Monday, August 31, 2015

Invasion Profile: Resistivity

This post is an elaboration of the "Mud Invasion Profile" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Mud Invasion Profile" slide at page 4

What is Resistivity?

Before going further to resistivity, we should know what resistance is first. Resistance can be defined as voltage divided by the current. The concept of resistance is parallel to the concept of  ”friction” in mechanical. It defines how hard electric current can pass. The Ohm's Law states that ideally the ratio of voltage and current are always constant. That’s why it is believed that the resistance depends on material and condition being used to flow the current. Unlike density, which will not change as the material is cut, resistance will change if the material’s geometry or volume changes. That is why more than just the material, the geometrical properties also affect the resistance.

After we know about resistance, we’ll talk about resistivity. Resistivity is the resistance of a unit material in a certain volume. Imagine that we have a cube of ice. When we apply current which flow through the cube (for instance, from left side to right side), resistivity can be known by measuring the voltage for the unit cube's length, (for instance the distance from left side to right side) Volts per meter, V/m AND by knowing the current which flow through the cube's cross sectional area (the area of right side) Amps per meter squared, or A/m2. Dividing these two terms will result in units of Ohm-m2/m or Ohm-m.

Why do we need to calculate resistivity?

Water is such a good solvent it almost always has some solute dissolved in it. Even the de-ionized water will always have some part of ion solute.  Specifically ground water, always contain ion which is most likely salt. When water has salt, it can conduct electricity readily, since salt separate into free ions in aqueous solution by which an electric current can flow. That is why water has low resistance.
Unlike water, organic compounds like oil, phenol, alcohol, and sugar do not conduct electrical current very well and therefore have a low conductivity when the current applied to them, which means high resistance. From this point things are getting interesting.

Because we know the resistance of oil is high and water is low, then we may conclude that the resistivity of rock containing oil will be high and rock containing water will be low. That is why formation resistivity is very important to know whether the formation contains oil or water. We don’t want to produce water, since we have enough water from the ocean, so we need a tool to see which part of the ground contains oil, by seeing the resistivity. When the tools detect rock which has low resistivity, we may believe that the rock contains hydrocaarbon.

How do we know the resistivity, and how the resistivity measurement tool works?

If only we had created a tool which accurately locates the exact location of rock containing oil, we have been in the world war 3, since every country may see which other countries have more oil, and force them to give their oil. Fortunately, the best man can make is a tool to locate the oil location after they drill the correct location. Only less than 70% exploration drilling in America find oil zone nowadays. That’s why after they drill, they put in a tool to the wellbore that may show which formation layer contains petroleum. As discussed above, this tool should be able to measure the resistivity of the rock, and this tool is called resistivity log. The simple explanation, resistivity log works by injecting current into the formation from a single electrode, then the current spread out radial from the electrode. Other electrodes, which is measuring electrodes approximates the measurement of a constant-voltage spherical shell around the injection electrode. The measurements of voltage and current are converted to a resistivity measurement.

Why we need to differentiate resistivity in a borehole?

A drop of poison will alter a glass of milk. In drilling a well, we need to put mud, which has different resistivity from the formation water or maybe the oil (if the formation contains oil). That’s why measurement of resistivity from the borehole must not represent the true resistivity of the formation, since mud, mudcake, flushed zone, and zone of transition will affect the measurement. There is no direct measurement to get resistivity of the formation. This problem should be handled so the true resistivity can be obtained.  

How do we differentiate resistivity in a borehole?

The resistivity measuring log can be divided based on the depth of measurement to the formation around them that they can investigate. There are logging tools which are specifically made to measure resistivity of flushed zone (e.g. Microlog, microlateralog), invaded zone (e.g. short normal, spherically focused log) and uninvaded zone (e.g. deep induction log, long normal). Flushed zone measurement is corrected for the influence of the mudcake by estimating mudcake thickness and resistivity. invaded and uninvaded zone resistivity measurements are corrected for environmental effects using the charts for the tool used. There are corrections in measurement since the borehole size and bed thickness variations may affect the measurement. Then charts are used to know parameters of resistivity in every zone.It is like service companies will use charts to know the resistivity of the uninvaded zone, which is the most important measurement, with the correction of resistivity measurement in flushed zone to estimate the real uninvaded zone resistivity.

