Reamer

A tool used in drilling to smooth the wall of a well, enlarge the hole to the specified size, helps to stabilize the bit, straighten the wellbore if kinks or doglegs are encountered, and to drill directionally.

There are cost advantages to reaming while drilling. The pilot hole offers good directional control while the enlarged hole from reaming creates the right conditions for running casing or completion equipment which relates to savings in time and cost.

Types of Reamers

Under reamers

An under reamer is device designed to run in conjunction with a drill bit. It has cutters that can be expanded or contracted by mechanical or hydraulic means and used to enlarge or ream a borehole below the casing. Baker Hughes and Statoil jointly developed the industry’s first on-command digital reamer as a single size prototype in 2007. The prototype was exclusively used in the Norwegian and UK sectors of the North Sea. The new remotely controlled hydraulic-electric reamer was developed using hydraulic blade activation, which was controllable from the surface.

Roller reamers

Roller reamers significantly improve performance with chronic stick-slip or whirl-induced borehole which commonly limit drilling in hard formation and extended reach wells. Historically roller reamers are also used for conditioning the borehole

RFID reamers

An RFID reamer uses radio-frequency identification (RFID) technology to active the reamer. The electronically activated reamer provides increased flexibility for hole-enlargement-while-drilling (HEWD) operations. Weatherford’s Riptide RFID tags are inserted into the drill pipe ID at surface level and carried downhole in the drilling fluid. An electronic reader on the tool’s controller interprets instructions embedded in the tags to unlock the controller, thereby permitting the cutter blocks to extend fully from the reamer body. A surface-level pressure gauge confirms that the tool is in the open position. RFID features enables operators to activate and deactivate the tool at any time while drilling or tripping. Specific benefits RFID technology offers include:

·         Prevents activation of the cutter blocks during jarring operations, significantly reducing the risk of getting the reamer stuck in the hole and incurring non-productive time as a result;

·         Helps protect casing and the tool’s cutting structure from damage while cleaning the hole at full circulation and rotation;

·         Permits running of multiple drilling reamers in tandem, all of which can be independently controlled.

Near-bit reamers

Near Bit Reamer (NBR) can be used to drill oversized holes. It is designed to enlarge the hole while drilling. They have been used in both conventional mud motor and rotary steerable directional assembles.

Bi-centre reamer bit

A bi-centre bit is composed of a pilot bit and a reamer. The reamer extends from one side of the bit so that when the pilot hole is drilled the hole is immediately reamed to a larger diameter. 

Expandable reamers

Expandable reamers were developed to improve efficiencies and reduce risk while drilling through problematic formations in deep water and other offshore wells, where wellbore stability is a major challenge. The first concentric expandable reamers were ball-activated. Once expanded, they could not be closed without stopping circulation. The second iteration was modified so that the reamer could be deactivated to allow circulation after reaming for better hole cleaning. The advantages of the concentric expandable reamer over its mechanical-arm predecessors included the attainment of a larger borehole size that could accommodate larger bottom hole assembly (BHA) components, flexibility in pilot bit selection, higher flow rates, and the ability to ream a previously drilled hole or to back ream a hole after drilling. The drawbacks to traditional expandable under reaming systems. Activation cannot be confirmed. Additionally, the placement of the reamer at 100 to 300 ft. above the bit means that the equivalent portion of the hole, known as a rat hole, cannot be enlarged without a dedicated trip that typically requires 1 to 2 days of rig time at a daily cost ranging from USD 1 million to USD 2 million.

Matching bit and reamer

          Reamers and bits should be matched for cutter size and aggressiveness (depth of cut) to balance loads more closely while drilling through homogeneous formations. For example, if the drill bit out-drills the reamer, some inner formation stresses may have been released by the time the reamer starts drilling a new formation. In this situation, there is no WOB and the weight is transferred to the reamer, which can damage the cutters. Additionally, with no WOB, lateral and whirl vibrations can damage the lower BHA and quickly compromise wellbore stability. Matching the bit and reamer for aggressiveness can alleviate or eliminate these risks. 

