The Effect of Oil on Foam Stability During Foam Flow in Pipes a report by Sneha Panchadhara,1 Rahul N Gajbhiye,1 Gyu S Lee2 and Seung I Kam1 1. Craft & Hawkins Department of Petroleum Engineering, Louisiana State University; 2. Rural Research Institute
Foam has been used widely in many engineering applications. This article is devoted to understanding the change in foam properties during shear flow in pipes, which has significant implications for foam-assisted underbalanced drilling (UBD).
Underbalanced Drilling
UBD refers to a drilling process where the net pressure exerted by the circulating drilling fluid in the annular space between the drill string and the formation is less than the effective pore pressure in the formation adjacent to the wellbore.1
UBD offers various advantages
such as reducing lost circulation, minimising formation damage, increasing penetration rate and avoiding differential sticking.
The concept of UBD was first patented in the US in 1866. In the early 1950s, air drilling was applied to the oil field and by the mid-1950s was established as an effective drilling method, providing high penetration rates and thereby reducing the cost per foot of drilling.2
Research
was diverted towards finding the volume requirements of gas or air for cleaning the hole, as shown by RR Angel.3
With the advent of air drilling came problems associated with it. The influx of formation water into the hole affected the high drilling rates, also the high annular velocities resulted in hole enlargement. This led to a decline in using this technique for drilling.
In the 1960s, the US atomic energy initiated the study of foam as a drilling fluid to clean wellbores with large diameter. Large wellbore diameters require large volumes of circulating fluid for solid cutting removal hence there was a need for a circulating fluid with low density to effectively remove the cuttings without causing formation damage.2 Foam is one such fluid which has the properties of low density and high viscosity which is proven to be an effective drilling fluid. With its relative success, the use of foam became popular and is being used worldwide for drilling operations.
Foam Rheology
Foam is characterised as a dispersion of gas phase in a continuous liquid phase. Due to its structure, foam offers many unique advantages such as – but not limited to – great lifting capacity for the solid cuttings, reduced lost circulation, higher drilling rates, reduction in the bottomhole pressure and minimising reservoir damage.
Although foam has been successfully applied in various oilfields, it is not easy to predict its rheology accurately. This is because of complicated interactions between bubbles, between gas and liquid phases and between fluids and pipe walls. Many aspects should be taken into consideration for reliable foam modelling, which includes foam quality, foam texture, liquid phase properties, foam generation method, history and profile of shear stress, pressure and temperature, presence of foreign
© TOUCH BRIEFINGS 2011
fluids and chemical additives, type and concentration of surfactant and wall slip effect. Many attempts have been made previously to characterise foam rheology. They roughly fall into three categories such as Bingham-plastic fluid,4
Power-law fluid5 and Herschel-Bulkley fluid.6
Figure 1 shows a schematic of UBD processes during which foam circulated from the drill pipe to the annulus comes in contact with fluids flowing from the formation. For proper execution of UBD, it is important to maintain the bottomhole pressure slightly lower than the formation pressure. This task can be quite challenging because the influx of formation fluids (such as brine, oil and gas) constantly changes the rheological properties of foams. Among them, a special emphasis should be made on the oil phase because foam rheology is very sensitive to the presence of oil.
Foam – Oil Interactions
Past studies have shown that the presence of oil can be detrimental to foams. Various researchers have studied foam–oil interactions. It has been observed that the sensitivity of foam to oil strongly depends on the type of surfactant being used,7
the type and the amount of oil present and the formation of pseudoemulsion.8
Foam stability in the presence of oil is often explained in terms of spreading and entering coefficients. For oil to destabilise foams, it should enter the interface between gas and water (i.e. the entering coefficient is positive) and spread at the interface (i.e. the spreading coefficient is positive). The entering and spreading coefficients are defined as:
E = σwg S = σwg - σog - σog + σwo - σwo
(1) (2)
E = entering coefficient; S = spreading coefficient; σwg = interfacial tension at gas-water interface; σog = interfacial tension at gas-oil interface; and σwo = interfacial tension at water-oil interface. From equations one and two, it is obvious that if S>0, then E>0.
Previous studies have already quantified the values of spreading and entering coefficients by measuring the interfacial tension of various surfactant/oil systems.9
Lamella number (L) is another method used to predict foam stability in presence of oil,10
which is given by:
L = σwg x ro σwo x rp
(3)
L = lamella number; σwg = interfacial tension at gas-water interface; σwo=interfacial tension at oil-water interface; ro = radius of oil droplet; and rp = radius of curvature at the plateau border of an aqueous film.
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Subsea & Pipelines
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