CHAPTER ONE

BACKGROUND OF STUDY

1.0 Introduction

A drilling fluid, or mud is any fluid that is used in a drilling operation in which that fluid is circulated or pumped from the surface, down the drill string, through the bit, and back to the surface via the annulus. (Growcock et al., 2005).The successful and cost of a drilling process is known to depend extensively on the asset of the drilling fluid used (Gray et al., 1980). Drilling mud circulates in a loop, from the platform, where it is forced down into the formation by entering the drill string, and pushed up to the surface again via the drill bit. The fluid characteristics such as density and temperature are variables that need to be regularly monitored for perfect drilling conditions of the well (Issham et al. 1985). They provide primary well control of subsurface pressures by a combination of density and any additional pressure acting on the fluid column (annular or surface imposed). They are most often circulated down the drill string, out the bit and back up the annulus to the surface so that drill cuttings are removed from the wellbore. Drilling fluids have a number of alternative names, acronyms and slang terms used within the industry. The most widely used name is “mud” or “drilling mud” and both these terms will be used interchangeably throughout this chapter. Other drilling fluid names and acronyms are: water-based mud (WBM), oil-based mud (OBM), synthetic-based mud (SBM), non-aqueous fluid (NAF), invert emulsion fluid (IEF), high performance water-based mud (HPWBM), drill-in fluid (DIF) and reservoir drilling fluid (RDF). Similar to drilling fluids are so-called completion fluids that are used to finish the well after drilling is completed. The fluids used during completions are often referred to as work over and completion (WOC) fluids, clear brines and/or packer fluids. Drilling fluid is a major factor in the success of the drilling program and deserves careful study.

Figure 1.0: Drilling fluids stored in the mud tank under agitation.

The performance of the drilling fluid is critical to everyone involved with the operation and to all aspects of the drilling operation. The drilling fluid is the primary means to keep the well from blowing out and it is responsible for keeping the hole in good condition such that drilling operations can continue to the desired depth. Drilling and completion fluids are one of the most important parts of the well construction process and ultimately the performance of the fluid will determine the success or failure of the operation. The responsibility of the proper selection and application of fluid is held jointly between the fluids supplier, the drilling contractor and the operator.

The research work focused on the use of Nigeria local materials in enhancing the drilling mud pH, its performance and contribution as drilling fluid properties and ensuring quality in hole making. Drilling fluid pH measurements and pH adjustments are fundamental to drilling fluid control because clay interactions, solubility of additives, and contaminant removal are all pH-dependent.

PH is a value representing the hydrogen ion concentration in liquid and it is used to indicate acidity or alkalinity of drilling mud. The pH is presented in a numerical value (0 – 14), which means an inverse measurement of hydrogen concentration in the fluid.

The pH formula is listed below;

pH = - log 10 [H]

Where: H is the hydrogen ion concentration in mol.

According to the pH formula, the more hydrogen atoms present, the more acidity of substance is but the pH value decreases. Generally speaking, a pH of 7 means neutral. Fluids with a pH above 7 are considered as being alkaline. On the other hand, the fluids with pH below 7 are defined as being acidic.

In the drilling mud, there are three main chemical components involved in Alkalinity of drilling fluid, which are bicarbonate ions (HCO3--), hydroxyl ions (OH --), and carbonate ions (CO3-2). The Alkalinity means ions that will reduce the acidity.

In order to get accurate measurements for the pH, using a pH meter instead of using a pH a paper is recommended because it will give more accurate pH figures. Additionally, pH meters must be calibrated frequently.

1.1              Drilling Fluid Classifications

             Drilling fluids are separated into three major classifications (Figure 1):

1.      Pneumatic

2.      Oil-Based

3.      Water-Based

Figure 1.1: Drilling Fluids Classification (Source: Drilling Mud Rheology and the API Recommended Measurements)

1.1.1    Pneumatic Fluids

Pneumatic (air/gas based) fluids are used for drilling depleted zones or areas where abnormally low formation pressures may be encountered. An advantage of pneumatic fluids over liquid mud systems can be seen in increased penetration rates. Cuttings are literally blown off the cutting surface ahead of the bit as a result of the considerable pressure differential. The high pressure differential also allows formation fluids from permeable zones to flow into the wellbore. Air/gas based fluids are ineffective in areas where large volumes of formation fluids are encountered. A large influx of formation fluids requires converting the pneumatic fluid to a liquid-based system. As a result, the chances of losing circulation or damaging a productive zone are greatly increased. Another consideration when selecting pneumatic fluids is well depth. They are not recommended for wells below about 10,000 ft because the volume of air required to lift cuttings from the bottom of the hole can become greater than the surface equipment can deliver.

1.1.2    Oil-Based Fluids

A primary use of oil-based fluids is to drill troublesome shales and to improve hole stability. They are also applicable in drilling highly deviated holes because of their high degree of lubricity and ability to prevent hydration of clays. They may also be selected for special applications such as high temperature / high pressure wells, minimizing formation damage, and native-state coring. Another reason for choosing oil-based fluids is that they are resistant to contaminants such as anhydrite, salt, and CO2 and H2S acid gases.

Cost is a major concern when selecting oil-based mud. Initially, the cost per barrel of an oil-based mud is very high compared to a conventional water-based mud system. However, because oil mud can be reconditioned and reused, the costs on a multi-well program may be comparable to using water – based fluids. Also, buy-back policies for used oil-based mud can make them an attractive alternative in situations where the use of water-based muds prohibits the successful drilling and/or completion of a well. Today, with increasing environmental concerns, the use of oil-based muds is either prohibited or severely restricted in many areas. In some areas, drilling with oil-based fluids requires mud and cuttings to be contained and hauled to an approved disposal site. The costs of containment, hauling, and disposal can greatly increase the cost of using oil-based fluids.    

1.1.3    Water-Based Fluids

Water based fluids are the most extensively used drilling fluids. They are generally easy to build, inexpensive to maintain, and can be formulated to overcome most drilling problems. In order to better understand the broad spectrum of water-based fluids, they are divided into three major sub - classifications:

1.      Inhibitive

2.      Non-inhibitive

3.      Polymer

Figure 1.2: Classification of Water-Based Fluids (Source: Drilling Mud Rheology and the API Recommended Measurements)

1.      Non-Inhibitive Fluids

Those which do not significantly suppress clay swelling, are generally comprised of native clays or commercial bentonites with some caustic soda or lime. They may also contain deflocculants and/or dispersants such as: lignite, lignosulfonate, or phosphates. Non-inhibitive fluids are generally used as spud muds. Native solids are allowed to disperse into the system until rheological properties can no longer be controlled by water dilution.

2.      Inhibitive Fluids

Those which appreciably retard clay swelling and, achieve inhibition through the presence of cations; typically, Sodium (Na+), Calcium (Ca+2) and Potassium (K+). Generally, K+ or Ca+2, or a combination of the two, provide the greatest inhibition to clay dispersion. These systems are generally used for drilli

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