Friday, September 11, 2015

Porosity Log Overview


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



It is very important to understand that the resistivity log which has high resistivity measurement doesn't always detect the hydrocarbon containing zone. There are two possibilities: first, the low resistivity zone might contain low resistivity fluid, which is hydrocarbon, and if that is the case, we have found our sweet pay zone. But second, the formation might be very small in porosity that the resistivity measurement becomes low.
Most of the rock is nonconductive to the electricity, so when the porosity of the rock is very small, there will only small quantity of fluid in the rock. Even when the fluid is very conductive, since the quantity is small, the log will show low resistivity measurement. That is why, the result from porosity log should be analyzed.

There are 3 types of porosity log: Sonic log, Density log, and Neutron log. Although the results will represent the porosity condition of the formation, the truth is none of them measure porosity directly. The density and neutron logs use nuclear to interpret the fluctuation as their porosity measurement. On the other hand, sonic log use acoustic measurements to be interpreted as porosity. Rather than choosing one of them, service companies may use combination of these logs to give better indications of lithology and accurate estimation of porosity, since lithology should be specified first if using a single porosity measurement for calculating the correct porosity.

Sonic Log

Sonic log is a log that measures interval transit time (Δt) of a compression sound wave travelling through the formation along the axis of the borehole. The Δt is the transit time of the wave and is the reciprocal of the velocity. The acoustic pulse from a transmitter s detected at two or more receivers. The time of the first detection of the transmitted pulse at each receiver is processed to produce Δt. The Interval transit time itself will depend on both lithology and porosity. Important thing to be noted is when we use the sonic log, porosity calculated in gas zones will be slightly higher than the actual values. It could be happened since the travel time in gas is higher than in water.



Density Log

The density log will count the returning gamma rays which represent electron density in formation. Gamma rays emitted from a chemical source by the logging tool, and then the gamma ray will interact with electrons of the elements in the formation. The detectors in the tool will count the number of returning gamma rays which represent formation electron density. There will be relation between  electron density to the formation density. This relation will be represented as a constant. Formation bulk density, which is a function of matrix density and formation fluid density will be used in an equation to find the porosity of formation. that is why it is very important to choose representative matrix density and fluid density to make accurate porosity measurement. Matrix density will vary  with lithology and it will make the porosity measurement become demanding. It should be noted that presence of gas in the pore space causes porosity from density log is higher than the actual porosity. We’ll see later that Density-Neutron combinations will create cross-over in the gas zone.




Neutron Log

Neutron logs will measure the hydrogen content in a formation by emitting neutron. Neutrons are emitted from a chemical source from the logging tool. Then, the emitted neutrons will collide with nuclei in the formation. The collision will make neutrons loses energy, neutron is absorbed and a gamma ray is emitted. The element which closely approximates the mass of a neutron is hydrogen. When collision between neutron and hydrogen happened, average energy transfers to the hydrogen nucleus is about ½ that of the original energy of neutron.  It may be concluded that materials with large hydrogen content like water or hydrocarbons will slow down neutrons. Since hydrogen in a porous formation is concentrated in the fluid-filled pores, energy loss can be related to the formation’s porosity.

Neutron-Density cross plot often to be used to detect gas zone. The characteristic of gas zone are Density porosity measurement is too high and Neutron porosity measurement is too low. It means the Neutron porosity will be lower than density porosity, thus creating Cross-over line between them in the gas zone.


Those are some basic informations about porosity log. We’ll discuss about them in detail in the future.


[CLICK HERE FOR THE FREE SLIDE]


Do you love this post? Thank you! I feel appreciated. This post is under the category of Petroleum Engineering,  you may love to see other similar posts HERE

Irianto Petrus Binsardo

Wednesday, September 9, 2015

Unconventional Reservoir Overview



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



Basic: Why the Logging Results Should Show The Permeability?

Because the permeability of the rock represent the capacity of the rock to flow the fluid through it, it is very common that a wellbore be checked by logging tools to measure the permeability of some interesting place of the zone. For more detailed technical and scientific information about permeability, see my previous post here. So since a low permeability rock will hardly flow fluid through them and high permeability rock will flow fluid easily, people will check the logging result whether they can find high permeability zone with low resistivity.

So, People Will Look For High Permeability Zone Which Contains Hydrocarbon!

Exactly! Permeability is usually expressed in millidarcys and people will commonly look for zones which permeability is about ten to several hundred millidarcys as the oil reservoir. It will be categorized as conventional reservoir which procedure, technology, and knowledge have been widely used. Also the economical return is still profitable to the company.
Unfortunately, the party is over! As oil getting harder and harder to be searched, people start to consider gas as one of the main power source. conventional oil resources are estimated to be around 70 billion barrels, while the conventional natural gas is about 590 trilion SCF! Gas is cheaper because it is harder to be handled and less popular, so when the price of oil is getting higher, the price of gas becomes more interesting than years before.

WAIT, YOU ARE WRONG! The oil price is lower now than years before!

