ABSTRACT
This experiment is to determine the capacity of oil reservoir if map showing contour lines (isopach) for the area of the reservoir is available. This experiment is carried out using planimeter. Firstly, the planimeter was switched on and one set of contour was chosen to be measured. Secondly, a mark was made on the contour map as a starting point of measurement. Thirdly, the tracer lens of the planimeter was placed exactly on mark and START key was pressed. Fourthly, the tracer was moved in a clockwise direction. MEMO key was pressed p ressed when the tracer was reached the starting point of measurement. The reading was recorded. As a result, 3 the total volume of the reservoir is 740 891 162 ft with the highest volume of reservoir is at 3 deprth of 50 ft which is 120 525 931.50 ft and the lowest reading is at 90 ft which is 107 3 15850.90 ft .
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INTRODUCTION
When discover a reservoir, a petroleum engineer will seek to build a better picture of the accumulation. If an isopach map is available, the volume or capacity of the reservoir can be determined using planimeter. Isopach is a contour that connects points of equal thickness. Commonly, the isopachs, or contours that make up an isopach map, display the stratigraphic thickness of a rock unit as opposed to the true vertical thickness. The planimeter is a simple instrument for the precise measurement of areas of plane figures of any shape. In this experiment, planimeter is used to determine the capacity of the reservoir. The capacity (in terms of volumetric value) can be determined by multiplying the area and the depth/structural elevation of the reservoir. Volumetric estimation is also known as the “geologist‟s method” as it is based on cores, analysis of wireline logs, and geological maps. Knowledge of the depositional environment, the structural complexities, the trapping mechanism, and any fluid interaction is required to estimate the volume of subsurface rock that contains hydrocarbons. The volume is calculated from the thickness of the rock containing oil or gas and the areal extent of the accumulation with these reservoir rock properties and utilizing the hydrocarbon fluid properties, original oil-in-place or original gas-in-place volumes can be calculated. An isopach is a line representing equal stratigraphic thickness, and an isopach map is one that shows bv means of isopachs the variations in true stratigraphic thickness of a stratum, formation, or group of formations. The subsurface isopach map is based primarily on formation thicknesses determined from well cuttings, cores or geophysical logs. Although isopachs must be drawn to agree with thicknesses plotted on the map, their spacing and the nature of thickening and thinning may be guided by other known facts concerning the source of sediments, their relative rates of deposition, truncation, and so forth. An isopach map drawn strictly to the numerical values and without regard to the geologic reasons for thickening and thinning of formations, is likely to present a picture difficult to integrate or reconcile with other geologic facts.
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OBJECTIVE
The objective of this experiment is to determine the capacity of oil reservoir if map showing contour lines (isopach) for the area of the reservoir is available.
THEORY
The geometry of the reservoir is projected onto a map called the isopach map which consists of several contour lines. From this map, the petroleum engineers are able to determine and estimate the volume of the reservoir. This data can be used to determine the available quantity of hydrocarbons in the reservoir.
Isopach and isochore maps are generally used: 1. for predetermining drilling depths to specific horizons in wildcat wells; 2.
To locate buried structures in regions where formations habitually become thinner over structural crests.
3.
In estimating the elevation of a datum bed below the total depth of a well that penetrated a higher known stratigraphic horizon.
4. To calculate the volume of oil in a formation
Two methods will be applied in this lab to estimate volumes from an isopach map. The first will be to use a planimeter to estimate the average thickness within the isopach area. The second method utilizes a software package that includes digitizing the isopach map and applying geostatistics to determine the volumetric. When a map showing contour lines (isopach) is available, it is possible for us to then measure the area of each reservoir. This process is called planimetry. Planimetry measures the area of the property. Therefore, with the area and thickness know,a volume of reservoir can be calculated, and from this,a total tonnage can be deprived. Planimetry can be done by hand using several methods but the one we are using in this experiment is a mechanical device called a planimeter. Planimeter, also known as platometer is a measuring instrument used to determine the area of an arbitrary two-dimensional shape. 3
After the area of each contour is computed, the volume is computed by various techniques. Basic volume calculations can be viewed as dividing the structure into layers of a common thickness. The different techniques define the layer geometry in different ways. The Trapezoid Rule computes the layer volume by computing the average area of the layer from the top and bottom layer and then multiplying the average area by the layer thickness. (Mathematically, this is also equivalent to computing the area underneath a thickness vs. area graph). The Trapezoid Method has historically been commonly used for computing map volumes because of its ease in computing, understandability, and accuracy.
Because isopach maps always have smaller contour areas as the thickness increases, the Trapezoid Method though accurate always slightly overestimates the reservoir volume. Another common volume method is the Pyramid Rule. This volume technique computes the areas of the frustum of a pyramid or cone.
An isopach map illustrates thickness variations within a tabular unit, layer or stratum. Isopach are contour lines of equal thickness over an area. Isopach maps are utilized in hydrographic survey, stratigraphy, sedimentology, structural geology, petroleum geology and volcanology. The gross rock volume or the capacity of reservoir is obtained from the area
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measured from the isopach maps. For measuring a common reservoir contour which is in normal pattern, the perimeter is trace in clockwise direction. The value obtained is the area of the whole space that is within the contour lines. The capacity in terms of volumetric value can be determined by multiplying the area measured by measured by planimeter with the depth elevation of the reservoir.