References

1. Al-Essa Ahmed. Digital Reamer Enhances Drilling Efficiency, Economics, and Safety. Journal of Petroleum Engineering.

2. Sowers, S. F., Dupriest, F. E., Bailey, J. R., & Wang, L. 2009. Use of Roller Reamers Improves Drilling Performance in Wells Limited by Bit and Bottom hole Assembly Vibrations. Society of Petroleum Engineers.

by: Rathna Bai B

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Classification of Reservoir Using Phase Diagram

     A petroleum reservoir or oil and gas reservoir is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. Petroleum reservoirs are broadly classified as conventional and unconventional reservoirs. In case of conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability. While in unconventional reservoirs the rocks have high porosity and low permeability which keeps the hydrocarbons trapped in place, therefore not requiring a cap rock.

Types of reservoir:

         Based on phase diagram the reservoirs are classified into:

·        Single phase gas reservoir
·         Retrograde condensate – gas reservoir
·         Dissolved gas reservoir
·         Two phase gas reservoir

       In order to understand the phase diagram we need to understand some key concepts which includes:

Cricondentherm: It is defined as the maximum temperature above which liquid cannot be formed regardless of pressure.

Cricondenbar: It is defined as the maximum pressure above which no gas can be formed regardless of temperature.

Phase Envelope (two-phase region): The region enclosed by the bubble-point curve and the dew point curve wherein gas and liquid coexist in equilibrium.

Bubble-Point curve: The bubble-point curve is defined as the line separating the liquid-phase region from the two-phase diagram.

Dew-Point Curve: The Dew-point curve is defined as the line separating the vapor-phase region from the two-phase region.

Critical point: The point at which bubble point and dew point curve meets. The corresponding pressure and temperature at that point is said to be critical pressure and critical temperature.

Single phase gas reservoir:

·         If reservoir pressure and temperature are such that they:

          - lie outside the two phase region

          - lie on right hand side of the cricondentherm;

then the reservoir is a single gas phase reservoir.

·         During production the reservoir pressure drops and the reservoir temperature remain constant. So the reservoir fluid follows the path AA₁ during production.
·         Both temperature and pressure decline along the wellbore while producing to the surface, the reservoir fluid follows the path AA₂.
·         The path AA₂ crosses the two phase region, the produced fluid may be into two phases - gas or liquid. But the fluid composition will be the same.
·         Hence condensate liquid produced at the surface, but the reservoir fluid remains as single phase gas

Retrograde condensate – gas reservoir:

•If reservoir pressure and temperature are such that they:

    -lie outside the two phase region

    -lie on left hand side of the cricondentherm and right hand side of critical point.

then the reservoir is a Retrograde condensate gas reservoir.

·          The reservoir fluid is initially in single phase gas state. During production reservoir pressure drops thus, the reservoir fluid follows a path BB₁B₂B₃.
·         The reservoir fluid enter the two phase region at B₁.Below that pressure the liquid condenses out from the gas. The condensed liquid adheres at the walls of rock pores.
·         The liquid is immobile until certain saturation is achieved. The gas produced at the surface will have lower liquid content thus producing gas oil ratio rises.
·         At point B₂ the liquid content is maximum. The retrograde condensation process continues till B₂.
·          The retrograde condensation is the process in which some of the gas condenses into a liquid under isothermal conditions instead of expanding or vaporizing when pressure is decreased.
·         The composition of produced fluid changes after the dew point and the composition of remain reservoir fluid also changes. Hence there is small right shift in the two phase region.
·         From B₂ to B₃ the vaporization of liquid takes place, thus gas oil ratio decreases.

Dissolved gas reservoir:

·         If reservoir pressure and temperature are such that they:

    -lie outside the two phase region

    -lie on left hand side of the critical point

then the reservoir is a dissolved gas reservoir.

