A significant portion of the U.S. gas resources is located in low-permeability, bypassed pay zones, within multilayered sandstone-shale sequences. Acquiring these resources leads to operational and design difficulties in stimulation, particularly if the target zone is bounded above and below by existing producing zones. The objective of this work was to evaluate the impact of adjacent, existing producing zones on the stimulation design and hence production performance of the bypassed payzone.
To investigate this problem, a 3D planar, hydraulic fracture propagation model was constructed and superimposed on a 3D flow model. The physical model was comprised of three layers, the top and bottom representing previously stimulated and producing layers, and the middle layer the target or bypassed layer. The impact of lithology, fracture length and total stress variations over time on the fracture conductivity, fracture efficiency and average reservoir pressure were investigated.
Evidence of pressure depletion of the target layer was observed due to production of the upper and lower layers. The degree of depletion is dependent on the fracture length and lithology of all of the layers. That is, the ability to propagate a fracture in the target layer was a strong function of the shale content and to a lesser extent, on the hydraulic fracture length of the bounding layers. Increased shale content in the target as well as the bounding layers resulted in a decrease in fracture conductivity of the target layer. However, an increase in fracture length did not necessarily result in a decrease in fracture conductivity of the target layer.
The study includes examples of stimulating the Menefee formation in the San Juan basin.