Considerations for Pore Volume Stress Effects in Over-pressured Shale Gas under Controlled Drawdown Well Management Strategy
- Jesse Adeniyi Akande | John Spivey (Phoenix Reservoir Software)
- Document ID
- Society of Petroleum Engineers
- SPE Canadian Unconventional Resources Conference, 30 October-1 November, Calgary, Alberta, Canada
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 5.8.1 Tight Gas, 5.1 Reservoir Characterisation, 1.2.2 Geomechanics, 3.3.6 Integrated Modeling, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.1.5 Geologic Modeling, 5.5 Reservoir Simulation, 5.6.9 Production Forecasting, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.2 Shale Gas, 5.5.8 History Matching
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The high decline rate observed in over pressured shale has attracted the attention of the industry, and better well management procedures for long term productivity improvement are still evolving. Operators are recognizing some benefit in controlled rate (controlled drawdown) production as one way of improving well performance for the wells in over pressured stress sensitive formations.
During uncontrolled rate production because of high drawdown, the permeability in stress sensitive shales decays faster because of increased stress. Often high initial gas rate is accompanied by high decline rate as the permeability reduction takes effect. In addition, proppant could also be produced back, crushed or embedded in the formation. However, controlled rate production minimizes the rate decline, albeit at reduced initial gas rate. Modelers resort to using different stress permeability decay coefficients for these two production strategies. Higher values are assigned to uncontrolled rate production to produce lower EUR. This approach, although convenient, requires different permeability versus stress tables depending on the production strategy.
Porosity and pore volume reduction in shales could be as high as 20 percent due to changes in net stress. The pore volume reduction provides in situ energy for gas recovery. The increased rate of permeability decay due to changing in situ stresses reduces the effectiveness of pressure support from pore volume reduction as fractures close under stress.. Controlled rate production strategy slows down permeability decay rate and this enables better use of pore volume energy. The pore volume consideration could provide additional gain to EUR for controlled rate.
Our analytical simulation model couples geomechanics permeability and porosity stress coefficients and evaluates well performance under moderate and low net stress sensitivity. Haynesville and Marcellus shales were evaluated. The importance of pore volume stress effect from the stand point of well performance evaluation and reservoir characterization is assessed.
The Haynesville shale is located in east Texas and northwest Louisiana at depths up to 14,000 ft. It is over-pressured shale and exists at very low initial net stress, less than 1,500 psi. High drawdown created by uncontrolled rate production increases the rate of net stress buildup in the formation. The high initial production rates are soon followed by drastic production decline. The Marcellus shale is located in the Appalachian basin. The initial pressure gradient in overpressure regions in the Marcellus such as Marcellus fairway in northwest West Virgina is about 0.56 psi/ft. Initial net stress could exceed 3,000 psi. Stress effects on permeability and pore volume for stress sensitive shale are rapid at low net stress and gradually taper down at increasing net stress.
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