Stimulation of Bypassed Pay Zones in Existing Wellbores
- John W. Moreno (New Mexico Tech) | Thomas W. Engler (New Mexico Tech)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- February 2009
- Document Type
- Journal Paper
- 147 - 155
- 2009. Society of Petroleum Engineers
- 5.8.6 Naturally Fractured Reservoir, 3 Production and Well Operations, 5.5 Reservoir Simulation, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.4.3 Sand/Solids Control, 2.2.2 Perforating, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.6.4 Drillstem/Well Testing, 4.1.2 Separation and Treating, 1.2.2 Geomechanics, 2.5.1 Fracture design and containment
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A significant portion of the US 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 therefore 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 comprises 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 caused by 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.
The purpose of this paper is to analyze the overall behavior of a multilayer formation, paying special attention to the effect of the current productive zones on the production and stimulation of the bounded, middle layer. To evaluate the general lithologic sequences, it is necessary to properly define both the design goals and the design variables that affect the overall process. Because both production and stimulation are considered, concepts on hydraulic fracturing (i.e., elasticity theory, rheology, continuity, and fracture mechanics) and fluid flow (i.e, continuity, flow rules, and state equations) must be incorporated for the design goals to be fully coupled. Also, because the common element in both hydraulic fracturing and fluid flow theories is the sensitivity of the medium to stress, the incorporation of nonisotropic permeability tensors and stress maps is necessary to properly describe the system.
The model considers a formation consisting of three layers, of which the top and bottom have been stimulated and producing for a period of time. After several years of production, the second (middle) layer is stimulated, and the entire system (three layers) produce to the wellbore. The proportion of propped target length among layers has been set to one (i.e., 750 feet [ft]) or two (i.e., 1,500 ft), and therefore in an X:1-2-2 sequence, the propped length for the first layer is 750 ft., whereas it is 1,500 ft. for the second and third layers, respectively.
The impact of four different types of rocks are investigated as shown by Fig. 1. These impacts, along with the number of layers that compose the system, lead to the selection of only eight possible lithologic sequences, on the basis of the following restrictions:
- The first and third layer is never either "sandy shale" or shale.
- Two contiguous layers cannot share the same type of rock.
Fig. 2 depicts the considered sequences.
From the previously mentioned considerations, a methodology for the development of a fully coupled model is proposed, and the corresponding lithologic sequences are evaluated and optimized.
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