Wednesday, August 26, 2015

Invasion profile: Permeability VS Porosity FAQ

This post is an elaboration of the "Mud Invasion Profile" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Mud Invasion Profile" slide at page 2

These are very basic questions that many people outside Petroleum Industry ask. We’ll discuss these terms as simply as possible

What is Porosity?

Porosity is the ratio of the volume of the pores or empty space/void in the rocks to the total volume of the mass.

What is Permeability?

The ability of the rock to transmit fluids

Are they connected to each other?

By definition, there is no connection between porosity and permeability. The porosity might get higher but with the same permeability. The tendency will show higher permeability, but there is no universal correlation between permeability and porosity.

But there are people researching the relationship between porosity and permeability empirically for a certain area. Since the permeability tests quite take time, people may get porosity from log measurements, which are faster and easier, then convert it to permeability by the empirical equation to obtain permeability estimation.

What makes rock get porosity?

Rock always has original porosity from the void between every tiny grain building the rocks. Sometime it is too small because the rocks are well cemented naturally. But then, the void might be formed because of chemical leaching of minerals or the generation of a fracture system after the rock had deposited.

What makes rock get permeability?

Porous rock always has its own permeability. But to test the permeability, pressure difference should be applied between two ends of the rock and let the fluid inside flow between the two ends.

Does permeability depend on the fluid type?

 Permeability is a property of the rock and not the fluid, so it won’t be different. But remember that permeability measurements are under Darcy’s Law. The conditions for Darcy’s Law are
  1. The displacing fluid behavior is Newtonian (Viscosity is independent of applied shear force)   

  2. The porous medium is fully saturated and medium grained

  3. The porous media is inert

  4.  The flow is under isothermal conditions

  5.  The grain geometry is stable

  6.   Inertial forces are negligible (Flow is laminar)

I can’t see how permeability works. How could permeability be the same when you see flow of viscous fluid is not the same at all with the aqueous one!

In simple answer, viscous fluid will flow in different flow rate than the aqueous one for the same pressure difference applied. Simply when there are different type of fluid, it will change other flowing properties which will result in the same permeability in the Darcy’s equation, which is the properties of rock.

So the permeability will depend on viscosity of the fluid?

No. Permeability is the property of the rock, which defining a specific rock being used. In other hand, viscosity is the property of the fluid, which defining a specific fluid going through the rock. Changing the fluid, which will change the viscosity, won’t change the rock, so it won’t change the permeability too. Changing the fluid only changes the flow behavior.

How could I imagine porosity vs permeability?

Here’s a good illustration for you. Imagine you are in a classroom filled with big yoga ball. It will take about 6 yoga ball to fill the classroom. Then there’s another classroom with exact same size, filled with marbles. It takes thousands of marbles to fill the whole class. Do you know that both of class will have the same percent of void space between these round things? Yes they are, but the difference is in yoga ball’s class the voids are more centered, and in marble’s class the voids are spread between thousands of marbles. This is the example of two rooms with same porosity.

Then imagine both rooms have door at one side and a window on the wall across the door. You would like to flow water from the door to window. Which room will flow water faster from door to window? The answer is the room of Yoga balls. It takes time to flow water through thousands of marbles! That’s how permeability works. The yoga ball classroom has bigger permeability. The water should flow through twisty path to the window. In the rock, this twisty path is called turtuosity, which is one of parameter which composes permeability. That is how the porosity could be same between rocks, but different permeability

If you have any questions or correction, feel free to comment. Thank you!

Get The FREE Presentation [CLICK HERE]

Sunday, August 23, 2015

Production System: Packer

This post is an elaboration of the "Petroleum Production System" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Petroleum Production System" slide at page 12


The main purposes of Packers in a well are to provide structural (anchor the tubing to casing) and sealing purpose. From The isolation and structural functions can be detailed as:
  • To isolate the annulus between well and tubing so it will provide sufficient barriers or to prevent the corrosion of the well casing corrosion. The important part for the sealing is large, cylindrical rubber elements around the packer. In situations where the sealed pressure is very high (above 5,000 psi), metal rings are used on either side of the elements to prevent the rubber from extruding. 

