RESERVOIR ESTIMATION

If there is one word that describes the oil and gas business is Uncertainty. Oil and gas business is full of risk. Therefore once exploration starts there is lot of stakes involved especially in early stages of the project. When the project starts one of the major questions is how much oil and gas is present? The answer to this question is a billion dollar answer.

Let us look at some of the estimating techniques-

There are 2 techniques used for estimation a) Deterministic b) Probabilistic. We will discuss only deterministic method.

Estimate= Area X Thickness x N*q*S*Rf/G*B   (1)

The variables are

q=Porosity

S=oil saturation in pore space

B=the formation volume factor of the oil

Rf= recovery factor

Every variable in the equation comes with level of uncertainty. Geologist try to eliminate the uncertainty by rightly guessing the values, this done by on ground results obtained from reservoir  and mathematical modeling.

Ground data from wild cat wells include log data, core data, geochemistry of rocks etc. From these measurement a set of values are obtained and fed into mathematical model such as Monte Carlo, Decision trees etc. These software then give estimates for worst, best and high outcome.

Depending upon the drive mechanism and production strategy the recovery factor is general

For Oil- 25-60%

For Gas- 50-80%

If there is very high level of uncertainty with certain parameter then an appraisal well is drilled to reduce the uncertainty. The purpose of an appraisal well is not to find oil and gas but to reduce uncertainty of parameters in equation (1). An appraisal well is terminated as soon as the desired parameters have been obtained. No further drilling and testing of an appraisal well is done once the desired parameter has been obtained.

The main aim of estimation technique is not to find oil and gas but to estimate probability of finding minimum quantity of hydrocarbon.