07122-41

Goal
This project will develop: 1) a methodology for re-fracture well candidate selection, 2) recommendations and tools for determining the optimal time for re-fracture treatments, and 3) re-fracture treatment designs that utilize novel fluids, proppants and injection strategies. The application of these methods and tools will improve reservoir access while minimizing unnecessary drilling and completion costs.

Performers
The University of Texas at Austin, Austin, TX 78712
Noble Energy, Denver, CO 80202
BJ Services, Tomball, TX 77375
Anadarko Petroleum Corporation, Houston, TX 77251
Pinnacle Technologies, Houston, TX 77064

Background
According to the National Petroleum Council the volume of technically recoverable gas from unconventional gas plays (tight gas sands, gas shales and coal bed methane) in the lower 48 states is in excess of 293 trillion cubic feet (TCF). Tight gas sands represent a significant portion of the unconventional gas resources in the US. In 2006, 6.5 TCF of gas was produced from unconventional resources in the lower 48 states, and this is projected to increase to 9 TCF by 2020. Over the next two decades tight gas sand production is expected to provide over 60% of incremental domestic production. As many of the US tight gas sand basins mature, an increasing number of wells could be re-fractured to provide significant savings on infill drilling and completion costs.

The primary challenge facing gas producers is the rapid depletion rate of new wells and their relatively high cost. Rapid decline rates require that many new wells be drilled just to maintain production. To address these concerns, this proposal aims developing methods for improving reservoir access while minimizing drilling and completion costs. This can be accomplished through minimizing the number of well bores drilled by suitably fracturing and re-fracturing both vertical and horizontal wells.

For this project, research and field work will be focused on the Codell Formation located in Colorado and the Barnett Shale located in Texas. In the past 10 years, over 5000 Codell wells have been re-fractured with economic success resulting in an estimated 90% of cases. Although the wells were originally drilled on 40 acre spacing, production analysis determined the effectively drained area to be only 10-20 acres and also produced evidence of an elliptical drainage pattern restricted to the reservoir volume near the propped fractures. Reorientation of the drainage area, with the propagation of new hydraulic fractures into poorly drained rock not drained by the original fracture treatment, is believed to be a dominant factor in the success of re-fracturing treatments.

Operators in the Barnett Shale have been on the leading edge in developing many of the completion techniques used in fracture stimulating shale reservoirs. BJ Services has fractured more than 2000 wells in the Barnett Shale with slickwater fracturing systems utilizing a variety of proppants including light weight proppants. This includes more than 800 horizontal wells and more than 4,000 frac stages. Hundreds of vertical wells that were previously fractured with conventional cross linked gelled fluids have been re-fractured. Horizontal well activity has expanded into the tight gas sands of East Texas and N. Louisiana including the Cotton Valley and Bossier Sands as well as the tight gas reservoirs in Mississippi. Fracturing treatments have evolved to include the most recent slickwater technology, low polymer fracturing fluids, and hybrid frac treatments.

Potential Impacts
The insights resulting from this project will provide guidance to service providers in the development of novel additives; light weight proppants and chemical systems for implementations in re-fracture treatments. The design tool and well selection procedures that will be developed can be applied to any type of re-fracture treatment in any gas shale or tight gas resource play in the US. Cost reductions due to fewer wells and smaller overall footage drilled will result in more economic wells and longer economic well lives.

Accomplishments
Work on this project began on August 27, 2008 and as of yet there have not been any major accomplishments to report with this initial summary.

Current Status
Work has begun on two initial tasks: the development of the Project Management Plan with work breakdown structure that concisely addresses the objectives and approach for each task with all major milestones and decision points, and the development of a Technology Status Assessment describing the state-of-the-art of the proposed technology. The key tasks to be undertaken following the submission of the Project Management Plan and Technology Status Assessment are outlined below.

1. Stress reorientation around fractured wells: implications for re-fracturing - Data from one tight gas play (the Codell Formation in the Wattenberg Field) and one gas shale (the Barnett Shale) will be collected so that reasonable estimates of formation properties and in-situ stresses are available for the study areas identified. These data will form the basis of re-fracturing analysis to be conducted using poro-elastic models. This task has three subtasks:

• Data compilation in the Codell Formation and the Barnett Shale – A database from BJ Services and Noble Energy of more that 4000 fracture and re-fracture treatments in the Codell Formation will be compiled with production data so that production performance can be linked with stress re-orientation calculations. Micro-seismic fracture mapping data supplied by Pinnacle will also be integrated.
• Stress re-orientation around fractured wells in shales and tight gas sands - Recently developed models will be run to simulate stress re-orientation in fractured tight gas wells in the Codell Formation. It has been observed that fracture re-orientation in fractured gas wells can cause the direction of maximum stress (the direction of fracture propagation) to become perpendicular to the original fracture direction.
• Models for stress reorientation in naturally fractured formations - In formations such as the Barnett Shale the presence of natural fractures (or planes of weakness) can result in the development of complex hydraulic fracture patterns. General conclusions will be derived by comparing simulation and field results obtained in naturally fractured formations with those obtained in un-fractured formations. Re-fracturing work carried out in the Barnett Shale and monitored with an array of surface tiltmeters indicated significant reorientation of re-fractures. Based on hydraulic fracture mapping it is apparent that complex fracture networks are being created with growth along the maximum principal stress direction. Production data indicates a substantial increase in production after re-fracture treatments. The mapping results will be compared with poro-elastic simulations and production response estimated from the simulations. Reservoir conditions under which such re-fracturing programs are expected to succeed will be estimated. An analysis of the field data set of 4000 refracs will be combined with a parametric poro-elastic simulation study to identify the parameters that most influence the success of refracs. This will lead to a better understanding of the physics of re-fracturing and better selection of candidate wells for refract treatments.

