Numerical and Analytical Modelling of the Gas Well Liquid Loading Process

Abstract

Liquid loading is a serious problem in areas where gas fields are maturing. This paper presents an analysis of the production behavior of liquid-loaded wells over time. This clearly shows that these wells can operate at two different rates-a stable rate, at which full production is taking place, and at a lower metastable rate, at which liquid-loading effects play a role.A model has been constructed that enhances the understanding of the process of water buildup and drainage in gas wells. It assumes a single gas-and water-coproduction point and a single water reinjection point. As expected, a water column is built up in the well as soon as production takes place below the critical rate. As observed in the field, for good inflow performance, a metastable flow rate can be observed. At this state, the water-reinjection and water-coproduction rate are equal to one another, and the water-column height stabilizes.A sensitivity analysis has been carried out to determine how well parameters influence the metastable flow rate, the time required to reach this metastable rate, the corresponding watercolumn height, and the shut-in time required to drain this water column. The results of the analysis indicate that significant metastable flow rates occur in wells that have good inflow performance, a low water/gas ratio, and a large distance between injection and production point.Furthermore, a steady-state analytical solution has been derived for the metastable rate and stabilized water-column height confirming the numerical-analysis results.

Lagrangian relaxation based decomposition for well scheduling in shale-gas systems

“…Compared to linear system identification, in which a prefilter tends to cut off model content in frequencies outside the bandpass, the prefilter applied in nonlinear system identification introduces a scattering of the frequency spectrum outside the bandpass. This property may impact the frequency weighting in (9) and hence the model match. See [25] for further details and analysis.…”

Shut-in based production optimization of shale-gas systems

“…Accumulated liquids may be due to some low saturation of water in the formation, gas condensates, or left-over water injected during HF stimulation . Onset of liquid loading in gas wells can be recognized by highly erratic and unstable gas rates, a sharp drop in the decline curve (Al Ahmadi et al, 2010;Lea and Nickens, 2004) or as semi-stabilization of the production rate at significantly lower levels (Dousi et al, 2006;Whitson et al, 2012), i.e. at so-called meta-stable rates, illustrated by the green erratic rates in Fig.…”

“…In this naive, commonly applied operational approach (Dousi et al, 2006), we assume that each well is produced until signs of liquid loading is observed, and further assume that the signs in this case is a sharp drop in the gas rate, or the onset of erratic rates (Lea and Nickens, 2004). For simplicity, we assume that liquid loading occurs once the rate drops below q gc , although some gas wells may produce stable sub-critical rates, including meta stable rates, for some time before signs of loading appears (Dousi et al, 2006;Li et al, 2002). The fictitious erratic rates from onset of loading are generated by simulation with uniformly distributed low-gain noise added to the rate once it drops below q gc .…”

“…Eventually, there will be insufficient pressure to lift the accumulated or coproduced liquids to the surface, and the well will exhibit liquid loading. This state can be recognized by erratic and unstable rates, sharp drops in the decline curve (Lea and Nickens, 2004), or stabilization of the production rate at significantly lower levels Dousi et al, 2006), so-called meta-stable rates. Liquid loading is frequently observed at late and intermediate times for shale-gas wells (Al Ahmadi et al, 2010;Awoleke and Lane, 2011;Cipolla et al, 2010;Kravits et al, 2011;Ilk et al, 2008;Mayerhofer et al, 2005;Sutton et al, 2010), and will additionally worsen the already low late-life rates.…”

“…All shale and tight formation gas wells will, however, eventually reach the state where the wells' pressure is insufficient to lift co-produced liquids to the surface, causing accumulation of liquids in the vertical wellbore . This state is known as liquid loading, and detected by a very erratic and unstable rate, sharp drops in the decline curve (Al Ahmadi et al, 2010;Lea and Nickens, 2004) or semistabilization of the production rate at significantly lower levels (Dousi et al, 2006;Whitson et al, 2012), i.e. at so-called meta-stable rates.…”