·         The reservoir fluid is initially in single phase liquid state. As the pressure declines it reaches the bubble point. Below this point bubbles and free gas will appear.
·         The free gas evolved will flow in to the wellbore. The flow of gas will be increasing and the flow of oil will be decreasing as the pressure drops during production.
·         On the phase diagram, the reservoir fluid path is CC₂. At CC₁ it crosses the bubble point curve and enters in to the two phase region.
·         Thus the volume of oil will be decreasing as the pressure drops.

Two phase reservoir:

     • If reservoir pressure and temperature are such that they:

-          lie inside the two phase envelop in P-T phase diagram

           then it would be a two phase reservoir.

·         It will consist of a liquid oil zone overlain by a gas cap. The compositions of oil and gas will be different.
·         The liquid or oil zone will be at its bubble point where as the gas cap will be at its dew point and it may be retrograde.

Summary of Reservoir Types:

	Type A Single phase gas	Type B   Gas condensate	Type C Dissolved gas	Type D  Two phase
Typical primary recovery mechanism	Volumetric gas drive	Volumetric gas drive	Depletion drive, water drive	Volumetric gas drive, depletion drive, water drive
Initial reservoir conditions	Single phase: Gas	Single phase: Gas	Single phase: Oil	Two phase: Oil and gas
Reservoir behavior as pressure declines	Reservoir fluid remains as gas.	Liquid condenses in the reservoir.	Gas vaporizes in reservoir.	Saturated oil releases additional gas.
Produced hydrocarbons	Primarily gas	Gas and condensate	Oil and gas	Oil and gas

    Reference: Applied petroleum reservoir engineering. 
 
  by: Mohammed Zubair Ahamed M  

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Distillation Column

     Distillation columns are key unit operations in traditional chemical engineering, especially in the oil and gas industry. They are usually tall structures filled with heated flammable fluids usually under pressure, and are consequently unavoidably hazardous. Many serious accidents have centred on columns and their secondary operations. Where they are present, the layout of distillation columns should receive early investigation since the layout of a number of other major items of equipment usually depends upon their placement, and they can have a high potential for initiating a situation in which one incident is sets off a chain of accidental incident from fire, explosion or collapse.

Main Components of Distillation Columns

        

     Distillation columns are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation contains several major components:

o        a column internals such as trays/plates and packings which are used to enhance component separations

o        a vertical shell where the separation of liquid components is carried out

o        a reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column

o        a condenser to cool and condense the vapour leaving the top of the column

o        a reboiler to provide the necessary vaporisation for the distillation process

          
     The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. A schematic of a typical distillation unit with a single feed and two product streams is shown below:

                                           Schematic of distillation column

Basic Operation and Terminology

         The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray. The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler.

       Heat is supplied to the reboiler to generate vapour. The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns.  The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottom product or simply, bottoms.

     The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product.

     Thus, there are internal flows of vapour and liquid within the column as well as external flows of feeds and product streams, into and out of the column.

Crude Oil Distillation

           A crude oil refinery is a group of industrial facilities that turns crude oil and other inputs into finished petroleum products. A refinery's capacity refers to the maximum amount of crude oil designed to flow into the distillation unit of a refinery, also known as the crude unit.

       The diagram presents a stylized version of the distillation process. Crude oil is made up of a mixture of hydrocarbons, and the distillation process aims to separate this crude oil into broad categories of its component hydrocarbons, or "fractions." Crude oil is first heated and then put into a distillation column, also known as a still, where different products boil off and are recovered at different temperatures.

o   Lighter products, such as butane and other liquid petroleum gases (LPG), gasoline blending components, and naphtha, are recovered at the lowest temperatures.

o   Mid-range products include jet fuel, kerosene, and distillates (such as home heating oil and diesel fuel).

o   The heaviest products such as residual fuel oil is recovered at temperatures sometimes over 1,000 degrees Fahrenheit.