  • A packer will isolate different production zones for zonal isolation. Sometimes, we could produce different fluid from different layers just by using one well. The fluid produced from a layer can be a problem if mixed with fluid from other layer, since the properties are different and there’s a possibility it will create plugging scale or solid.

  • Isolate gravel and sand.

  • Isolation capability from any fluid from the bottom fluid, e.g. straddle packers. It is usually used by the geologist if they miss the payzone (drill too deep) and want to do drill stem test.

A packer without a seal is called an anchor. They have various applications:

    • Prevent tubing movement. In a well which has lost its natural energy, an artificial lift should be used. Artificial lift such as sucker rod pump will make a lot of movement of the tubing, and the tubing need anchor to prevent movement

    • It may also reduce associated stresses of the tubing and transfer the load to the casing.

After a logging has been done, a packer will be run in the casing on production tubing or wireline. Packer will be run into borehole until the determined depth is reached. The slip and rubber must be expanded to contact the casing. There are various methods to expand the rubber and slip. By mechanical set, slip and rubber will expand because of rotation or upward /downward motion on the tubing. However, in completions, they are often hydraulically set, which will use fluid pressure to drive the cone behind the slips

The packer could be either permanent or retrievable. The choice will depend on the future plan of the well. The permanent packers are lower in costs and greater in sealing, but they need to be milled to be removed. When the well is planned to be intervened in near future, maybe recompletion or artificial lift, a retrievable packer must be used. Even though retrievable packers generally lower sealing and gripping capabilities, but after removal and subsequent servicing, they can be reused.

Packers are also used in Drill Stem Test. There are 3 types of packers used in Drill Stem Test:

  1. Straddle packer. The straddle packer requires two inflatable packers that are set by rotating the surface pipe. It is usually used to isolate the bottom and upper zone of payzone if the geologist misses the payzone when drilling.
  2. Standard packer. This is used more often than other methods. The packer is placed above the pay zone. Sometimes the company uses two packers above payzone in case one packer doesn’t hold. Rerunning the test will be more expensive than investing on two packers.
  3. Cone packer. Sometimes the well is drilled just above the payzone, then drill the core from the payzone with core bit, which is smaller in size. In that case, Drill Stem Test can be done by using cone packer. Put weight on the packer and it will seal the area over the core hole. Too make sure a good seal, a standard packer coan be used above the cone packer.

(source: Davenport, B. Handbook of Drilling Practices, Gulf Publishing Company:1984)

Friday, August 21, 2015

Production System: Why There are Different Petroleum Types in Reservoirs

This post is an elaboration of the "Petroleum Production System" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Petroleum Production System" slide at page 7

The layer of formation which has been drilled will be classified as reservoir if it traps sufficient amount of hydrocarbon. Hydrocarbon, or commonly known as oil and gas, is a molecule with composition of chemical elements hydrogen (H) and carbon (C). Oil and gas made up of these two elements, with very diverse proportions. The Oil deposits found in one place will very rarely be found in other places with the same composition.
There are many factors effecting the composition of the hydrocarbon, including the history of the maturation. Hydrocarbon itself consists of a liquid phase (oil) and a gas phase, depending on the reservoir condition it takes place (pressure and temperature). Changes in reservoir conditions will result in a phase change as well as the physical properties change of reservoir fluids.

Because there are very different composition of hydrocarbon, also because of the reservoir conditions are vary from one reservoir to other reservoir, that is why we cannot expect type of hydrocarbon in the reservoir we found. The hydrocarbon properties we need to measure extensively will be about the density and specific gravity, viscosity, formation volume factor, gas solubility, compressibility, and bubble /dew point pressure. The measurement can be done in the field or in laboratories.