As I say higher price, it means as supply and demand relation. That means, as unrenewable energy, the more oil demand now will make the less supply in the future, and when the demand in the future still high although we only have few supply, the price will be high. The low oil price now is the impact of political decision and oversupply from the unconventional gas. We may enjoy the oversupply now, but when we realize that we have depended on oil and gas too much, the price may be much higher than before when the supply decrease abruptly.

So there’s conventional gas reservoir and unconventional gas reservoir? What is the definition of unconventional?

Yes there are differences of conventional gas and unconventional gas. The differences of gas and oil are about the properties of the fluid, e.g. the percent of methane, the phase of the fluid. In other hand, conventional and unconventional is all about how easy we may produce handle, and sell it from its reservoir. So the unconventional oil reservoir will contain extra heavy oil, bitumen (a little bit solid), or shale oil (oil from shale, which is hard to flow); while unconventional gas will be about low quality gas (high percentage of CO2 and sulfur), tight sand gas (low permeability), and gas hydrates (from the permafrost of ocean)

Why searching unconventional reservoirs when there are plenty conventional gas reservoirs?



The conventional gas reservoir resources were estimated around 590 trillion SCF, but it doesn’t mean they are widely spread all over the world! Arabian countries have a lot of proven reserves in their land that they could produce oil easily and sell them cheaply. In America, such oil is very low in number, but they have a lot of gas in their shale, which is tight in permeability and harder to produce, but the revenue will be very big. It is very important to depend on every country’s own power source, instead expensively buy from other countries.  Just imagine, Venezuela would be the largest oil reserves in the world if their 225 billion barrels of extra heavy oil considered as recoverable.

Back to the picture, how they could get oil/gas from low permeability zone?

Hydraulic Fracturing (“fracking”) is the technology to create artificial permeability to the zone. Advances in fracking techniques create  a tremendous increase in shale oil and gas production.  The permeability will be increased by artificially introducing small fractures into the formation. The pressure from the fluid used in fracturing will create fracture to the rock. Additional pressure injected from the surface will make the fluid penetrate deeper to the formation, and create longer fracture. This fluid contain many “granules”  named proppant which will support the fracture to stay opened when the pressure released. This way, the permeability of the zone has been increased.

First Fluid which is at first in hydraulic fracturing is called "pad". The pressure applied to get the pressure at which the rock break, called the breakdown pressure. Then fracturing fluids is used to a wide fracture, wide enough for the proppant to flow in without bridging or settling. That is why the fracturing fluid should have high viscosity.

Proppant will make high permeability flowing channel into the well. The permeability  contrast between fracture and the formation will determine the increase in the production of a fracturing project. The more fracture permeability contrasted with the real permeability, greater productivity will be gained. But it does not mean that we always use the biggest proppant,we still look for the most economical way, in the selection of size and types of proppant. When proppant stressed  more than its strength, it will be crushed and it is a disadvantage in terms of productivity. There are many things could be used as proppant, such as sand, ceramic, and resin coated proppant.

How about the unconventional reservoir permeability measurement? Will we use the same log?

We also use log to know the permeability of the reservoir. But an important complication is there is different resolution between the logs relative to the heterogeneity often exhibited at the core scale. Core measurements are often performed on a small sample while the log will represent the data for feet depth. Some attempt to scale up the core measurements should be done to allow for calibration of log to core data. Also the problem of the calibration is the heterogeneity of the zone is very high that scale up might be hard. These heterogeinity also makes the unconventional reservoir characterization become harder than the conventional one.





The unconventional reservoir studies are very important nowadays since it is the future of our energy sources, which is agreed by many consultants. The unconventional reservoir engineering and training are developing to create more economic method of production, with the prime safety consideration. Also, young engineers should enrich themself in unconventional reservoir since it might be the future of world's energy


Do you love this post? Thank you! I feel appreciated. This post is under the category of Petroleum Engineering,  you may love to see other similar posts HERE

Irianto Petrus Binsardo
Resume

Saturday, September 5, 2015

Well Logging Procedure Overview


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





Well logging is an important job in Oil and Gas industry.  It uses sophisticated tools, equipments, devices and software, The interpretation will be done by professional earth scientists with precision as it is very important in formation evaluation. That is why there are many trainings, seminars and handbooks of well logging, which are reasonably high in price, to make sure people contributing in well logging jobs are capable in using the software and basic procedure to get the representative results of the formation. The slide ppt (ppsx) of basic interpretation in well log results has been shown in this blog, but to elaborate it, we should know the tools, procedures, and simple facts in the field that is important to get the best results.