       The simplest refineries stop at this point. Most refineries in the United States reprocess the heavier fractions into lighter products to maximize the output of the most desirable products using more sophisticated refining equipment such as catalytic crackers, reformers, and cokes.

Types of Distillation Columns

     There are many types of distillation columns, each designed to perform specific types of separations, and each design differs in terms of complexity.

Batch Columns

      In batch operation, the feed to the column is introduced batch-wise. That is, the column is charged with a 'batch' and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced.

Continuous Columns

     In contrast, continuous columns process a continuous feed stream. No interruptions occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs and are the most common of the two types. We shall concentrate only on this class of columns.

by:  Mohamed Burhan E

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Properties of Crude Oil

INTRODUCTION

Crude oils are made up of liquid paraffin hydrocarbon compounds ranging from pentane to pentadecane (C₅ – C₁₅). These hydrocarbon compounds consist of different groups such as the normal paraffins, iso-paraffins (branched chain paraffins), alkyl paraffins, naphthenes (or cycloparaffins), alkylbenzene and nuclear aromatics. The normal paraffins are the saturated, low molecular weight hydrocarbons. The associated gaseous phases are within this group. The naphthenes (or cycloparaffins) are highly bonded, high molecular weight hydrocarbons. All crude oils contain some appreciable amount of the naphthene compounds, (10% by composition).

Crude oils also contain a great variety of heteroatomic chemical constituents, comprising of sulphur, oxygen, carbon dioxide, nitrogen and trace metals. Nitrogen varies from 0.01 to 2% as dissolved gas in the crude oil (Levinson, 1974). Oxygen occurs in different forms in oxygen-bearing resinous substances.

Crude oils accumulate in geologic structures called 'traps'. A trap can be stratigraphic, paleogeomorphic or a combination of these. Paleogeomorphic traps includes structural folds and stratigraphic traps are those caused by lateral changes in reservoir rock properties within a stratum.

Physical Properties of Crude Oils

The physical properties = quantitatively measurable characteristics of crude oils. They vary according to the composition of the oil, the relative abundance of the groups of hydrocarbons, and essentially depend on reservoir temperatures and pressures.

           

Specific (or A.P.I) Gravity

            This is the weight of a given volume of crude oil. It is measured in two gravity scales, as stated below:

            i. A.P.I.

            ii. Baume gravity

•      The A.P.I. (American Petroleum Institute) gravity scale is more commonly and widely used than the European Baume gravity scale.

•      The A.P.I. gravity of a crude is influenced by the composition of the oil. Crude oils characterized by high amount of dissolved gases, are less dense thus, light in weight and therefore, possess high A.P.I. gravities, while denser crude oils of low amount of dissolved gases are characterized by low A.P.I. gravity values.

•      The group of hydrocarbons predominating in a crude oil also influences the A.P.I. gravity.            

           

For example, paraffin crude oils (45-60% paraffin hydrocarbons and less amount of naphthenes and aromatics) are light, thus, high A.P.I. gravities. But naphthenic base crudes (consisting predominantly of naphthene hydrocarbons 60-75%, with lesser amount of paraffins and aromatics) are heavy and have low A.P.I gravities.

•      The A.P.l. gravities of crude oils usually increase with depth. This is because a combination of source and reservoir maturation processes associated with slow but continuously increasing geo temperatures, cause the generation of lighter (or High A.P.I gravity) oils at greater depths of burial.

Viscosity

            This is the measure of resistance to flow in crude oils due to internal friction. It is expressed in 'poise' or ‘centipoise’.

•      The viscosity of a crude oil is influenced by the amount of dissolved gases at the prevailing temperature. Crudes characterized by high amount of dissolved gases have high A.P.I gravities and low viscosity or moderately high fluidity.

•      At high temperatures molecular agitation (or velocity) of the crude increases, making for a volumetric expansion and reduction in internal molecular friction, thus, reducing the viscosity of oil.

