Mining for Petroleum

Although Mining Engineering and Petroleum Engineering are two different fields of study, the two fields cross paths when it comes to the Oil Sands Industry. Oil Sands refers to the deposits of petroleum in the form of heavy bitumen being present in a mixture of sand or loosely consolidated sandstone. Because this heavy oil is too thick to flow and because it is thoroughly mixed with sand, the conventional methods of extraction of petroleum by drilling cannot be applied here. For this reason and also if the oil sand deposit lies relatively close to the surface, open pit mining is used to extract the oil sands. Once mined, the sands are processed to produce synthetic oil.

The mining for oil sands begins like in most mines by the overburden removal process. Common machinery such as excavators, bucket scrapers, and bulldozers are used to remove the overburden and the overburden is hauled by trucks and piled separately for refilling purposes. Once the overburden is removed and the oil sand layer exposed, the mining process begins. Since the oil sand deposits are unconsolidated, they can be easily be extracted by shovels or excavators. For this reason and for obvious safety reasons blasting is not performed. In addition to these machines, draglines and sometimes continuous excavators such as bucketwheel excavators are also used for mining the oil sands. The mined sand is hauled to a plant where the petroleum is extracted and refined by a variety of processes including crushing, froth flotation and distillation.

Although previously termed as an unconventional source of petroleum, the depletion of "conventional oil" and the associated rise in oil price has resulted in a shift of attention towards the oil sands industry. The largest oil sands deposits are located in Canada and Venezuela and the total volume of oil contained in the sands exceeds the conventional oil reserves of the world.  Oil sands have been commercially extracted in the Athabasca region in Canada since the 1930s and since have developed along with the advances in extraction technology. The following graph shows the value of sales  of oil sands and conventional crude oil in Canada and it can be seen that the oil sands sales have shown a significant rise during recent years.

value of sales of crude oil and oil sands (data taken from : http://www.capp.ca/library/statistics/Pages/default.aspx)

New methods have been developed to extract oil sands deposits that are too deep to extract using surface mining. These methods involve the reduction of the viscosity of the bitumen and the separation from the sand and enables the oil to be pumped to the surface. These methods are termed "In-situ methods". Cyclic Steam Stimulation (CSS), Steam Assisted Gravity Drainage (SAGD) and Toe to Heal Air Injection (THAI) are some of these methods. With advances in in-situ methods like these, the the recovery percentages of oil sands deposits have increased and now it can be considered as a viable alternative to conventional oil.

Limestone Mining in Pictures

In this post I have attempted to summarize the limestone mining process in pictorial form (and a video). Short descriptions have been included where necessary.

Overburden Removal

once the overburden is removed, the beds are cleaned and are prepared for drilling

Drilling

drilling is performed according to a pre determined pattern to accomodate explosives for blasting

Blasting

Rock Blasting is performed to fragment and loosen the consolidated limestone.

Loading and Hauling

Dispatching 

Tyre Maintenance of Mine Machinery

The performance of machinery and their availability plays a major role in the productivity and output of a mine. For this reason, every aspect of the machines should be properly maintained. The tyres of these machines are what keeps the machine in contact with the ground and bears the weight of the machine. While tyres for mining machinery are designed to withstand a beating, incorrect maintenance can reduce lifetime of the tyres, or ultimately lead to catastrophic accidents. Therefore, the proper maintenance of the tyres is as important as the maintenance of any other component of the machine.

Handling large tyres of the gigantic mine machinery is not an easy task. Even a simple task such as changing the tyre is difficult and results in a considerable amount of downtime. Therefore the best strategy in this case is preventive maintenance. This means checking tyre inflation daily, ensuring that valve caps are on, checking tread depth and removing stones and other debris lodged between the treads.

changing tyres is a difficult task

When it comes to daily maintenance, the first thing that comes to mind is the inflation pressure. Both over-inflation and under-inflation are detrimental to the tyre, and therefore, it must be ensured that the inflation pressure lies within the accepted limits. It should be noted that when running the machine the tyre pressure increases by about 10-20 percent. Therefore, this must be taken in to account when setting inflation pressure bounds. Tyres kept in good condition with the proper inflation will last longer and perform better.