In the petroleum industry, there are 5 types of fluid reservoirs that have different types and characteristics. 5 types of reservoir fluid are

A. Dry gas

In the dry gas, the main component is methane, which is very light.  That is why the hydrocarbon state in the reservoir will be gas. in fact, it remains in gas state from reservoir to the surface. All properties from the reservoir to the surface does not change. Based on field data, this reservoir has an initial GOR ≥ 100,000 scf / STB and content of heptane plus of 0.7% mole

B. Wet gas

The main content of the reservoir is generally similar to the dry gas, just more portion of intermediate hydrocarbon content (C2 - C4). In reservoir, the state of hydrocarbons is gas, but at the time it flow to the surface, condensation is happened due to the decrease in pressure and temperature. Condensate that forms on the surface from  the wet gas is fairly expensive because  it contain  short-chain hydrocarbons which have a greater heating value. Based on the results of field data, this reservoir has a GOR of 70000-100000 scf / stb with more than 50 degrees API

C. Retrograde gas

The components are mostly filled with methane and intermediate hydrocarbon. The phase in reservoir is gas. Retrograde gas is unique because the liquid phase will be increased as the temperature and pressure decreased (whether the pressure in reservoir decreased or because it flow to the surface), but if the pressure continues to fall, then some of the liquid back to gas. That is why the properties in reservoirs will be different than on the surface. The GOR is from 8000 to 70,000 scf / stb with initial Specific Gravity Stock Tank Oil> 40 API.

D. Black Oil

The majority of the oil reservoir is in the form of black oil. The other name for black oil is low shrinkage oil, which means a slight reduction of pressure resulted in a slight decrease in the percentage of liquid phase.

E. Volatile Oil

The other name is high shrinkage oil which means a slight reduction of pressure resulted in a large decrease in the percentage of liquid phase. The GOR is from 2000 to 3300 scf / stb, The oil SG is about 30-50 API.

Tuesday, August 18, 2015

Invasion Profile: Transition Zone

This post is an elaboration of the "Mud Invasion Profile" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Mud Invasion Profile" slide at page 5 until 6.

Please download the FREE slide in my previous post [CLICK HERE] to get the whole analysis, colorful demonstration, and easier understanding.


When we see a wellbore design, the hole diameter of the borehole is translated as the outside diameter of the drill bit.
Unfortunately, the borehole diameter can be larger or smaller than the diameter of the drill bit and it is quite tricky to be measured. It might changes because of wash-out and/or collapse of the shale, poor cementation in porous rocks, or the thick mud cake on porous and permeable formations. The solution for real hole size diameter is by using the caliper log.

Caliper log (source)

While drilling, there’s always drilling mud inside the borehole, except special case like underbalance drilling or the worst of all, kick and blow out. Drilling mud helps move cutting of boreholes, lubricate and cool the drill bit, drill and maintain the excess pressure on the formation pressure, keep the wellbore stability, keep the drill cutting sustain when the drilling stops, and in mud motor, it can move the drill bit.

Mud density is kept high so that the hydrostatic pressure of the mud column is always greater than the formation pressure. This pressure difference pushes some mud seeping into the formation. But when the mud seep into the formation, solid particles stuck on the side of the hole and form a mud cake. They block the pores so no further seepage can go into the formation. The failure of mud cake to block more seepage of mud fluid will lead into mud loss, which will start well kick if not treated properly.

(mud cake formed when mud flow through filter paper. mud cake will prevent further seepage)

The mud fluid into the formation called mud filtrate (mud filtrate).  Small seepage of mud fluid will separate the formation condition into 2 zones. A zone infiltrated by mud filtrate is called invaded zone. Invaded zone consists of Transition/Anulus Zone and Flushed Zone. The zone that is not contaminated by mud filtrate is called uninvaded zone. It is only saturated by formation water, oil, or gas. Water saturation in this zone is very important, because it is used to determine the hydrocarbon saturation in the reservoir.

Invaded Zone Profile

Flushed zone is only a few inches from the borehole, this zone is usually cleared of formation water. All moveable formation fluid has been replaced by drilling mud. If there is oil, the invasion of mud filtrate can be determined from the difference between the water saturation in this zone versus the water saturation in uninvaded zone. Usually about 70-95% of oil moved. 

Transition or Annulus Zone appears when formation fluid and mud filtrate mixed. Transition zone occurs between the flushed zone and uninvaded zone.
Ideally, there are three types of fluid invasion distribution in the borehole.