V = Ah 3
Where: V = Volume of reservoir (m ) 2
A = Area of the contours (m ) h = Depth (m)
APPARATUS
1. Planimeter 2. Isopach Map 3. Ruler
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PROCEDURE
1. The power was switched on. 2
2. The unit was set (cm ) 3. Starting point was marked with x as reference point at each of contour line. 4. Start key was pressed and figure 0 displayed. 5. The tracer point was traced on the circumference clockwise until it meets the starting point. 6. Pressed „Memo‟ key, and then pressed „hold‟ ke y and 3 readings was taken. 7. „Avg‟ key was pressed to get 3 readings average and the value was recorded. 8. Step 4 until 7 was repeated for the rest of the depth.
RESULTS
2
Depth
2
Area (cm )
(ft)
1
0
st
nd
rd
3
Actual area (ft )
Volume (ft )
2
3
Average
86.8
85.8
86.9
86.50
6 318 480.64
0
10
71.3
71.8
71.5
71.53
5 224 981.74
52 249 817.40
20
61.4
60.1
59.9
60.47
4 417 092.77
88 341 855.40
30
51.1
49.1
50.6
50.27
3 672 023.38
110 160 701.40
40
40.4
40.6
40.1
40.37
2 948 867.79
117 954 711.60
50
32.4
33.7
32.9
33.00
2 410 518.63
120 525 931.50
60
25.1
26.7
27.7
26.50
1 935 719.50
116 143 170.00
70
16.5
16.4
17.1
16.67
1 217 677.14
85 237 399.80
80
6.3
6.8
7.2
6.77
494 521.55
39 561 724.00
90
1.7
1.7
1.5
1.63
119 065.01
10 715 850.90
28 758 948.15
740 891 162
Total
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Depth vs Average Area 100 90 80 70 ) t f ( h t p e D
Depth vs Average Area
60 50 40 30 20 10 0 1.63
6.77 16.67 26.5
33
40.37 50.27 60.47 71.53 86.5
Average Area ( ft2)
Graph Depth vs Average Area
SAMPLE CALCULATIONS
Basic formula/conversion:
1. Basic map scale:
2. Conversion from area in contour line map to actual area of geological structure:
( ) (
) ( )
3. Volume determination for every 10 ft of depth
( ) () ( )
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Sample calculation (from 90ft of depth):
1. Taking data from 90ft depth by using planimeter
(
)
2. Volume determination for 90 ft of depth
( ) () ( )
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DISCUSSION
In this experiment, the area of each of the reservoir contour was obtained by using the digital planimeter. The elevation height difference between each of the contour was estimated to be 10 ft each. In order to get the volumes of the reservoir rocks, graphs of area versus depth were plotted. The areas under the graphs were the total volumes of the reservoirs. The area under the graph was divided into a few sections, where the area under the graph was calculated for each elevation of 10 ft before it was all sum up together. The number of contours in each reservoir represents the height of the reservoir. The more line the contours have, the higher the reservoir will be. From the data analyzed, it was determined that the volume of the reservoir is 740 891 3
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162 ft with largest volume of the reservoir obtained at depth of 50ft which is 120,525 931.50ft . From the result, it can be conclude that even the reservoir shape is smaller as the depth increases, it does not mean the volume also decrease with depth, although measured at fixed interval. The volume is depend on the area of the contour. Since the planimeter is a sensitive device, it must be used slowly and the tracing process should be accurate to get a very accurate area reading. There are lots of method in calculating the volume of the reservoir rock and planimetering can be considered as one the accurate ones.
CONCLUSION
In this experiment, the depth of the contour map used are from 0 – 90 (ft). The calculated 2
area was given in (cm ). Then, the calculated area was converted into (ft) in order to get the actual area. The actual area in this experiment was decreased as the depth increased. The volume 3
also decreased as the depth increased. The volume was calculated in (ft ). The volume was calculated from thickness of rock that containing oil or gas and the area extent of accumulation with these rock properties and utilizing the hydrocarbon fluid properties. The highest volume of reservoir at 50 ft which was 120 525 931.50 (ft3) while the lowest reading at 90 ft which was 107 15850.90 (ft3).
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RECOMMENDATION
1. Make sure the scale in planimeter is correct based on studied isopach map. 2. Make sure the table is steady to prevent miscalculation in planimeter. 3. Use planimeter in bright room to have a better tracing on isopach map. 4. Take three (3) reading for each depth and take the average area to diminish human error and random error.
REFERENCES
1. http://en.wikipedia.org/wiki/Isopach_map 2. http://infohost.nmt.edu/~petro/faculty/Engler370/fmevlab4-isopach.pdf 3. http://en.wikipedia.org/wiki/Contour_line 4. http://infohost.nmt.edu/~petro/faculty/Engler370/fmevlab6-isopach.pdf 5. http://www.scribd.com/doc/136465502/Volume-and-Capacity-of-Reservoir-Lab-Report 6. CGE 558 lab manual
APPENDICES
Figure 2.1: Planimetering process (left) and the digital planimeter tools (right).
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Appendix 1
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