External conditions of the mine also affect the tires. The road condition for example is what determines how fast the tyres will wear out. While it is not practical to maintain spotless roads, roads should be kept clear of scattered rocks and sharp pits as driving over these can cause a crack in the rubber, and subsequent running will cause it to propagate and destroy the tyre. Therefore clearing fallen debris off the roads and maintaining the road surface is also a part of tire maintenance. Wet rubber cuts better. therefore it is best to avoid wet road conditions. While most haul roads in mines are watered regularly to minimize dust, it must be ensured that the roads don't get too wet.

Bad road conditions can cause cracks to develop

The handling of the machines and their usage are two other areas that need to be considered. Driving too fast, especially in the case of dump trucks can cause several types of tire damage. In the case of heavy machinery used in mines, a combined parameter combining both speed and weight is assigned to tyres. This parameter is called the TKPH rating (Ton Kilometer per Hour). The TKPH rating should be considered when selecting and using tyres. When loading dump trucks, they should be loaded symmetrically so that the load is distributed over tyres equally. asymmetric loading can cause uneven wear and later result in more severe damages. It should also be noted that shear forces generated when steering the trucks while stationary at the loading points could cause ruptures in the tyres.

When dealing with damaged tyres, it is always best to attend to it them at the start of the problem. Neglecting small faults like cracks and running with them can cause these small faults to quickly propagate and result in serious and irreversible damage. When a crack appears it can be removed altogether by cutting around it. This however has to be done with proper equipment and knowledge of the tyre. When replacing tyres, it is always best to replace it with a tyre of similar amount of wear.

In summary, daily maintenance goes a long way in increasing the lifetime of the tyre. Inflation pressure must always be within the specified limits. The road surfaces, loading and dumping points should be free of fallen debris and excessive water, because wet rubber gets cut better. payload weight and hauling speed must be controlled. The TKPH rating assigned to the tyre must be adhered to. Proper handling of the machinery such as symmetric loading, avoiding steering while stationary and avoiding excessive tyre spinning will reduce the likelihood of damages. when repairing, the first stages of the problem is always easy to cure.

Jet Skiing at Unawatuna

Unawatuna is known for its pristine beaches and is one of the most popular tourist destinations in Sri Lanka. For thrill seekers going on holiday to Unawatuna, The Unawatuna Water Sports Club offers a wide range of water sports. On my recent visit to Unawatuna I tried Jet skiing. Having ridden a jet ski on a river before, this was a totally different experience. The waves and undulations of the sea makes it much more exciting.

Located in front of the Cormoran Beach Club, the friendly staff at Unawatuna Water Sports Club will guide first timers. A safety jacket is provided and an instructor will accompany you if needed. The Jet Skis at Unawatuna Water Sports Club are easy to operate as you only have to accelerate and steer. It only takes a while for you to master the controls and start skiing like a pro.

In addition to Jet Skiing, Unawatuna Water Sports Club also offers, Surfing, Sailing, Banana Boat Rides, Doughnut Rides and Kayaking.

Turquoise Water in a Limestone Mine

Clear turquoise water is often associated with tropical beaches. An open cast mine is probably the last place one would expect to find clear blue or turquoise water. The pictures below were taken at the Aruwakkalu Limestone mine in Puttalam Sri Lanka.

When limestone is excavated from the base of the quarry, and when the excavated pit reaches the ground water table, water starts seeping in. It is this water that has a spectacular turquoise colour. The reason for this colour is not the blue reflection of the sky like in the case of seas and lakes. It is due to the scattering of light by minute Calcite crystals.

Limestone is essentially Calcium Carbonate and the water in the limestone pit contains a large amount of it dissolved. Once the water gets saturated by calcium carbonate, it starts to crystallize and forms very tiny crystals which remain dispersed in the water. This crystalline form of calcium carbonate, is known as Calcite. These micro Calcite crystals scatter light selectively giving it a turquoise colour.

Since Calcium Carbonate is alkaline, this water has a high pH value. Therefore, as inviting as it looks, taking a dip in this water is not advisable. Prolonged exposure to water with a high pH can cause skin and eye irritations.

The Intricacies of Mud Crack Formation

Patterns found in nature are most often astoundingly complex while also being elegant and aesthetically pleasing. Snowflakes for example consist of extremely delicate, symmetric structures of frozen ice and are always different from each other. Yet these infinite patterns are influenced by only two main controlling factors, temperature and humidity. A  more obvious example of patterns in nature are mud cracks. Although they are not as delicate and complex as snowflakes, they also exhibit some patterns that are interesting to observe. What follows are some facts I gathered from a small study about mud cracks and some  photographs taken during my training period at the Aruwakkalu limestone mine.