Why we need Well Logging?




in nontechnical language, Well logging is the method to see the condition around the borehole so we can have the data for some feet into the formation from the well. Formation means a rock unit, so the well log will see the condition of the rock around the well, and from these conditions we may conclude whether the rock will contain petroleum or not. Thus, still in nontechnical term, to measure the condition of the rock around the well, logging tools should “emit something” and measure the respond of the rock OR the logging tools may “measure emitted things” from the rock to get the properties of the rock. Whether the tools “emit something” or simply “measure emitted things” will differentiate the types of logging tools as active OR passive logs a. And from these measurement we may answer many questions from engineers in the field and the office. It is very important to the geologists as it will answer their prediction whether there is hydrocarbons, whether the potential zone porous, whether the top of formation as predicted, and what kind of hydrocarbon in the formation. It is a method for the reservoir engineer to know the pay zone thickness, homogeneity of zone, and porosity and permeability of the zone. It will help the decision of production engineer on which zone to complete, the production rates, whether there will be water production in following years, and even what stimulation should be used in the future.

Logging seems very powerful!




Indeed. It provides many information of the formation near the well. It will allow us to see changes in formation properties along the well. But it also has its shortcomings too. The biggest short coming is that the measurement is representative only to the rocks some feet around the well. That means we will have no clue if in 20 ft near the wellbore, the rock and fluid properties have changed. That is why the result is quite unrepresentative in the non-homogenous zone.

So, what are the tools? 


There are many logging tools that are used based on whether they are active or passive, and based on the properties they measure. The tools should be chosen based on the condition of the well, and the most important one, economical consideration. It’s all about money after all. The more equipment we use will cost more money. The longer we use the tools, the more we pay the rig (exploration rig, offshore rig and bigger rig will cost higher money. Exploration rig may cost $8000-15000 per day!).  The type of well will impact on the economical consideration too. Horizontal well will cost higher since the tool can’t be lower by wireline (should use pipe-conveyed logging), deep well will cost higher because the tools should be modified to resist the high pressure, and hot well (e.g. geothermal well) will cost higher because the tools should have high temperature design.

The active well log will emit electromagnetic, nuclear, or acoustic to the formation, then measure the respond to get the formation properties. For example, in Acoustic log, the logging tool will emanate acoustic and see the respond from the formation. The sound might travel through rock depends on the matrix’s physical structure. So, when the respond come to the acoustic receiver, the amplitude, speed and phase are function of all of the combined matrix densities, interconnections, cementation, fracturing, and porosities within the matrix.

The passive well log will respond to the conditions of the rock as the sources to measure the properties. They may respond to Natural gamma ray emitted by the formation (gamma ray log), spontaneous potential difference of formation and reference (SP log), and changes in borehole size (caliper log).

How do they work?


The well logging companies will operates the logging trucks with computer programs and software to gather the measurement data from the logging tool in real time. The wireline/cable is made of conductor material with outer armor to protect the cable from borehole conditions. As the measurement continue, the cable winch on the truck will be rolled and the measuring tool will move upward, creating the measurement along the wellbore depth. The data either recorded in the truck or transmitted directly from the wellsite to the office. The accuracy of the result also depend on the speed of the logging tools, as the faster upward movement of the logging tools, the result will less accurate.

Those are some overview of well logging job. It needs special persons who understand the fundamental of oil logging to give the correct interpretation, as the logging interpretation quite subjective, depend on experience, proficiency, and care of observer in interpreting the data. The well logging result analysis of every companies will also varies based on their own criteria, programs and experiences. But over all. the safety and economic should be the real consideration since the logging tools have radiation safety issue and need special training, also the cost of data misinterpretation will be greater than the cost of hiring major experienced logging service company.


[CLICK HERE TO DOWNLOAD THE FREE PRESENTATION]


Do you love this post? Thank you! I feel appreciated. This post is under the category of Petroleum Engineering,  you may love to see other similar posts HERE

Irianto Petrus B
Resume

Tuesday, September 1, 2015

Basic Logging Interpretation



Introduction

Logging is one powerful method to measure physical properties of the formation around borehole to make sure whether we are lucky enough to drill through petroleum bearing zone!


 Logging is done by lowering a logging tool into the borehole and recording formation properties while the tools are rolled up to the surface. These logging tools can be divided into broad categories based on the physical properties measured. The tools use variety of sensors to measure different properties such as gamma ray, electrical, acoustic, radioactive responses, electromagnetic, pressure and other properties of the formation rock and fluid. Since the tools are rolled up to the surface while recording the data, the record could be seen continuously as a function of depth. These data will represent a lot of formation properties depend on what properties are the tools measure from the borehole. These data then should be interpreted to get its main goal: to know the condition of the formation and to decide whether the prediction of geologists about the petroleum potencies are right or wrong. That is why an accurate logging interpretation is very important to the life of Geologists, and other engineers who predict the hydrocarbon bearing zone. 

(By saying "logging" here I mean WIRELINE LOGGING, which is very common to be called as "logging", and not MUD LOGGING)

Preview

In this slide, we'll try to understand how the logging result could be achieved. After that, we will talk about the header of the picture, we'll see what kind of logging tools are usually used for every properties in the result. From the result, we'll discuss which location can be interpreted as petroleum bearing zone. And further more, we will see how from this simple picture we may find the petroleum reserve estimation.

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 you might learn about logging interpretation easier, also it will be very useful if you want to teach other about logging interpretation interestingly..


DOWNLOAD THE PRESENTATION HERE

Irianto Petrus Binsardo