The shape of transition Zone

We will see how the distribution of seepage in the invaded and uninvaded zone and its relationship with the relative resistivity.

 Step Profile, mud filtrate is distributed as a cylinder around the borehole. In the outer diameter of flushed zone, we can see a sudden contact with uninvaded zone, which is mud filtrate free. Uninvaded zone occupied by formation water or hydrocarbon. In this example uninvaded zone filled with 100% water (no hydrocarbon) so the resistivity reading is low. Thus, we see that there’s no transition zone, since mud filtrate doesn’t go to uninvaded zone partially.

(from George Asquith & Charles Gibson)

Transition Profile, This is the most realistic models. Distribution is still a cylinder, but the invasion of mud filtrate reduced gradually (gradation) in transition zone, then connect with uninvaded zone at the outer part. At Flushed Zone, pores filled mud filtrate, and the resistivity measurement will be high. On the Transition Zone pores filled with mud filtrate, formation water and, if any, residual hydrocarbons. At Uninvaded Zone, water filled pore formation, and if there is, hydrocarbon (in this diagram hydrocarbons does not appear since the resistivity measurement is low at uninvaded zone).
(from George Asquith & Charles Gibson)

Annulus Profile, describes the temporary fluid distribution when the logging operation is stopped for a moment (not recorded in the log). Annular profile describes the fluid presence between invaded and uninvaded zone. It is a sign of the hydrocarbons presence. This profile can only be detected by induction log (ILD or ILM) as soon as the well is drilled and show high resistivity measurement. When the mud filtrate seep into the zone, formation water is pushed out, then the pushed formation water form a ring (annular ring) on ​​the invaded zone boundary, this profile can only occur at the hydrocarbon bearing zone. At the Flushed Zone, pores are filled by mud filtrate and residual hydrocarbons, so the value of resistivity is high. On Transition Zone, pores are filled with mixture of mud filtrate, formation water, and residual hydrocarbons. Outside those zone is the Annulus Zone, where the pores are filled with formation water and hydrocarbons. At the time of the annulus profile appear, the resistivity will suddenly decreased on the outer boundary of invaded zone, due to the high concentration of formation water. Formation water seepage is pushed out by mud filtrate into the annulus zone. This leads to the temporary absence of hydrocarbons in annulus zone. Basically, most of the hydrocarbon bearing rock is water wet, and the relative permeability of the oil is higher than the water. That’s why it is easier to remove the oil in annulus zone because the mud filtrate pushes the formation water to annulus zone. It is temporary because at the later time the hydrocarbon will push back the formation water to create balance.  The formation water resistivity measured in annulus profile zone will be higher than in real condition because it is affected by high resistivity hydrocarbon.
(from George Asquith & Charles Gibson)

Transition of Resistivity Profile

Whether the resistivity shown in invaded zone is higher or lower than the uninvaded zone will depend on the drilling mud being used in the wellbore and the formation fluid detected in the penetrated zone.

When using fresh water drilling muds, in water bearing zone, the mud filtrate resistivity will be lower than that of formation water. That’s why the transition zone will have lower resistivity than the flushed one, lower and lower until the uninvaded zone.

 Using the salt water drilling mud, the resistivity of mud and formation water in this zone will not have significant difference. Invaded zone, flushed zone and uninvaded zone will have low resistivity result.

resistivity transition through water bearing zone; upper: using fresh water mud. Lower: Using salt water mud (from George Asquith & Charles Gibson)

The drilling somehow will penetrate the hydrocarbon bearing zone (water saturation less than 60%). Using fresh water mud, the resistivity of flushed zone is high, since fresh water have higher resistivity than the formation water. In transition zone, typically the resistivity is still high, since it contains mud filtrate and hydrocarbon, even though some portion of formation water would be there. In uninvaded zone, there’s no portion of mud filtrate and more portion of formation zone. Thus, the resistivity is lower.

