Summary of recent work on the project

Fracture propagation modeling:   Aspects of a semi-analytical model describing propagation of a flat lying hydraulic fracture were refined.  The model includes a closed-form analytical solution for the growth of a flat-lying hydraulic fracture with no leakoff.  The results are a set of expressions for the driving pressure, length and aperture as power functions of time.  Similar power functions have been derived for hydraulic fractures with different geometries.  A numerical procedure was developed to solve an expression for leakoff, and the analysis was implemented as an EXCEL spreadsheet.  The analysis is easy to use and should facilitate the design and interpretation of shallow hydraulic fracturing procedures.

Fracture propagation field work:  Field tests of hydraulic fracture propagation continued from last year.  The equipment required to make hydraulic fractures was unavailable for most of the year, so the activities were suspended for that time.  A new set of fracturing equipment has been made available recently and we shaking down the functionality of the new equipment.  The equipment appears to be functional and we have resumed field tests.  Jim Richardson is leading the field tests.   

Flow to a well intersecting a fracture (water):  Field experiments of water flow to a hydraulic fracture were initiated last year.  The research was suspended for most of the past year, but they have recently been reinitiated.  The current tests will provide baseline data with which we can evaluate the effects of hydraulic fractures on well performance.  Cindy Agee is conducting these tests. 

 Flow to well intersecting a fracture (air):  Considerable progress was made on both field experiments and theoretical work describing air flow to a hydraulic fracture, which will have applications to vapor extraction methods.   Two field facilities have been built which contain control wells and wells containing hydraulic fractures.  Each site currently contains one control well, and two wells containing hydraulic fractures of different grain size.  The facilities are underlain by saprolite derived from different parent rock.  We expect that the results from tests at this site will provide data on the effects of permeability (grain size) of material filling fractures, and the effects of parent rock type and foliation on air flow. 

Those data will be used with the theoretical analyses described below to obtain insights into the effects of hydraulic fractures on air flow.  Detailed measurements of air pressure will be the primary data set that will be used to compare to the theoretical analyses.  To this end, we have found that conventional methods for creating multi-level pneumatic piezometers are cumbersome and fall short of the data resolution that we want for the theoretical work.  As a result, we have been developing new devices for creating multi-level pneumatic piezometers.  A recent test of one design has given encouraging results, and we have an alternative design that will be tested soon.  

The field work is supported by the development of several analytical solutions to the pressure in the vicinity of a conductive circular fracture.  One solution considers the fracture as a uniform flux source, which gives a simple approximation to a circular fracture.  An semi-analytical solution has been developed recently that considers the fracture as a disk of arbitrary radius and pneumatic conductivity.  This analysis uses the pressure distribution around a ring-shaped source, which uses a complete elliptic integral of the first kind.  The ring-source solution is integrated over the disk using a mass balance expression for air flow through a permeable slot.   A solution to this problem has been derived, but a detailed evaluation is planned for next year.  Additional theoretical analyses have been conducted using numerical simulators (T2VOC and RADMOD).   Graham Bradner has been conducting this research.

Flow to wells near ideal lateral heterogeneities:   Understanding how hydraulic fractures affect the performance of wells has led to a more general problem of understanding the effects of idealized lateral heterogeneities on well performance.  In particular, we have considered the effects of an aquifer that consists of two domains separated by a vertical contact, and the effects of a vertical strip embedded in material of differing permeability.  The effects of idealized heterogeneities on well tests have been addressed in earlier publications, but this treatment has been brief and appears to be poorly known.  My intent was to summarize the characteristics of the two idealized heterogeneities and examine the feasibility of characterizing those heterogeneities using data from well tests. 

This work was motivated by a need to characterize vertical layers that are created at depth using injection techniques similar to hydraulic fracturing.  These layers have potential environmental applications, but a method to verify their hydraulic properties using neighboring wells is needed to support the development technology.

We have completed this review and the work is currently being summarized by Vasi Passinos in her MS thesis.

Chapman Ross is conducting this research for his MS Thesis.