, water, certain polymers and surfactants to make a micro-bubble suspension. The polymer is used to increase the viscosity of water and a surfactant is required to generate micro-bubbles by reducing surface tension of the base fluid. Variety of scientists have contributed to a deeper insight into the performance of Aphrons and the controlling parameters. Save and Pangarkar [
2] studied the effect of viscosity on CGA based fluids stability half-time and drain rate (the rate of drained liquid from CGA dispersion). Due to the difference between density values of the bubbles and liquid, the bubbles tend to move upward and the liquid is drained downward. The drain rate refers to the rate of liquid, which is separated from the Aphron fluid. They observed that any increase in viscosity increases half-time of the CGA fluid sample due to drain rate reduction in presence of increased viscosity. Jauregi and co-workers [
3] studied the CGA stability in terms of the surfactant concentration. They concluded that, as surfactant concentration increases, CGA stability increases. Brookey [
4] used a Xanthan gum (XG) biopolymer as the best stabilizer of CGAs and enhancer of the low shear rate viscosity (LSRV) that has a positive impact on wellbore cleaning, cuttings suspension and invasion control. An excellent drilling cutting suspension reduces the wellbore erosion especially in highly deviated and horizontal wells; this excellent cutting suspension is achievable by elevating low shear rate viscosity value. Ramirez et al. [
5] proposed non-ionic polymers to increase viscosity at low shear rates. Growcock [
6] stated that clay/polymer blends can be used as viscosifier. Bjorndalen and Kuru [
7] proposed the XG biopolymer to stabilize aphron and also mentioned that viscosity has the most impact on Aphrons stability. Keshavarzi et al. [
8] used three surfactants SDS, Triton X-100 and CAPB for investigation the formation and stability of CGAs. Their study reported that, CGAs made by SDS and CAPB surfactants have significant stabilities in contrast to the poor stability of Triton X-100. Huang et al. [
9] investigated the fluid loss of surfactant viscoelastic fluids in different temperatures and permeabilities. Their results revealed that a filter cake was created on the porous media that caused the reduction of fluid loss. Arabloo et al. [
10] studied CGA fluid properties by using a mixture of surfactant and xanthan gum biopolymer. Their study reported that, as the polymer and surfactant concentration increases, the yield point, plastic viscosity (which will be named in this paper as viscosity) and apparent viscosity of CGA based drilling fluids increase. Tabzar et al. [
11] investigated various bio-polymers including Carboxy Methyl Cellulose (CMC), Starch and Xanthan Gum and two types of surfactant, namely Cetyl Trimethyl Ammonium Bromide (CTAB) and Sodium Dodecyl Sulphate (SDS) to make aqueous CGA drilling fluids. The outputs of their research showed that a mixture of XG and SDS has a great performance in decreasing filtration loss and improvement of stability of CGAs. All the mentioned works have been performed on the water based CGAs, which is not proposed to be used in high temperature and shale formations. In such formations the oil based Aphrons will have better efficiency. Unfortunately, there is a lack of study on oil based CGAs in literature. Only two works of Growcock et al. [
12] and Alizadeh and Khamehchi [
13] are available in literature related to oil based CGAs. Growcock et al. [
12] introduced the oil based Aphron fluids and investigated its application in drilling fluids. Alizadeh and Khamehchi [
13] experimentally investigate the oil based CGAs stability. They used different kinds of vegetable oil in their study due to environmental consideration. It should be mentioned that, the oil based Aphron is suitable for shale formation drilling because it does not cause shale instability; also, oil-based fluids are good lubricants, which reduces the drilling torque.
Another important parameter, which controls the behaviour of CGA based drilling fluids, is the bubble size and size distribution of CGAs. Parthasarathy et al. [
14] estimated the average bubble diameter of CGAs based on Kolmogorov’s theory. They planned to experimentally study the small bubbles created by gas dispersion mechanism in presence of a ventilated cavity attached to a cylindrical vessel. Therefore, they needed to predict the size of bubble before the experiment for optimizing the experimental parameters and count, for reducing cost and time. In this state, they used Kolmogorov’s theory of turbulence to predict the size of bubbles. As the result of the study, the predicted diameters generally were in 20% of their measurements. Roy and co-workers [
15] applied a Particle Size Analyser to estimate the bubble size distribution of CGAs. Their instrument projected a laser beam through a transparent cell containing a stream of moving particles suspended in the liquid. Majority of CGAs produced from HTAB had a mean diameter of 125 microns and the CGA bubbles from the SDBS surfactant had a mean diameter of 88 microns. Dai et al. [
16] employed optical microscopy and image analysis to determine the size of the generated CGAs. The mean diameter of 55.02 microns was reported in their study. Also, they investigated the effect of PH on the stabilization and size of CGAs. Arabloo et al. [
10] employed a charged coupled device camera and a light microscope to investigate the size distribution of CGAs of different aphronized drilling fluids. Alizadeh and Khamehchi [
17,
18,
19,
20,
21,
22,
23,
24] proposed a new and novel diffusion model for CGA fluids including the mass transfer, which is described as diffusional phenomena in the presence of convective motion. In this model the mass transfer resistance for all phases and interfaces of a bubble was taken into account. Result of their study illustrated that the bubble sizes decrease and stability increases until a threshold value in case of increasing surfactant concentration in the base fluid. This threshold value varies with base fluid type, pressure, temperature and surfactant type. In their researches, a first-order differential model was used for liquid drainage modelling from the dispersion, while bubble size distribution was predicted using a population balance model. Moreover, the result shows that increasing the surfactant concentration usually increases the stability of the produced micro bubble fluid, due to the increase of viscosity, yield point, apparent viscosity and gel strength. As polymer and surfactants concentration increase, the viscosity of the base fluid and the bubble population increases respectively, which will increase the overall viscosity of the CGA drilling fluid.
In the current study, the significant focus is to investigate the effect of salinity, type of polymer and surfactant and also their concentrations on stability, size distribution, rheological properties and filtration loss of water based CGA drilling fluid. Taguchi design of the experiment is designed for understanding the effect of mentioned parameters on the stability of the CGA fluids.
Section 2 will describe the methodology. Then the results will be presented and discussed in
Section 3. Finally, we will end this research with main concluding remarks.