If we use the salt water drilling mud, which is low in resistivity, we can see that in flushed zone the resistivity is low, then the transition zone have higher resistivity since hydrocarbon has appeared in this zone, and the uninvaded zone will have the highest resistivity since the hydrocarbon saturation in uninvaded zone is the original hydrocarbon saturation.

resistivity transition through hydrocarbon bearing zone; upper: using fresh water mud. Lower: Using salt water mud (from George Asquith & Charles Gibson)

Friday, August 14, 2015

Petroleum Production System


This is the schematic picture of petroleum production system. The production system is the system that transports reservoir fluids from the subsurface reservoir to the surface, processes and treats the fluids, and prepares the fluids for storage and transfer to the market.
As shown in the picture, the basic elements of the production system start from Reservoir, Wellbore, Surface wellhead, flowlines, and processing equipment. A ‘‘reservoir’’ is a porous and permeable underground formation containing an individual bank of hydrocarbons confined by impermeable rock or water barriers and is characterized by a single natural pressure system. Oil and gas wells are drilled like an upside-down telescope. The large-diameter borehole section is at the top of the well. Each section is cased to the surface, or a liner is placed in the well that laps over the last casing in the well. Each casing or liner is cemented into the well (usually up to at least where the cement overlaps the previous cement job). A well is composed of casings, tubing, packers, down-hole chokes (optional), wellhead, Christmas tree, and surface chokes.  The equipment at the top of the producing wellhead is called a ‘‘Christmas tree’’ and it is used to control flow. The fluids produced from oil wells are normally complex mixtures of hundreds of different compounds. A typical oil well stream is a high-velocity, turbulent, constantly expanding mixture of gases and hydrocarbon liquids, intimately mixed with water vapor, free water, and sometimes solids. The well stream should be processed as soon as possible after bringing them to the surface by separator.


The slide will discuss about every important part of the production system. It will discuss step by step petroleum flow from Reservoir  to the surface, it will focus on different kinds of reservoir, why the reservoir can flow the hydrocarbon to the wellbore, the detailed picture about the wellbore, the structure of the wellbore, different parts of wellhead and christmas tree, their functions, and finally different kinds of separator which are used in petroleum industry. The slide is focused on color and moving object to help the imagination of the viewer, so the learning process will be accelerated. Therefore, We advice the presenter to give a little time learning about petroleum system, OR this slide also good for those who want to imagine how does it works. Because everything's less boring when they are moving and colorful!

You'll be amazed how a single picture may talks a lot of things! 

 AND THE BEST PART IS THE PRESENTATION IS FREE!! You can download the presentation below, and see how a single picture can tell you a thousand words. It is quite impossible to analyze ALL the perspective of the picture (example: from the point of view reservoir engineer, drilling engineer, production engineer, geologist, etc) So feel free to add your perspective and additional data about the picture in the comment! Thank you!

Monday, August 10, 2015

Mud Invasion Profile


Invasion, the process by which wellbore fluids leak off into permeable formations, is a potential source of damage to well productivity.  

 When mud filtrate is forced into the porous zone during the fluid invasion process, it creates blockage due to one or more of several mechanisms that may reduce the absolute permeability or restrict flow due to relative permeability or viscous effects.
Hydration or dehydration of swelling clays, dispersion or flocculation of swelling or non-sweI1ing clays and formation particles or dissolution of cementing materials results in formation of fines which move within the pore spaces. thereby adversely affecting its permeability.

 As the filtrate invades the formation, it creates a zone very close to the borehole wall. in which most of the original water and some of the hydrocarbons may be flushed away by the invading mud filtrate. The zone is referred to as "Flushed Zone” 3. It contains either mud    filtrate or residual hydrocarbon, depending on whether the formation was originally water bearing or hydrocarbon bearing. Away from the flushed zone, the displacement of the formation fluids by the mud filtrate is less and leSS complete. resulting in a transition from mud filtrate saturation to original formation water saturation. Beyond transition zone exists the “uninvaded” zone or virgin zone


 I analyze quite many aspects from the picture. I make it better with color for deeper understanding, analyze the impact to the logging result, I analyze what's wrong with the picture.

You'll be amazed how a single picture may talks a lot of things! 

And you all can get my presentation FREE from the link below!!! It is very unique, full color, and funny moving chart that you might learn about invasion profile faster, and it will be very useful if you want to teach other about invasion zone interestingly..