•      The greater the quantity of a high-molecular weight hydrocarbon group in a crude, the denser and more viscous it is.

Refractive Index

            The refractive index n, of a crude oil is measured from n = sin i / sin r

where; i = incidence angle

                                            r = angle of refraction

            It depends on the density of the oil. Heavy crudes (of low A.P.I) have high refractive indices. This is because a dense crude would create a dense medium for a passing ray of light, which is refracted towards the normal at a low angle (r). On the other hand, light oils have low refractive indices.

Optical activity

            It is the power of crude oils to rotate the plane of polarization of a polarized light. It is commonly expressed in degrees per millimetre. If any crude oil causes the plane of polarization to rotate to the right, it is called a 'dextrorotary', but if is to the left it is known as a 'levorotary’.

•      This property is destroyed at high temperatures (250 - 300°C).

•      Optical activity is also exhibited by some organisms that contain cholesterine substances (such as cholesterol C26 H45 OH).                                                         

According to Amosov (1951), the amount of optical rotation shown by a crude oil depends mainly on its sterane- pentacyclic and triterpane content. And these are hydrocarbon compounds derived from the microbial decarboxylation of organic cholesterine substancestical activity.

           

Cloud and Pour points

The pour point

            The temperature at which a crude oil will no longer flow, when a tube containing it is first heated in a bath, in order to dissolve all its wax content and then gradually cooled. At this temperature, the crude oil is in semi-solid to solid form, and thus loses its fluidity.

            If the pour point of a crude is above the surface temperature, it will precipitate its paraffin waxes on approaching the surface of the ground will only flow on heating.

The Cloud point

             The temperature slightly above the pour point, with an appearance of cloudy substances in the crude: which is due to the settling out of the solid paraffin waxes contained in the crude oil.

            This property determines the influence of low temperatures on crude oils. It provides information about the amount of solid paraffin waxes contained in the oil. This property is common in paraffin base crudes but wax-free naphthenic oils do not show cloud point.

Volume

            The volume of a crude oil in its reservoir rock differs from the volume it occupies in the surface.

            This is due to formation gas-oil ratio and reservoir pressures.

            The formation gas-oil ratio expresses the volume of gas contained in one barrel of a crude oil as it comes from the reservoir rock.Under high reservoir pressure, the volume of oil in the reservoir increases because of the influence of dissolved gases. But on release of the reservoir pressures, the dissolved gases escape, leading to the shrinkage of the volume of the crude oil at the surface.

Fluorescence

            It may be yellow, green or blue.

            For example, when a paraffin base crude oil (gasoline-rich) is exposed to ultraviolet fluorescence light, it emits yellow colour, while naphthenic oils emit brownish colour.

            This property is important in testing for cutting, core and drilling mud samples and in well-logging interpretation, for location of different oil horizons.

Colour

            This is the light transmitted through crude oils. It is yellowish to red for light oils and dark or even opaque for heavy (or low A.P.I gravity) oils.

           

  

Some other Physical Properties

           

Odour

           

            This varies greatly in crude oils. High content of light hydrocarbons (paraffins and naphthenes) in a crude gives rise to a gasoline-like odour. A pleasant odour is produced if the crude has abundant aromatic hydrocarbons. But with high amount of unsaturated hydrocarbon compounds, sulphur and nitrogen compounds in the oil, it produces a repugnant odour.

Coefficient of Expansion

            This is the measure of volumetric increase of a crude under thermal influence. It increases with increase in A.P.I gravity. Oils containing high amount of dissolved gas and possibly high A.P.I gravities possess high values of coefficient of expansion. Heavy crude oils (low A.P.I gravity) have lower coefficients of expansion.

Aqueous Solubility

            The aqueous solubility of crude oil and its fractions increases linearly with temperature.

