Volumetric Reserves Estimation

 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 estima￾tion 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 approxima￾tion 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 delin￾eation 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 perfor￾mance data are available, volumetric estimation provide valuable checks on oil

and gas in place estimates obtained from material balance and reservoir simula￾tion 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

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.

Trapezoidal Rule 

Pyramidal Rule

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.