Deterministic Versus Probabilistic Volumetric estimation reserve

The aspect of uncertainty in hydrocarbon reserves estimation cannot be

overemphasized since the estimation of reserves is done under conditions of uncer￾tainties. There are basically two methods of returning the results of reserves estima￾tion for any of the techniques such as volumetric, material balance, decline curve etc.

employed for reserves estimation. These methods are the deterministic and proba￾bilistic methods. Thus, if a single best estimate of reserves is made based on known

geological, engineering and economic data, the method is called deterministic whose

procedure is to select a single value for each parameter to input into an appropriate

equation (volumetric, material balance, decline curve etc.), to obtain a single answer.

In volumetric method, all input parameters are exactly known and variability is

sometimes ignored.

On the other hand, when the known geological, engineering, and economic data

are used to generate a range of estimates and their associated probabilities; the

method of estimation is called probabilistic. This method is more rigorous and less

commonly used; it utilizes a distribution curve for each input parameter and through

the use of Monte Carlo Simulation. In this method, all input parameters are not

exactly known and variability cannot be ignored.

Since the oil and gas business is associated with some inherent uncertainties, it

implies that a quality control and assurance should be made before making any

decision to develop the hydrocarbon prospect because a wrong evaluation of the

hydrocarbon initial in place leads to a wrong decision which in turn leads to an entire

failure of the field development. However, a comparison of the deterministic and

probabilistic methods can provide quality assurance for estimating hydrocarbon

reserves. This means that when the values of the reserves calculated deterministi￾cally and probabilistically agree with minimal deviation or tolerance of error, then

confidence on the calculated reserves is increased. On the contrary, when there is a

significant difference in value, then the assumptions made need to be reexamined.

A Monte-Carlo technique is employed to evaluate hydrocarbons in place where

each input parameter required for the reserves estimation are represented by statis￾tical distributions. Monte-Carlo methods are mainly used in three distinct problem

classes, such as optimization, numerical integration and generating draws from a

probability distribution. There are basically five types of statistical distribution used

with this method. 

 

What is a Contour

 Contour is an imaginary line on the ground surface joining points of equal elevation

or a line on which every point is at the same level above or below a chosen reference

surface. In most maps, the reference surface is sea level. This line on the map

represents a contour and is called contour line.

Therefore, a map showing contour lines is known as Contour map. Contour maps

are one of the most effective means of displaying information about the geologic

structure (i.e. the degree of buckling and faulting of the layers) of an area. A contour

map gives an idea of the altitudes of the surface features as well as their relative

positions in the plan. A map showing structure contours for a certain rock layer

throughout an area is called a structure contour map (Fig. 3.1). Such maps are used

to illustrate the size, shape and location of geologic structures.

Contour lines are drawn as fine and smooth freehand curved lines. Sometimes

they are represented by broken lines. They are inked in either in black or brown

colour. A drawing pen gives a better line than a writing pen and French curves

should be used as much as possible. Every fifth contour is made thicker than the rest.

The elevation of contours must be written in a uniform manner, either on the

higher side or in a gap left in the line. When the contour lines are very long, their

elevations are written at two or three places along the contour. In the case of small￾scale maps, it is sufficient to figure every fifth contour. Therefore, the constant

vertical distance between two consecutive contours is called the contour interval.

The contour interval is constant between the consecutive contours

Methods of Contouring

There are basically two main methods of locating contours; these are the Direct

Method and Indirect Method.

Direct Method

This method requires a lot of time to be invested in searching for points of the same

elevation on the ground surface. This implies that it is very slow and tedious but it is

the most accurate method of contouring, thus suitable for small area and where great

accuracy is required. In this method, the contours to be located are directly traced out

in the field by locating and marking a number of points on each contour. These

points are then surveyed and plotted on plan and the contours drawn through them

(Fig. 3.2).

For a radial line, temporary benchmarks are first established at the centre and near

the ends of the radial lines. The contour points are then located and marked on these

lines and their positions are determined by measuring their distances along the radial

lines. They are then plotted on the plan and the contours drawn by joining all the

corresponding points with the help of a plane table instrument (Fig. 3.3).

3.3.1.2 Indirect Method

In this method, the points located and surveyed are not necessarily on the contour

lines but the spot levels are taken along the series of lines laid out over the area. The

spot levels of the several representative points representing hills, depressions, ridge

and valley lines and the changes in the slope all over the area to be contoured are also

observed. Their positions are then plotted on the plan and the contours drawn by

interpolation. This method of contouring is also known as contouring by spot levels.

Conversion from Planimeter Unit to Field Unit

For a map scale of 1:10,000

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.