Unit 9 - Geostatistics

Question 1

What is a population ?

Answer 1

• The properties of a reservoir unit for which the geologist or engineer is required to infer (or estimate or guess) values

Question 2

How do geologists usually estimate the population parameter?

Answer 2

• by an average from a sample

Question 3

What are the most commonly used descriptive statistics that are determined from a sample?

Answer 3

• Measures of central tendency → this means , the tendency of measurements in a sample to centre around a particular value rather than spread themselves across a range of values

Question 4

What kind of variables occur in petroleum engineering?

Answer 4

• Category Variables
• Quantity variables

Question 5

What are category variables?

Answer 5

• any variables that involve putting individuals into categories

these can be:

• nominal (e.g. names such as make of bit or formation)
• ordinal (e.g. a number to represent degrees of bit wear or stratigraphic location)

Question 6

what is the difference if Ordinal data is ranked or ordered?

Answer 6

• Ranked: e.g., bits ranked by relative condition of wear, low= 1 to high = 7
• Ordered: e.g., stratigraphic unit 1 is older than stratigraphic unit 2

Question 7

How can quantity variables be subdivided?

Answer 7

have a numerical quantity that can be either discrete (e.g., fossil specimens, numbers of channels) or continuous (e.g., permeability, porosity, etc.)

Question 8

What are extensive or intensive variables?

Answer 8

Extensive variables: Are additive (e.g., volume)

Intensive Variables: e.g. Permeability, which is normally non-additive. The average of two plugs, e.g., a shaley core plug (1mD) and a clean sandstone (1000mD), combined, would depend on how they were combined.

Question 9

the average permeability depends on what?

Answer 9

• flow conditions
• flow geometry

Question 10

What is an effective property?

Answer 10

• property of an equivalent homogeneous medium.

Question 11

What means Bias? Give examples

Answer 11

• bias is a systematic error in the estimator and may occur due to several reasons:
  1. “Sands” may be sampled more than other lithologies leading to high values for the arithmetic average
  2. It may not be possible to plug very loose sands - often the best reservoir zones - leading to a pessimistic estimate of permeability
  3. Thin low permeability baffles (faults, thin siltstones) may not be sampled leading to optimistic estimates of permeability

Question 12

What is the median?

Answer 12

• the median of a distribution is equal to the value above and below which equal numbers of samples lie (k50 see below).
• the median is most easily determined by ranking the data and determining the middle value
• the median is rather insensitive to the tails of the distribution and, therefore, is often less sensitive to outliers (potential errors).

Question 13

what is the mode of a distribution?

Answer 13

• the most commonly occurring value
• if you have a bimodal or multimodal system, a single mode is not appropiate

Question 14

How do you call a distribution that is not symmetrical?

Answer 14

• skewed distribution

Question 15

How do you define well testing?

Answer 15

as a perforated flowing interval over which permeability is estimated from the analysis of pressure data whilst the well is flowing or once the flow has been stopped.

Question 16

Measures of permeability and variability can?

Answer 16

• Define the level of heterogeneity
• Determine the number of samples required
• Indicate likely recovery process
• Suggest likely flow performance

Question 17

When do you use the term uniform?

Answer 17

• when a single constant property is used

Question 18

When do you use the term homogeneity?

Answer 18

• low variability → Cv<0.5

Question 19

when do you call something heterogeneous?

Answer 19

when 0.5 < Cv < 1.0

Question 20

How do you calculate anisotrophy?

Answer 20

• ratio of vertical to horizontal anisotrophy