            The rate of solubility becomes significant at temperatures of about 100°C. At temperatures above 180°C, crudes occur as molecular solutions in mixed phase with water. According to Cartmill and Dickey (1970), at such high temperatures, the nature of the phase enhances primary migration of oil by molecular solution mechanism. And salinity of about 150,000 ppm of sodium chloride results in the separation of liquid hydrocarbons from the aqueous phase.

Surface tension Effect

            Crude oils possess some intermolecular forces of cohesion, expressed as force per unit peripheral outline. Because of this force, oil in dispersed state cannot move through water- wet sand, much less, fine-grained shales.

            The small forces created by natural hydrodynamic gradients do not overcome those created by surface tension. Consequently the oil is dispersed in the form of globules.

Flash Point

            This is the temperature at which the volatiles rising off the surface of heated oil will ignite with a flash, on passing a flame over the surface. This provides some clue about the gaseous content of the crude oil.

Chemical Properties

           

            The chemical properties of crude oil deal with the chemical nature and the changes in composition in relation to temperature and pressure variations occurring at all times within the oil pool. Some of the chemical properties are related to the origin, migration, and accumulation of the crude oil.

Hydrogenation of crude oils

            During the early stages of crude oil formation, there is a remarkable thermal cracking of the organic materials into decomposed complexes, and the coupling activity of anaerobic bacteria processes, and the catalytic influence of such available trace metals as vanadium and nickel, lead, to the transformation of the complex organic matter into alkene rich paraffinic oil.

            And according to Zobel (1947), reservoir catalytic chemical reactions lead to the dissociation of avail sulphides into free sulphur and hydrogen. The elemental hydrogen would convert the alkene rich paraffinic crude oil into an accumulation of gaseous paraffinic oils (of high A.P.I gravity), in relatively close association with the kerogen (or organic source rock).

Paraffin wax content

            Paraffin waxes in crude oils are semi-solid to solid forms of hydrocarbons, consisting mainly of normal paraffins. These n-paraffins range from about C₅ – C₃₀. Hedberg (1968), described waxes as complex petroleum substances whose complexity is caused by molecular mixture of branched chain and n-paraffin hydrocarbons, with molecular weights, high enough to be solids at ordinary temperatures.

            The amount of wax in crude oils varies very greatly. High content of paraffin waxes in oils may lead to the clogging of pores of the reservoir rocks. Such oils congeal at atmospheric temperatures and exhibit high pour points.

Odd carbon Chain Lengths

            Chemical analysis shows that some crude oils exhibit a detectable predominance of n-paraffin of odd number carbon chains over those of even Members. The odd carbon chains range from C₁₇ to C₃₃. The ratio of the sum of the mole percentages of odd carbon n-paraffin to the sum of even Carbon n-paraffin in a specific molecular weight range, serves as an index of odd carbon preference.

Welte (1965) believed that crude oils from different environments

Paraffin waxes in crude oils have melting points above 30 degree Celsius possess correspondingly different odd carbon preference indices.

Porphyrins in Crude Oil

            Porphyrins are complex hydrocarbon compounds that originate from living organic matters such as chlorophyll and hermins.

            According to Hodgson et al (1967), porphyrins have high affinity for trace metals of vanadium, nickel, and iron. Chemical investigations have shown that paphyrins are one of the important constituents of crude oils.

            According to Hunt (1968), these hydrocarbons are derivatives from marine phytoplankton.

            Porphyrin substances are of moderately low temperature origin, usually destroyed at temperatures of about 200°C. Thus, their presence in crude oils is an indication that the

 crude was formed at temperatures below 200 C

Trace Metals in Crude Oils

            Crude oils contain varying amounts of trace elements some of which include, iron, aluminium calcium, magnesium, copper, lead, tin, astemum, antimony, zinc, silver, nickel, chromium, molybdenum and vanadium. But the most important of these trace elements are vanadium, nickel and iron. The concentration of any of these trace metals is so small that the value is expressed in parts per million. The concentration of trace metals in any crude oil is found to be inversely proportional to the A.P.l gravity of the oil.

by: Syed Inayathullah

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