SPE-153073-MS Cyclic Shut-in Eliminates Liquid-Loading in Gas Wells
Curtis Hays Whitson (Norwegian U. of Science & Tech) | Silvya Dewi Rahmawati (NTNU) | Aleksander Juell (Norwegian U. of Science & Tech)
SPE/EAGE European Unconventional Resources Conference and Exhibition, Vienna, Austria, 20-22 March 2012
This paper presents a method to eliminate production loss due to liquid-loading in tight gas wells. Cyclic shut-in control is a simple production strategy that particularly benefits lower-permeability stimulated wells, including but not limited to shale gas wells.
Comparison is made between a gas well producing (1) in a “ideal?? situation where 100% of liquids entering from the reservoir or condensing in the tubing are continuously removed (without shut-ins), (2) in a meta-stable liquid-loading condition with low gas rate, typical of most wells today, and (3) by the proposed strategy of cyclic shut-in control. We show that cyclic shutin control of stimulated low-permeability vertical wells to ultra-low-permeability horizontal multi-fraced wells can produce without ever experiencing liquid loading, and with little-to-no delay of ultimate recovery.
Cyclic shut-in control can be applied to all stimulated, lower-permeability gas wells, from the onset of gas rates that result in liquid-loading. The strategy can also be used for wells which already have experienced a period of liquid-loading , but the expected performance improvement may be less because of near-well formation damage caused by historic liquid-loading – e.g. fresh-water backflow and liquid-bank accumulation. In historically liquid-loading wells, an initial period of liquid removal and/or light stimulation may be needed prior to initiating cyclic shut-in control.
We show that the shut-in period should optimally be as short as operationally possible.
Cyclic shut-in control is shown to work equally well for layered no-crossflow systems with significant differential depletion at the onset of liquid loading.
Minimizing rate and recovery loss of liquid-loading gas wells is of international interest. We believe that cyclic shut-in control will become an industry standard practice for shale gas wells, and should lead to a significant ultimate increase in worldwide gas reserves. The method is extremely simple and requires only a rate-controlled wellhead shut-in device.
Every gas well will, at some point in its life, reach a condition where the gas rate is insufficient to carry co-produced liquids to the surface. These liquids may flow from the reservoir, or condense out of the produced gas on the way from bottomhole to surface. The liquids may be condensed water, free water, condensate or oil. After this condition is reached, some fraction of the produced liquids will flow counter-current to the gas, and accumulate in the bottom of the well. As liquids accumulate, the backpressure on the formation increases. This results in a sharp reduction in the gas production rate, and in the worst case the well might die completely. A more common result of liquid-loading is that the well stabilizes at a lower production rate, called the “meta-stable rate??, a term introduced by van Gool and Currie (2007). Fig. 1 shows a gas well that illustrates the impact of liquid-loading. A sharp drop in production is seen when the well reaches a gas rate of about 200 Mcf/D (typical for 2-3/8?? tubing), eventually stabilizing at a meta-stable rate of about 20 Mcf/D for some 20 years.