Monday, October 5, 2015

Skin Zone




In this article, we will discuss the graph above, whose Y axis displays pressure and X axis displays distance from well.  Rw represents the radius of the well, Re represents the distance of the reservoir limit from our well, while Rs is the distance of the skin zone from the well. The picture explains about the skin zone which is located near the well, created as the result from damages of our interventions to the formation. This skin zone will give vast amount of problems, as in the picture, the pressure will alter to be smaller near the well. But, the skin itself does not always make the pressure become lower. The skin also may be engineered to make the permeability near the wellbore become even higher than the natural permeability, thus increase the pressure higher than what supposed to be naturally. That might be the method to solve the problem which may be seen from the picture, as the higher pressure near the well will be advantageous for us. But first, let’s see why these conditions may be happened to our well.

The concept of skin zone was first proposed by Van Everdingen and Hurst in 1953, stating that the permeability may be reduced near the well due to damages during drilling and completion. This skin zone may be obtained from well test analysis such as pressure drawdown analysis or pressure build-up analysis. Skin factor play a key factor in the calculation. Skin factor is a number used to analytically model the difference from the pressure drop predicted by Darcy's law due to skin. The formula leads to the general results: if permeability near the wellbore altered to be lower than natural permeability, the skin factor will be positive, if permeability near the wellbore altered to be higher than natural permeability, the skin factor will be negative. If the permeability does not alter, the skin factor is 0.


The damage might come from a lot of things. In drilling, the mud particle & filtrate invasion and cement filtrate might be the reasons. Perforation problems, such as the perforating fluid, compacted zone, and perforation debris also have contributions.

Although the welltest analysis may find out the skin zone, it is not only determining the alteration because of drilling and completion damages such as stated by Van Everdingen and Hurst. The result such as shown in the picture is a composite parameter. Other parameters are borehole deviation, non-Darcy effect, partial penetration of well completion, reservoir shape, perforating, and perforation geometry. Each of these factors may be calculated partially using other data that we have beside the welltest result, thus we may find the biggest contributor of our skin problem.

Deviation angle may contribute in the skin factor calculation. The permeability of the rock is different horizontally and vertically, thus in a deviated well, the result of permeability will consist of vertical permeability and horizontal permeability. The result will be different in vertical well, where the horizontal permeability is the only one accounted.

Why we really concern about this skin zone? What will happen when the pressure near the wellbore become lower than what it should be?

The skin zone will bring quite a headache because it will decrease the pressure near the wellbore. As a common sense, the fluid will flow naturally from place with higher pressure to lower pressure, and the lowest pressure in this production system will be at the storage, as close as atmospheric pressure. Every places which are flown by the petroleum should have higher pressure than this atmospheric pressure, with the highest pressure is in the reservoir, so the fluid will flow from reservoir to storage. Unfortunately, the reservoir pressure cannot be maintained stable for years. The reservoir pressure will be depleted, and also the pressure near the wellbore.  As the pressure difference between the bottom hole and wellhead become smaller, the rate of production become smaller, until nothing can be produced and we need artificial lift such us pump to add pressure near bottom hole.  This condition might be explained with Flow Efficiency. Flow efficiency/ Productivity Ratio/ Completeness Ratio is the ratio between pressure drawdown with formation damage to without formation damage.


When the flow efficiency is high, then it might be implied that the skin is insignificant and the production might be accomplished as predicted.

When the skin zone has lower pressure, it might be expected that we will need use the pump sooner than scheduled.  We will need to have extra expenditure faster, and even worse, we need to abandon the well faster than it should be. We may see that this problem is very distinctive and need professional handling.

There are some efforts we may do to make things better. Hydraulic fracturing and acidizing might fix this skin zone to the level of natural permeability, or even higher, thus the skin factor might be less than 0. Hydraulic fracturing will be used to bypass the skin zone and create fracture which will help the fluid flow easier to the well. Acidizing might be used to try to remove the damaging substances, such us the filtrate from mud.

In a conventional homogeneous reservoir, there are some common skin factor values to describe the condition of the well. Skin factor ranged 0-2 means slightly damaged, 2-5 means medium damage, 5-10 means significant damage, and above 10 will be a serious damage. In a fractured reservoir, a non-damaged reservoir will have skin value of -3 since the permeability have been increased as the fracture created. 


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Sunday, October 4, 2015

Volumetric Reserve Estimation: Isopach



Picture source: Permadi,A.K.2004.Diktat Teknik Reservoir 1.Bandung

This picture shows the isopach map, a map which is used in original hydrocarbon in place calculation, namely volumetric estimation. Original hydrocarbon in place is the hydrocarbon which is stored in reservoir prior to production. By this definition, the hydrocarbon in place will not show how much volume of hydrocarbon we may obtain from the formation. Although the original hydrocarbon in place does not show how much we can gain, There is thing called Recovery Factor, a percentage range about how much hydrocarbon we might obtain typically, whose value depend on the drive mechanism of the reservoir and the reservoir fluid type. We will see why isopach map is important here, in volumetric reserve calculation.

The volumetric method is a method to count the volume of hydrocarbon from geological, log, or core data. This is a good estimation method before the production data can be achieved. This method is very sensitive to the accuracy of geological data, also often leads to overestimation of reserves, for it does not consider the heterogeneity of the reservoir, as the geological data are obtained from samples, which may not represent the average properties of the reservoir. From the data, then a isopach map may be created.

First, it is very important to have logging data or seismic data, from which we can get the shallowest and deepest depth on the well where the hydrocarbon can be found. These depths are reservoir top and bottom. By using planimeter, the area of limit can be estimated, and the area difference of them in every depth elevation implies the reservoir area.

