Overview of Reserve Estimation
The estimation of hydrocarbon reserves for a producing field is a process that
continues throughout the entire life of the field. This process is usually associated
with some level of uncertainties in calculating the reserves. These reserves estimation methods are affected by the reservoir type, sources of reservoir energy (drive
mechanism), quantity and quality of the geologic, engineering and geophysical data,
the assumptions adopted when making the estimation, available technology, the
experience and knowledge of the evaluator(s). The oil and gas reserves estimation
methods can be grouped into the following categories: analogy, volumetric, decline
analysis, material balance calculations for oil and gas reservoirs, and reservoir
simulation.
The selection of appropriate method to estimate reserves and resources, and the
accuracy of the estimation, depend largely on the following factors: The type,
quantity, and quality of geoscience, engineering, and economic data available for
technical and commercial analyses, the complexity of the formation geology, the
recovery mechanism, the stage of development, and the maturity or degree of
depletion. More importantly, reserves and resources assessment rely on the integrity,
skill and judgment of the experienced professional evaluators (PRMS
Guideline 2011)
In the early stages of development, reserves estimations are restricted to the
analogy and volumetric calculations. The analogy method is applied to reserves
estimation by comparing factors for the analogous and current fields or wells. This
implies that in analogy method, the reserves are estimated on the basis of a
relationship of resemblance or equivalence between two fields. This method directly
compares a poorly or newly discovered reservoir to a known reservoir that has
similar geologic and petrophysical properties such as lithology of the formation,
depth, porosity to mention a few. Hence, the accuracy with this method is the least
among other methods of reserve estimation.
Furthermore, a close-to-abandonment analogous field is taken as an approximation to the current field. This method is the most useful technique when running the
economics on the current field; which is supposed to be an exploratory field
(Petrobjects 2003).
3.2 Volumetric Method
The volumetric method is probably the easiest method used by engineers to estimate
reserves. It requires a limited amount of data for the estimation, this implies that
immediately after discovery of the hydrocarbon accumulations, during initial delineation and development of a field, the volumetric method is the key to hydrocarbon
volume estimation. Reserves estimation is often high with this method, because it
does not consider the heterogeneity of the reservoir and it includes the undrained
compartments that do not account to flow and are included in making up the bulk
rock volume of the reservoir or accumulation. At this stage, the level of inherent
error can be reduced if the reservoir is accurately described or characterized.
3.2.1 Errors in Volumetric Method
Volumetric method is subject to considerable error because it is often used to
evaluate reserves when little data are available; it requires the estimation of the
reservoir rock and fluid properties and the reservoir volume from spot measurements
of the properties that are then applied to the entire reservoir. The porosity and
saturation are measured either from core samples or logs that are measured from a
small portion of the reservoir and under best circumstances, it only approximates the
condition in the reservoir. The areal extent of the reservoir is rarely known until
many wells are drilled while the volume is estimated using zone thickness measured
at one or more points in the reservoir. The volumetric method is only seen as a gross
estimate of oil or gas in place.
Application of Volumetric Method
• The volumetric result is useful in reserves estimation of the initial oil and gas in
place.
• The volumetric result is useful in reserves estimation of oil and gas in place at any
time of depletion.
• Volumetric estimation is useful during the development period before reservoirs
limit have been defined.
• Later in the life of the reservoir, when reservoir volume is defined and performance data are available, volumetric estimation provide valuable checks on oil
and gas in place estimates obtained from material balance and reservoir simulation methods.
The volumetric method is a straightforward approach which requires determination
of the areal extent of the reservoir or bulk volume (calculated as area times pay
thickness), the rock pore volume, and the fluid content within the pore volume to
calculate the amount of hydrocarbons-in-place. The ultimate recovery can thus be
estimated by applying an appropriate recovery factor. Each of the variables used in
the volumetric reserves calculation above has inherent uncertainties, and when
combined; cause significant uncertainties in the reserves estimate (Petrobjects
2003). Therefore, the following steps consist the volumetric method of reserves
estimation:
Step 1: Determination of hydrocarbon rock bulk volume (hydrocarbon saturated
portion) from area and thickness (isopach map). Explanation of this method is
presented in the next page.
Step 2: Determination of average porosity either from core analysis or well logs.
From core analysis
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Calculation of Reservoir Bulk Volume (Table 3.1)
The volumetric method of reserves estimation largely depends on the bulk volume,
calculated as follows:
(a) Prepare a structure map with contours from top to bottom of the reservoir, in
subsea depths
(b) Mark out a small square on the map e.g. (10 cm 10 cm). Use the scale on the
map to determine the area of the square in acres. Planimeter the square and
determine the area in planimeter units. Then determine the planimeter constant in
acres/planimeter unit by dividing the actual area in acres by the area into
planimeter units. Use the planimeter constant to covert the areas of the map
from planimeter units to acres.
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Trapezoidal Rule
Pyramidal Rule
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To calculate the bulk volume of the reservoir from Isopach or contour map, there
is need to understand the concept of contouring which can be defined as the process
of tracing contour lines on the surface of the earth. This is not only applicable to
petroleum engineers but contour survey is also carried out at the beginning of any
engineering project such as a road, a railway, a canal, a dam, a building etc.