2. Selecting Timing and Candidate Wells for Re-fracturing – The poro-elastic model will be further tested and used to model re-fracture treatments that have been conducted by BJ Services and Noble Energy. These simulations will be compared with surface tiltmeter data obtained during the treatments. The stress reorientation model will be run by systematically varying each one of the important input parameters to study the impact of these parameters on fracture design. The emphasis in this task will be on evaluation and identifying important parameters that affect re-fracture design and not on trying to history match field fracture designs. The following parameters are thought to be critical to the success of re-fracture treatments and will be specifically studied:

• The injection rate and volume of the pad
• The composition and volume of the proppant stage
• The rheology of the proppant stage
• The composition of the chase fluid and its composition
• The temperature and rate of injection of each of the stages listed above
• The relative permeability and capillary pressure characteristics of the reservoir
• The water saturation, temperature and pressure in the reservoir

Each of these parameters will be varied over a broad range and the impact of each will be evaluated.

The effect of depletion and reservoir properties on the timing of re-fracture treatments is of particular interest. Clearly, in very low permeability sands the propagation of pore pressures occurs slowly and results in fracture reorientation that is time dependent. The research team will investigate the ideal timing for re-fracturing under different scenarios (production rates, permeabilities, in-situ stresses etc.)

At the end of this process the researchers will develop simple charts and spreadsheets for candidate well selection and re-fracture timing based on the poro-elastic simulations and comparisons with field experience.

3. Re-fracture Designs for Deviated and Horizontal Wells – The research team will carry out 3-dimensional modeling of the process of re-fracturing in an altered stress region around horizontal wellbores to identify the important parameters for sucess. Guidelines for re-fracturing horizontal wells will be developed for use by operators for both candidate well selection and the timing and design of re-fracturing treatments.

4. Proppant Placement in Re-fracturing Treatments (Vertical and Horizontal Wells) - The presence of an existing fracture in a well places certain constraints on the placement of proppant during the re-fracturing operation. If it is expected that altered stress fracturing will result in an orthogonal fracture and the possibility of a flow constriction and screen-out away from the well bore. Conditions under which this will occur will be clearly defined so that operators may be aware of the need for design modifications.

If extension of the existing fracture appears likely then the opportunity exists for placing the new proppant much deeper into the fracture and maximizing the length of the propped re-fracture. Strategies for accomplishing this will be studied through re-fracture simulations and field data. A fracturing simulator that accurately simulates proppant transport will be used to explore different proppant injection strategies during re-fracturing.

5. Use of Novel Proppant Placement Strategies in Re-fracturing Operations - The problem of proppant placement during re-fracturing becomes particularly challenging for horizontal wells. Strategies for using newly developed fracturing fluids and proppants will be explored so that re-fracturing in horizontal wells can become an accepted practice if certain technical criteria are met.

6. Field Design of Re-Fracture Treatments in the Wattenberg Field - Re-fracturing of Codell wells in Wattenberg Field has been successfully performed for the last decade and several reasons have been suggested for the success of Codell refracs including reorientation of fractures. The researchers will use the field observations of fracture treatments in the Codell to test the models and conclusions that have been developed under previous tasks.

7. Design, Implementation and Evaluation of Field Fracture Designs - Based on the work done on existing re-fracture treatments, the research team will implement four new re-fracture treatments in the Wattenberg Field. Well candidates must be in a part of the field which has relatively low stress contrasts, will have been drilled on 40 acre spacing, and will have shown unsatisfactory results in prior re-fracture treatments.

The design of the re-fracture treatments will be based on simulations as well as input from field engineers. The treated wells will be compared with companion wells drilled on similar spacing at a similar reservoir pressure so that the performance of the refracs can be compared directly with traditional re-fracture designs. The cost effectiveness of the treatments will also be evaluated. Post-frac and pre-frac evaluations will be conducted in a manner that is consistent with existing best practices. Anadarko and Noble have both agreed to provide two wells each for these treatments.

Project Start: August 27, 2008
Project End: August 26, 2011

DOE Contribution: $949,318
Performer Contribution: $527,550

Contact Information:
RPSEA – Charlotte Schroeder (cschroeder@rpsea.org or 281-690-5506)
NETL – Virginia Weyland (Virginia.Weyland@netl.doe.gov or 281-494-2517)
University of Texas at Austin – Mukul M. Sharma (msharma@mail.utexas.edu or 512-471-3257)

Additional Information

E&P Focus Article [PDF] Summer 2009