Then, we focus on the isopach map picture above. This method is the simplest way to estimate the initial hydrocarbon in place. As the result from planimeter has been obtained, we need to make the map into a form  of trapezoid pile.  the trapezoid will be created in interval of depth, in this case 5 ft depth. the top 5 ft of the reservoir could simply be assume like a “cone” or “pyramid”. Since the height of every interval is constant, then difference will be about the area of trapezoid above and below the focused trapezoid.

If the Area difference between between the trapezoid we measure and the trapezoid above it is less than 0.5 (this mostly happened in the top intervals), then the formula would be Pyramid bulk volume



If the Area difference between the trapezoid we measure and the trapezoid above it is more than 0.5, then the formula would be Trapezium bulk volume.

Let's see from the picture as the example. In case we supposed to know the volume of the lowest trapezoid, then we could see from the picture that the area of trapezoid we measure would be the area of OWC (oil water contact). The trapezoid above it will be the reservoir area 5 ft above the OWC, and next we need to divide this reservoir area with the area of OWC. If the result is less than 0.5, we apply pyramid bulk volume, or if it is more than 0.5, we apply trapezium bulk volume.

The basic of this explanation is for the purpose of volumetric estimation, thus Petroleum Engineer could understand as far as the usage and the basic method. Geologists are the specialists who are focused in either isopach map or the planimeter itself in detail. 


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Saturday, October 3, 2015

Binary Cycle of Geothermal Steam




Binary cycle of geothermal plant is a very fascinating geothermal engineering which utilize the geothermal energy to the extent of zero withdrawal from underground. It is a kind of method which might be dreamed by people concerning the environmental issues, such as pollution, spill, and perhaps, the answer for renewable energy. Indeed, the geothermal energy itself is a clean energy that should be developed, but one special feature of the binary cycle is that the temperature of water in the reservoir doesn’t need to be as high as usual geothermal energy. Most of the time, the power plant for geothermal energy will need steam with temperature of 200 deg C, but by using this scheme, the reservoir fluid will only need as hot as 100-200 deg C. Among other renewable energy that can be used, perhaps geothermal energy should be considered more since the efficiency and technical knowledge are simply modifications of the system of petroleum production.

 Looking at the picture (red->orange flow), we might see that the production well gain the fluid from geothermal reservoir, pumping it to the surface, thus the fluid will flow to the heat exchanger afterwards. One distinctive thing of geothermal from petroleum production is that the tools should be made by heat resistant materials. Also the pipes should be insulated to prevent the excessive heat loss. In heat exchanger, the geothermal heat will be transferred to the organic fluid in the other pipe. This is the main key of the binary cycle. The binary cycle uses the organic fluid whose boiling point is lower than water boiling point, so by using the heat provided by the water from geothermal reservoir, the organic fluid will change to vapor. This is the reason why this binary cycle is proper for medium temperature geothermal reservoir. When it is very hard to get the vapor from the water to go through the turbine, simply use this organic fluid instead. After flowing through heat exchanger, the water, which has been cooler, will flow to injection well. Sometimes, the injected formation will be drilled on lower elevation location, making the formation fluid flow naturally easier because of gravitation, thus needs less pumping power.

On the other system of heat exchanger (yellow-orange flow), the organic vapor will flow to the turbine. This turbine works from the energy of this organic vapor. There are spinning blades that turn when vapor blows past them. The turbine will turn electricity generators at incredibly high speed. This is another superiority of geothermal energy than other renewable energies: that the steam turbine rotates at 1800–3600 rpm, which is 100-200 times faster than typical wind turbine.  Also, about the turbine, we may see another advantageous reason to use the low boiling point organic fluid instead of water in medium temperature geothermal reservoir. Water induction into steam turbines may create bunch of problems, Even a small amount of water may lead to huge damage on the turbine blades, the cylinders, and the housing. The turbine will make the generator produce electricity, and the electricity can be used by people afterwards. 

After the turbine, the organic vapor will flow through the condenser. Condenser is another “heat exchanger”, which is used to exchange the heat from the organic fluid flowing from turbine and the water from cooling tower. But that is not the main function. The purpose of the condenser is to create the vacuum pressure (lower than atmospheric pressure). As we know, vapor will consume more extensive volume than liquid. When the heat exchange happened between vapor and water from cooling tower, the vapor will condense, thus decreasing in volume. Since the volume decreased, the condenser will have less pressure. This vacuum pressure in condenser is very important to make the energy transfer in the turbine become efficient, since the energy transfer in the turbine is related to the pressure difference of inlet and outlet of the turbine. In this binary cycle, the surface condenser, in which the vapor does not directly contact the cooling fluid, should be used. The cooling water itself should be obtained by water sources near the site, might be rivers or lake. Most of the time, these sources do not provide enough water to absorb all the heat. More common method is by utilizing cooling tower, either mechanical draft cooling water, in which the heat transferred to the air induced by fans; or natural draught cooling water, in which the heat transferred to the air induced by the shape and height of cooling tower.

The organic fluid then flow to the heat exchanger to gain heat from the formation water again and the cycle continues. It might be seen that both the formation fluid and organic fluid cycles in a closed loop, thus we may conclude that this binary cycle of steam is very environment friendly energy. This method has been successfully applied in Parantuka, Kamchatka Peninsula, and Otake.

One noticeable problem has been observed on this method usage is that the implementation should be done on medium temperature reservoirs instead of the high temperature one, since further intensive assessment on the heat resistance of organic vapor and heat exchanger should be done on the high temperature reservoirs. 


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