Comparative Study Between Overbalanced And Underbalanced Oil Drilling Methods

Introduction

1.1 Background Study

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. (S.Z. 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. (S.Z. Jilani et al.)

Drilling operation is considered as the primary initiator of formation impairment as virgin formation comes first time in contact with a foreign fluid, i.e. drilling mud, which invades the formation and plugs the pores around the well bore. Although drilling fluids are presently being designed in such a way to minimize solid and fluid invasion into the formation.

In this project we would be taking a look at some comparative analysis done between two drilling operational techniques; the conventional Overbalance drilling technique (OBD) and the Underbalance drilling technique (UBD).

1.2 Overbalance Drilling Operation:

Many definitions have been laid down for a conventional overbalance drilling operation usually referred to as conventional drilling. These definitions categorize the drilling fluid design used for an overbalance drilling operation.

The amount of pressure (or force per unit area) in the wellbore that exceeds the pressure of fluids in the formation is also considered as overbalance. 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 conventional overbalanced drilling operations, the hydrostatic pressure exerted by the drilling fluid in the well is designed to exceed the pressure of the hydrocarbon fluids in the reservoir. Since the pressure in the hole is higher than the pressure in the rock fluid therefore drilling fluid can lose into the formation. These losses cause damage to the near well-wall area resulting in reduced production, experience show that even a short exposure to overbalanced conditions can severely impair the productivity of the well.

In addition, high overbalance pressures coupled with poor mud properties can cause differential sticking problems in which the drill pipe is pressed against the wellbore wall and as a result the pipe becomes stuck to the wall. Because reservoir pressures vary from one formation to another, while the mud is relatively constant density, overbalance varies from one zone to another. (Schlumberger Oilfield Glossary, 2014)

Figure 1.1 illustrates an overbalance drilling operation. As we can see, the drilling fluid being circulated plugs the pores causing drilling fines to migrate and invade the formation.

A simple equation to illustrate this condition of drilling can be given as:

Pm ˃ Pf

Figure 1.1 : Conventional overbalance drilling (available in the complete material)

1.3 Underbalance Drilling Operation:

The growing number of depleted reservoirs around the world and the increasing necessity to recover hydrocarbons more efficiently has been forcing the oil and gas industry to continuously improve its drilling technology. Currently, the combination of drilling techniques that were conceptualized more than 100 years ago, have with recent technological innovations ended up in specialized drilling techniques. These techniques, when properly designed and executed, allow drilling a well more economically, safely, and successfully in almost any given environment. One such technique is called underbalanced drilling (UBD).

Underbalanced drilling, or UBD, is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. As the well is being drilled, formation fluid flows into the wellbore and up to the surface. This is the opposite of the usual situation, where the wellbore is kept at a pressure above the formation to prevent formation fluid entering the well. In such a conventional “overbalanced” well, the invasion of fluid is considered a kick, and if the well is not shut-in it can lead to a blowout, a dangerous situation. In underbalanced drilling, however, there is a “rotating head” at the surface – essentially a seal that diverts produced fluids to a separator while allowing the drill string to continue rotating guaranteeing safety. (Wikipedia.org)

Though not as common as overbalanced drilling, underbalanced drilling is achieved when the pressure exerted on the well is less than or equal to that of the reservoir. Performed with a light-weight drilling mud that applies less pressure than formation pressure, underbalanced drilling prevents formation damage that can occur during conventional or overbalanced drilling processes.

The negative differential pressure obtained during underbalanced drilling between the reservoir and the wellbore encourages production of formation fluids and gases. In contrast to conventional drilling, flow from the reservoir is driven into the wellbore during underbalanced drilling, rather than away from it.

Figure 1.2 describes an underbalance drilling operation showing less hydrostatic drilling fluid being circulated and we can see flow from the reservoir as well as formation fines into the wellbore as drilling operation is carried out.

A simple equation to illustrate the condition of drilling can be given as:

Pf > Pm

Figure 1.2 : Underbalanced Drilling (available in the complete material)

Although initially more costly, underbalanced drilling, also known as managed-pressure drilling, reduces common conventional drilling problems, such as lost circulation, differential sticking, minimal drilling rates and formation damage. Additionally, underbalanced drilling extends the life of the drill bit because the drilling gases cool the bit while quickly removing cuttings.

To establish pressure control, a rotating control head with a rotating inner seal assembly is used in conjunction with the rotating table. An important factor to successful underbalanced drilling, drilling and completion operations must remain underbalanced at all times during operations. To accomplish this, pre-planning and onsite engineering are critical to the success of underbalanced drilling procedures.

Typically used for only a section of the entire drilling process, underbalanced drilling cannot be used in most shale environments.

1.4 Underbalance Gases

Gases used for underbalance include air, nitrogen and natural gas. Although it is not typical, if natural gas is recovered from the well, it can be reinjected into the well to establish underbalance, resulting in the most cost-effective solution for underbalanced drilling.

Commonly used in underbalance operations, nitrogen is preferred for its somewhat low cost of generation, scale of control and minimal potential for downhole fires. While pure nitrogen can be purchased, it is cost-prohibitive. Therefore, nitrogen is more commonly produced onsite with a membrane unit, resulting in a 95% level of purity.

1.5 Underbalance Techniques

There are four main techniques to achieve underbalance, including using lightweight drilling fluids, gas injection down the drill pipe, gas injection through a parasite string and foam injection.

Using lightweight drilling fluids, such as fresh water, diesel and lease crude, is the simplest way to reduce wellbore pressure.

The method of injecting gas down the drillpipe involves adding air or nitrogen to the drilling fluid that is pumped directly down the drillpipe. Advantages to this technique include improved penetration, decreased amount of gas required, and that the wellbore does not have to be designed specifically for underbalanced drilling.

In performing the gas injection via parasite string, a second pipe is run outside of the intermediate casing. While the cost of drilling increases, as does the time it takes, this technique applies constant bottom hole pressure and requires no operational differences or unique MWD systems. Figure 1.3 shows a pictorial representation of the above procedure.

Figure 1.3 : Gas injection via parasite string (available in the complete material)

A less common underbalanced application, nitrogen foam is less damaging to reserves that exhibit water sensitivities. While the margin of safety is increased using foams, the additional nitrogen needed to generate stable foam makes this technique cost prohibitive. Additionally, there are temperature limits to using foam in underbalanced drilling, limiting using the technique to wells measuring less than 12,000 feet deep.

1.6 Statement of Problem

Due to the growing technologies associated with drilling operations, it is pertinent to look for affordable, convenient and more productive alternatives. In this project, we would analyze both drilling techniques and see its effect on various formations to be able to make a better choice for each drilling scenario.

1.7 Aims and Objectives

In view of the comparative analysis of overbalance and underbalance drilling operations in the Oil and Gas industry, this project work seeks to investigate the following:

To have an understanding of both the underbalance and overbalance drilling operations.

To know reservoirs suitable for either underbalance or overbalance drilling operations.

To also know the various effects and challenges associated with these operations.

1.8 Scope and Limitation

The scope of this study is to comparatively analyze underbalance and overbalance drilling operations with emphasis on reservoir types.

1.9 Methodology

In the course of the project, vital information was obtained from the library, review of related literatures, internet surfing, textbooks, petroleum journals and these formed the basis for analysis of both underbalance and overbalance drilling operations.