The Pros And Cons Of Underbalance Drilling

Over the past number of years, the most common and well- known method used to drill wells was the Overbalance Drilling method (OVD). Overbalance Drilling is a technique where the bottom hole pressure of the wellbore is kept higher than the formation pressure while performing the drilling process. To maintain an overbalance atmosphere, the mud weight in the drilling fluid needs to be heavier and denser. It is designed to just be above the formation fluid pressure but lower compare to the formation fracture pressure (Akdeniz, 2012). However, everything has its cons. There are a lot of problems and challenges faced while performing this method. The invasion of high oil and water saturations followed by solids from the formation have significantly

The question still remains whether Underbalance really is the best choice or otherwise just another method with flaws and disadvantages. Therefore, the Underbalanced upside can be seen through three main aspects which are the avoidance of formation damage, the increase in productivity and the amount of cost saved while performing this method.

The main and most important reason why Underbalanced Drilling was needed is to prevent and minimize the reservoir damage. Reservoir damage in this context means the damage done through the invasion of drilling fluid into the reservoir rock. This happens due to the high pressure of overbalanced drilling in the wellbore as it forces the drilling fluid into the pores of reservoir rocks that are being drilled. The drilling fluid of overbalanced drilling is a mixture of high-density mud content and water. Due to the differential pressure gradient, a stream of fluid which is a mixture of water with small suspended solid and mud propagate into the rocks. Subsequently, the mixture will form a barrier and eventually block the oil and gas flow from entering the wellbore. The term used to explain this phenomenon is called phase trapping or blockage (Bennion D. B. & Thomas,

In terms of drilling fluid, underbalance uses a way cheaper, lighter and more affordable fluid such as natural gas, water, light mud and even oxygen content air. In some certain extend where discoveries has been done, foam is used as the filtrate. With the use of foams lately, drilling bits life could be extend and it saves the financials of one’s drilling operation ( Ostroot, Shayegi, Zoontjes & Lovorn, 2007; Mitchell, 2007). Furthermore, as the underbalance condition is applied in the wellbore, the amount of mud and drilling fluid loss has been reduce dramatically. This will also reduce the skin factor and likewise, the cost disposal for cleaning processes of casings and pipes has also decline ( Ostroot, et al. 2007; Bennion D. B. , Thomas, Bietz & Bennion D. W. , 1998; Bennion, D. B., Lunan & Saponja, 1998; Mitchell, 2007; Akdeniz, 2012 ; Bennion D. B. & Thomas, 1994). On the other side of this discussion, overbalance drilling has a very high differential sticking rate occurring in the well. Differential sticking takes place when the drilling filtrate left the wellbore and forms an impermeable barrier or in its petroleum term, a cake. With this, a filter cake is form and it continues to build until the rotation of the drill pipe is disturb. Including in the overburden pressure of the wellbore combined with the lack of liquid in the drilling fluid causes the