Modeling and Simulation of Production from Commingled Shale Gas Reservoirs

The modeling and simulation of commingled production from multilayered shale-gas reservoirs is presented and the changing pressures and flow rates in various zones are simulated. An effective iterative numerical simulation is developed for the coupled wellbore and reservoir hydraulics calculations for multi-layered shale gas reservoirs. The performance of each layer communucating with the wellbore, in the absence or presence of formation cross flow, is evaluated and demonstrated by case studies. Changes in the permeability of shale with prevailing conditions is accounted for by considering the apparent gas permeability in shale depending on the pore proximity effects. This rigorous simulation method presented here enables an accurate evaluation of the pressure and production at each layer including the cross flow effects. Introduction Commingled production from wells completed in multi-layered reservoirs commonly exist in many fields, including the shale-gas reservoirs. Such reservoirs performance is usually monitored by means of the logging tools to obtain the downhole pressure and production measurements of different layers. Frequently, the temperature and noise logs are used to detect the wellbore crossflow, but no effective tool is available for measurement of the formation crossflow. Direct measurements usually require expensive and time-consuming approaches and interruption of production wells. Prediction of the flow rate and pressure profiles at separate zones and the contribution of each zone to overall well production is required for management of production from commingled reservoirs. The problem is more complicated when formation cross flow occurs between varios zones and because of permeability effects created in layers separating the pay zones by induced fractures resulting from stress deformation and other means. Stratified shale layers may also have different fluid and formation properties depending of their own sedimentary deposition processes and may involve different types of external boundary conditions, including full or partial water influx. Therefore, various stratified layers can have different contributions on well production performance, and well optimization and management. A rigorous phenomenological modeling is therefore required to rigorously quantify the contribution of each layer to the overall production and analyze the effect of cross flow for commingled shale gas reservoirs. Juell et al. (2011) used a backpressure equation to estimate the gas properties of multi-layer reservoir including wellbore crossflow. They assumed the well was produced at a constant bottom-hole flowing pressure. They compared the reservoir backpressure calculation and the results from the numerical simulator, and concluded that the backpressure equation can be used to predict the performance of layered reservoirs when coupled with material balance equation in the case of no-crossflow gas reservoirs. Also, solution of reservoir backpressure equation against reservoir material balance equation solution has been verified. Kuppe et al. (2000) developed a spreadsheet program to evaluate OGIP, layer productivity, and recoverable reserves for tight gas commingled reservoirs without crossflow using the p/z-plot method for two layer-reservoirs. They classified multi-layered reservoirs into two categories by grouping the high permeability layers into one composite layer and the low permeability layers into another composite layer. The p/z material balance method was used to determine the model layer properties. This method was successfully used for reservoirs with permeability in the range of 0.1 to 10 md to match and forecast the productivity index for various layers. Their study indicated that the material balance (p/z) technique is restricted and can be successfully used only under limited conditions, such as