Overview: Gas Production Technology (November 2007)
Authors: Wilson Scott J. 
Published: February 2015

For this feature, I have chosen three excellent papers that highlight the complex, but very common tendency of gas wells to "load up." Given the explosive growth in new low-rate gas completions and the persistent move of all older high-rate completions toward the same condition, this topic seemed fit for a little more attention. Although not specifically addressed in these papers, the recent widespread use of very long (or thick), multihydraulically fractured completions intensifies these problems. If you think you do not have a well with a liquid-loading issue, you probably have not looked hard enough.

Each of these papers addresses the issue from a different viewpoint. One uses video logging to "see" what is happening downhole, while another uses truly remarkable integrated transient wellbore and reservoir-simulation tools to predict mathematically the same behaviors observed in the first. The other paper stakes out a middle position of using comparatively simple analytical models to predict the exact location of a standing liquid level within a gas-producing section.

The video-logging paper describes a series of production logs run in several horizontal wells. In the best spirit of data collection and research, the results were far from the expected outcome and revealed the predrill assumption of no potential loading problems to be incorrect. Also, the authors specifically put to rest that long-debated question of what happens to liquid and solids accumulations in horizontal wells with a toe-up trajectory.

The paper on modeling the liquid-loading process builds on and complements N. Dousi et al. 2006 "Numerical and Analytical Modeling of the Gas-Well Liquid-Loading Process" [SPE Production & Operations21(4): 475–482 (SPE 95282-PA)]. In both papers, water loading up the wellbore is studied in detail. They both predict a "metastable" flow condition in wells that cannot lift all the liquid but have sufficient liquid injectivity (into the producing zones) to not die completely. This wellbore condition is now attributed to significant formation damage and well-performance deterioration after shut-in periods.

The integrated-wellbore/reservoir-model paper takes the subject even further, with an exhaustive transient mathematical treatment of the loadup process. Coupling a reservoir simulator with a time-dependent wellbore model, the authors were able to prove that mathematical models can match an erratic production profile, although that is rarely the objective of the typical modeler.

Whatever your mathematical inclination, one of these offerings should help you gain a greater appreciation of the complex process of liquid loadup in wellbores.

Gas Production Technology additional reading available at the SPE eLibrary: www.spe.org

SPE 107870 "Analysis of Condensate Buildup and Flow Impairment of Retrograde Gases in Fissured Reservoirs" by H. Ayala, Pennsylvania State University, et al.

SPE 107169 "Controls on Water Cresting in High-Productivity Horizontal Gas Wells" by R.P. Sech, SPE, Imperial College London, et al.

SPE 107274 "Economic Viability of Gas-to-Liquid Technology" by Ali A. Garrouch, SPE, Kuwait University

SPE 101497 "Applications of Acoustic Liquid-Level Measurements in Gas Wells" by J.N. McCoy, Echometer Company, et al.