Fast Optimization of the Net Present Value of Unconventional Wells Using Rapid RTA

Decline-curve analysis (DCA) is the industry standard to predict hydrocarbon production to then estimate the net present value (NPV) of unconventional wells. Conventional DCA assumes the bottomhole flowing pressure (BHP) is constant. This is an unrealistic assumption for many unconventional wells that can lead to incorrect estimates of ultimate recovery (EUR) and thus, erroneous NPV calculations. This work illustrates the application a novel technique that combines variable BHP conditions with decline-curve models (Rapid RTA) to estimate the NPV and select the optimal cluster spacing and number of fractures for a given well. We compare the results of DCA and Rapid RTA to estimate and optimize the cluster spacing for a tight-oil and shale gas well. There are six steps in the proposed approach. Step 1 gathers the well’s data. In step 2, we define the decline-curve model to be used. Step 3 applies DCA and Rapid RTA techniques. The outputs of step 3 are the fracture half-length and effective permeability derived from model’s production history-matches. Step 4 generates synthetic production histories using DCA and Rapid RTA varying the cluster spacing from 20 to 60 ft in 5 ft increments. In addition, we perform a sensitivity analysis for different total lateral lengths ranging from 2,000 to 14,000 ft in 2,000 ft increments. Step 5 evaluates the NPV for the different synthetic cases. In step 6, we determine the maximum NPV and thus, the optimal cluster spacing for each total lateral length. Finally, we compare the results of DCA and Rapid RTA. The Rapid RTA results show marked differences in terms of the shape and the optimum value of the NPV function from the simpler DCA method. For the two cases illustrated, DCA results are a direct consequence of the rate-time analysis failing to correctly detect the onset of boundary-dominated flow (BDF). Accurate detection of the onset of BDF is crucial when analyzing production of unconventional wells. In contrast, Rapid RTA correctly detects the onset of BDF and presents a defined maximum value of the NPV vs. cluster spacing (number of fractures) function. This optimum value is a trade-off between the fracture treatment cost, the speed of recovery of hydrocarbons, and the value of money with time (discount rate). The differences in the NPV function for the DCA and Rapid RTA techniques relate to the distinct future production behaviors observed in these methods. Our results highlight the importance of accounting for variable pressure effects in unconventional wells for accurate: production forecasting, NPV calculations, and optimization of fracture clusters and total number of fractures. The major contribution of this work is the implementation of the Rapid RTA technique allowing fast computation of pressure-rate-time analysis and NPV calculations with computational times comparable with DCA for optimizing fracture cluster spacing and the total number of fractures of unconventional wells. We have developed a web-based application to give readers a hands-on experience of this new technique and to analyze and optimize the NPV of unconventional wells.