Mud cracks are formed quite obviously, when  mud dries up. So the main factor controlling mud crack formation is moisture content. Unlike in glass and many other materials, cracks in mud do not originate at single point and then propagate. If the rate of drying is uniform across a surface of a layer of mud, cracks will start appearing simultaneously across the surface. Mud starts to dry from the exposed surface. So when the moisture content of the surface layer starts to drop, the mud particles come close to each other as the space occupied by water is emptied. This results in the contraction of the surface layer. This causes stresses to build up on the surface layer and ultimately results in the formation of cracks. First the main cracks divide the exposed surface into a series of "islands" then, secondary cracks may occur within these islands.

primary and secondary cracks

Since drying starts from the surface, the bottom layers still contain some water and still remain expanded. Therefore the width of the crack is lesser in bottom layers, like a cut made from the surface. As the moisture content further reduces and the bottom layers also dry up, the crack propagates to the bottom layers and the gap widens. 

If the moisture content in a single layer of mud differs at the top of the layer and the bottom, the amount of contraction also differs. The top portion will contract more than the bottom portion. This will result in an upward curving of the layer like in a bimetallic strip. If the wet mud has been standing for a long period, stratification of different layers will take place. Stratified layers can be observed when a portion of the cracked mud is viewed sideways. Stratified layers can have a bigger difference in contraction rates, and if the layers are strongly adhered to each other, a more prominent curving of the layers will be seen.

stratification and upward curving of layers

If the layers are not strongly adhered, instead of curving upwards, the layers will separately contract producing a step like effect of the cracks. 

step like contraction

Although the mechanics of mud crack formation has no immediate practical application, crack formation in other substances is studied extensively. Crack formation on thin surface layers in electronic circuits can lead to peeling off of printed components and cause problems. Therefore a lot of effort is put into preventing the formation of such cracks. A very fine network of small cracks can be seen on the surface of old paintings on close inspection. This fine set of cracks is called "craquelure".  The characteristics of these cracks depend on the type of pigments, oil and also the ambient conditions at the time of painting. For this reason, these cracks are used to authenticate old paintings. 

Creating a 3D Terrain Model using Surfer

A three dimensional model of a particular land feature can be very useful for visualization. A 3D model which can be manipulated in a digital environment is even more useful, especially if volume calculations need to be performed. A good software package for this purpose is, "Surfer". It has a relatively simple user interface and is easy to learn.

Essentially what Surfer requires to create a 3D surface model is a set of spot heights along with their northing and easting coordinates. The data can be imported into Surfer by many ways, of which using a MS Excel file or an ASCII text file are simplest. The software will interpolate the data points and create a surface. The algorithm used for interpolation can be changed according to the requirement.

Obtaining spot heights of the area can be done by performing a level survey. If high accuracy is not required, a quick and easy method of obtaining spot heights is to overlay a grid with known coordinates over a topo sheet with contours. Using the contours, the heights of the vertices of the grid can be determined. Since the northing and easting of the grid are also known, it can be fed into Surfer via MS Excel Another method of creating a 3D model using Surfer is to directly import a DEM (Digital Elevation Model) into the software. DEMs can be downloaded from the USGS website.

A simple volume calculation based on spot heights obtained by the gridding method is illustrated below.

First, the grid was drawn over the contour map in the AutoCAD environment. In this example, the contour interval is 2 m, and the grid spacing has been selected as 3 m.  

The spot heights were found by using the contours closest to the vertices of the grid and the data was entered as x, y and z coordinates into an MS Excel sheet.

Using Surfer, the excel sheet was input and a grid file was created. (Grid>Data). The gridding method used for this example is "Kriging". Other methods can be used depending on the requirement.

A 3D surface was created using the grid file previously created.

This 3D surface can be used for volume calculations as well. For example, Consider that the trough indicated in the above terrain model needs to be filled with water, up to the 8m contour level and it is required to calculate the volume of water required.

This means we need to calculate the volume between the modeled surface and the plane z=8. To do this, the volume calculation dialog box was opened (grid>volume..) and the upper surface was given as z=8 and for the lower surface, the grid file was selected. The calculated positive volume indicates the volume of water required to fill the trough. In this example the calculated volume is, 696.73 cubic meters.