Software

OxFilet

OxFilet is a FracSys/DOS module to derive the conductive fracture intensity the fracture transmissivity distribution, based on transmissivity measurements in packer and well tests. A Windows 95/NT version of OxFilet is currently under development.

OxFilet uses forward modeling of the well testing process to produce simulated well tests which can be compared against measured well test results. Since the simulated test results are based primarily on assumptions of fracture intensity and transmissivity distribution, OxFilet provides a consistent approach for derivation of these parameters.

The primary advantage of the OxFilet approach is that it recognizes that well tests in fractured rock generally depend on the properties of the fractures which happen to be intersected by the tested interval. These well test results do not reflect the "averaged" properties of a "representative elementary volume." Rather, they represent the specific properties of specific properties. As a result, the well tests results are best used to derive a statistical description of the tested fractures, rather than effective continuum properties.

OxFilet requires the following assumptions to derive fracture properties from well tests:

  • Well interval transmissivity is the sum of "at borehole" fracture transmissivity
  • "At borehole" fracture transmissivity can be related to the effective transmissivity of the fracture at larger scales as a function, incorporating effects of uncertainty and intersecting fractures
  • The spatial pattern and statistical properties of fractures are similar among tested intervals
  • Each fracture is represented only once in well test results

OxFilet derives the conductive fracture transmissivity and intensity by comparing simulated and well test results, and adjusts the assumed intensity and transmissivity distribution to match the measured well test transmissivity distribution.

OxFilet Features

Feature Description
Platform   DOS (FracSys Version 2.601)
  Windows 95/NT (under development)
Approach   Simulation of well test results within DFN model realizations
Transmissivity Distribution Assumptions   Constant
  Uniform
  Exponential
  Lognormal
  Normal
  Power Law (Pareto)
Intensity Assumption   Baecher (Poisson model)
  Fractal model (under development)
Well Test Assumptions   Well Test Transmissivity is Sum of the Transmissivity of Intersected Fractures
  Well Test Transmissivity is a Function of Intersected Fracture Networks
  Well Test Transmissivity is a Function of the Aperuture Field on Intersected Fractures at the Well
Goodness of Fit Criteria   Kolmogorov-Smirnov
  Chi-Squeared
  Moments
Search Algorithms   Conjugate Gradient
  Simulated Annealing
  Interactive Visual Optimization
Statistical Reporting   Well Test Transmissivity Mean, Std Dev. Skewness, Kurtosis
  Percent of Non-conductive intervals
Graphical Reporting   Histogram (PDF)
  Cumulative Distributions (CDF)
   

Algorithm Summary

The input for the OxFilet algorithm consists of:

  • The geometry of the wells from which the data was collected, and
  • The measured packer interval transmissivities.

Based on the packer interval transmissivity information, OxFilet calculates the following packer interval transmissivity statistics:

  • Mean, standard deviation, skewness, and kurtosis of interval transmissivity
  • Percentage of non-conductive intervals

OxFilet obtains an initial estimate of the conductive fracture intensity P10 (conductive fractures per meter) from the percentage of non-conductive packer intervals P0 as (Snow, 1965),

P10 = -ln(P0)/L

where L is the mean packer interval length.

The user then specifies the assumed conductive fracture intensity P10, fracture orientation distribution (Fisher, Bingham, bootstrap, etc) and transmissivity distribution (log normal, power law, exponential, etc.).

The following parameters are used to control the FracSize simulation:

  1. The transmissivity threshold corresponding to "non conductive" intervals
  2. The search space maximum, minimum, and step size for fracture transmissivity mean and standard deviation
  3. The number of realizations to be combined to calculate the simulated packer test transmissivity statistics for a given fracture transmissivity and intensity assumption
  4. The type of search (single point, grid search, conjugate gradient search, or simulated annealing search)
  5. The search criterea (Kolmogorov-Smirnov or Chi-Squared Statistic)
  6. The maximum number of iterations, parameter temperature, and jumping scale (for a simulated annealing search), or the damping factor (for a conjugate gradient search)

Based on the parameters provided, OxFilet generates stochastic fracture realizations, and calculates the packer interval transmissivity statistics and percentage non-conductive intervals.

OxFilet compares simulated packer interval transmissivity distributions against in situ measured test results using either Kolmogorov-Smirnov or Chi-Squared statistics. OxFilet can search for the fracture transmissivity distribution and conductive intensity with the best goodness of fit measures using either a conjugate gradient or simulated annealing search.

FracMan Program Modules