What is the difference between over balance and under balance

Finally, it can be concluded that UBD provides lower drilling costs, increased rate of penetration ROP and less hole problems when it was compared to overbalanced drilling of the same formations. Figure 3. The main purpose of drilling vertical or horizontal wells is to produce maximum recoverable oil at minimum cost. Traditionally, wells have been drilled overbalance with the hydrostatic pressure of the drilling fluid greater than the pressure in the formation. Unfortunately, drilled wells with overbalance are subject to formation damage with varying degree of severity, depending upon the drilling conditions.

This is mainly due to the invasion of drilling fluids during drilling operations. The formation impairment or skin due to drilling fluid invasion around the well bore reduces well productivity. This is especially true for wells completed either open hole or with slotted liner due to the difficulty to undertake any stimulation job to alleviate formation damage problems.

Jilani et al. Nowadays, horizontal wells are preferred over vertical wells because they offer a net productivity enhancement as well as an increase of the contact area with the reservoir. However, formation damage is more critical to horizontal wells because these wells have such long exposed interval that stimulation jobs are not efficient and feasible.

Drilling operation is considered as the primary initiator of formation impairment as virgin formation comes first time in contact with a foreign fluid, i. Although drilling fluids are presently being designed in such a way to minimize solid and fluid invasion into the formation. The primary question asked by an operator relates to the benefit of extreme overbalance EOB versus current perforation techniques of underbalance UB or modest overbalance for well control. Of course this question spawns a whole set of new questions about when and how to use and the benefits and risks of the EOB.

EOB, as used in this paper, includes extreme overbalance perforating and rapid overpressured perforation extension. This paper compares and contrasts UB and EOB strengths and weaknesses and provides recommendations of when and why to use the different techniques.

Field examples are provided of successful and unsuccessful application of EOB completions. Perforating, fracturing and acidizing are simply a means of providing and enhancing flow communication between the reservoir and the wellbore. The objective of these different completion operations is to cost effectively maximize the producibility of the well. Perforating is used to provide the basic fluid communication between the reservoir and the wellbore.

Underbalance perforating and acidizing are typically used to bypass or eliminate near wellbore damage. Fracturing is generally used to expose a larger reservoir flow area to the wellbore. Use of propellant tools and now extreme overbalance EOB produce short fractures and thus complements underbalance UB perforating and acidizing to bypass wellbore damage.

However, the process of perforating also creates a permeability damaged region around the perforation commonly called the "crushed zone. Since perforating is the primary means to provide the initial fluid communication with the reservoir and where applicable is performed underbalance, the relevant issue is when-where-how to use EOB as an addition to perforating.

Several field examples will then be discussed. Finally, based on the physics of EOB, recommendations of applicability will be made. One word of caution: the following discussion on underbalance perforating does not apply to unconsolidated or very weak rock because of the potential failure of the rock from a high underbalance.

The primary purpose of underbalance perforating is to minimize perforation damage. However, there are two different types of perforation damage. The primary one is a damage region that surrounds the perforation tunnel having reduced permeability. This damaged region has historically been called the crushed zone.

This damage is a result of fractured sand grains reducing the size of the pore throats. The other, and normally ignored, perforation damage is caused by residual comminuted sand not removed from the perforation tunnel. This material, except at the very end of the perforation tunnel, typically has high permeability and porosities and does not impede production.

However, it can restrict injectivity and require higher injection pressures. The amount of pressure or force per unit area in the wellbore that exceeds the pressure of fluids in the formation. This excess pressure is needed to prevent reservoir fluids oil, gas, water from entering the wellbore. However, excessive overbalance can dramatically slow the drilling process by effectively strengthening the near-wellbore rock and limiting removal of drilled cuttings under the bit.

In addition, high overbalance pressures coupled with poor mud properties can cause differential sticking problems. Because reservoir pressures vary from one formation to another, while the mud is relatively constant density, overbalance varies from one zone to another.