Although currently, large-scale and multilateral horizontal wells are an important way to improve the oil recovery in the unconventional reservoirs, the flow behavior of fluid from the reservoir into the horizontal wellbore becomes more challenged compared to the single small-scale horizontal well. One of the main challenges is that pressure loss from the well completion section and wellbore cannot be ignored in the coupling process between the reservoir and the horizontal well. In this paper, a new method is presented to solve the coupling flow between the reservoir and the horizontal well with different well completions. The new coupling model is compared with Ouyang’s model (1998) and Penmatcha’s model (1997), and the predicted data are consistent with each other at both early and late times. Meanwhile, four different cases have been proposed to verify the application of the new coupling model with different well completions, and the results indicate that the uneven inflow profile can be effectively alleviated via reasonable completion parameters and different well completions. Based on two types of flow-node units, it can quickly model and solve the coupling problem between the reservoir and the horizontal well with complex completion cases. It can also depict the inflow profile of the horizontal well with different well completions, which is conducive to understand the coupling process. The new coupling model can provide theoretical support for further optimization of completion parameters and well completions and finally improve oil recovery.

Trilinear flow model is an effective method to reproduce the flow behavior for horizontal wells with multistage hydraulic fracture treatments in unconventional reservoirs. However, models developed so far for transient analysis have rarely considered the inflow performance of wells. This paper introduces a new composite dual-porosity trilinear flow model for the multiple-fractured horizontal well in the naturally fractured reservoirs. The analytical solution is derived under a constant rate condition for analyzing transient pressure behaviors and generating the transient inflow performance relationships (IPRs). The plots of pressure profiles with time could provide insightful information about various flow regimes that develop throughout the entire production cycle. Sensitivity analysis of pressure and pressure derivative response was also performed by varying different parameters (such as hydraulic fracture width and permeability, reservoir configurations, etc.) and by which the impacts of different parameters on the durations of regimes as well as the productivity index can be confirmed. The main outcomes obtained from this study are as follows: (1) the ability to characterize naturally fractured reservoirs using a new composite dual-porosity trilinear flow model; (2) the application of analytical solutions of transient analysis to generate transient IPR curves for different flow regimes; (3) understanding the effect of reservoir configurations, fractures, and matrix characteristics on pressure distribution, flow regime duration, and transient IPR. More specifically, the pressure drop increases and the productivity index decreases with the decrease of the hydraulic fracture conductivity and the increase of matrix permeability and the skin factor. Also, the larger hydraulic fracture spacing and drainage area result in the later onset of the pseudo-steady-state regime. (4) A comprehensive study on transient pressure behaviors and transient inflow performance can provide valuable information to characterize the multifractured complex systems as well as some insights into the production.

Nitrogen (N2) injection-assisted flowback with coiled tubing (CT), CT flowback for short, is more and more widely used due to its convenience in implementation and flexibility in control. The unique feature of gas injection with CT is that the injection depth can be changed continuously (Zhou et al., 2011), which makes flowback become a more complex dynamic process. However, scarce research has been done on such a dynamic process, especially in the optimization of operation parameters such as CT running into hole (RIH) speed and N2 injection rate. In this paper, a numerical model is established using a transient multiple flow simulator to reproduce the dynamic process of CT flowback. On this basis, the characteristics of CT gas injection are explored by comparing fixed-point gas injection with variation-depth gas injection. The influence of formation flowback capacity, CT RIH speed and N2 injection rate on CT flowback process is analyzed. And finally the optimization suggestions of CT operating parameters are put forward. The simulation results show that, compared with fixed-point gas injection, variation-depth gas injection (i.e. CT gas injection) can achieve a more controlled flowback, significantly reduce the gas injection pressure, and realize the gentle unloading of bottom hole pressure. Formation flowback capacity has an important effect on the CT flowback process, different formation flowback capacities result in different gas well unloading degrees, with the increase of formation flowback capacity, the unloading degree of gas well decreases. CT RIH speed can to a large extent control wellbore unloading rate, i.e. the flowback intensity. The increase in RIH speed is at the expense of the increase in gas injection pressure and will lead to a more aggressive bottom hole pressure unloading. So in determining the RIH speed, both surface gas injection facility limitations and downhole pressure control requirements need to be taken into account. For a given CT RIH speed, there is a minimum N2 injection rate, which is required to achieve the continuous drainage, i.e., the lower limit for N2 injection rate. On the basis of this, further increasing the N2 injection rate will accelerate the wellbore unloading, and reduce the peak of gas injection pressure, but there is also an upper limit for N2 injection rate beyond which the benefits are no longer apparent.