(159h) Hybrid Dynamical Modeling of Artificial-Lift Operation in Shale Gas Wells

Oil and gas reservoirs can be classified as conventional and unconventional depending on the ease of recovery and the quality of reservoirs. Shale gas – an unconventional resource – is natural gas found in shale rock formations, typically at depths of 10,000 ft or greater. Recent advances in horizontal drilling and hydraulic fracturing have made shale gas extraction economically viable. Hydraulic fluid consisting of water, proppants and other chemical components is pumped at high pressures to induce fractures in the reservoir and thereby increase the net permeability. About a third of the pumped fluid returns to the surface, while the remaining is produced along with natural gas during normal well operation. With time, liquids (water and condensates) accumulate at the well bottom of the well, hindering the gas flow to the well-head. This phenomenon is called liquid loading. Artificial lift techniques are employed at this stage to remove the accumulated fluids, to operate each individual well in an optimized manner to extract most of the useful resources.

This contribution focuses on computationally efficient modeling of a class of Artificial Lift techniques for dewatering of shale gas well. Specifically, artificial lift techniques that use well’s own energy for de-liquefication of shale gas wells are considered. Examples of this include periodic well shut-in, plunger lift, gas lift, and a combination thereof. In the first two cases, the well is shut-in for a certain period of time, allowing the gas pressure to build up. Thereafter, a control valve is opened and the liquids accumulated at well-bottom are produced at the surface due to flow of the pressurized gas, either as a mist (periodic shut-in) or as a slug above a metal rod called plunger (plunger lift). In case of gas lift, some of the produced gas is pumped back into the well to augment well’s energy for dewatering.

In general, artificial lift operations such as periodic shut-in and plunger lift, consist of several modes of operation based on whether the production valve is closed or open. Each of these modes follows different nonlinear dynamics. The system switches from one mode to another mode based on occurrence of various discrete events e.g., switching on/off production choke, plunger reaching at surface, etc.  Thus, artificial lift model is inherently hybrid in nature. 

We propose a generic hybrid dynamical model for the above-mentioned artificial lift operations for de-liquefication of shale gas well. The model consists of reservoir flow component that accounts for the flow of fluids from the reservoir to the well-bore; a component for gas dynamics; and a component for liquid loading. In particular, we identified various modes of operation and derived computationally efficient dynamic model for each of these modes. Various conditions responsible for switching system dynamics from one mode to another are also identified and integrated with the continuous dynamic of various modes.