Natural gas production from unconventional gas reservoirs in North America rely on the technologies of horizontal drilling and multi-stage hydraulic fracturing. The cased and cemented completion technique, Plug-and-perf (P-n-P), has been utilized as a traditional completion in horizontal wells for many years. However, the openhole completion technique, Open Hole Sleeve Multi-stage System (OHMS), has gained favor in the past decade, because of its cost and time efficiencies. Production comparisons between these two completion methods remain a controversial subject, with ongoing debate regarding which method yields more gas production.
Historical studies compare production indicators, typically from a limited set of sample wells. These historical studies indicate either insignificant differences between the two completion methods, or that OHMS systems significantly outperform P-n-P. However, these historical studies are limited in a number of ways.
A computational fluid dynamics model (CFD) has been developed to compare P-n-P completions with OHMS for horizontal, multi-stage fractured wells. The numerical model uses a 6-inch borehole draining a tight gas reservoir (0.01 mD), under steady state flow with no formation damage. The P-n-P completion assumes 0.22 in., 180o phased perforations connecting to a planar fracture penetrating the height of the reservoir. The OHMS completion assumes sandface flow, and no orifice effect from the sleeve openings. Both completions assume the planar fracture intersects at the center of the reservoir model.
The results of the CFD analyses compare the productivity index ratio (J/Jo) verses dimensionless fracture conductivity (Cfd) for each completion over a range of fracture conductivity (kfw) obtained from commercial proppant data. Parametric studies were performed varying propped fracture width, fracture half-length and vertical to horizontal permeability ratio (kv/kh), to investigate the effects of these parameters on the completion comparison.
This paper presents the results of the numerical modeling and compares productivity index results to historical, production-based studies of the P-n-P and OHMS completions in horizontal, multi-stage fractured wells. This work verifies a slight production advantage in OHMS systems versus P-n-P completions used in horizontal wells today. The work is significant because few, if any, numerical models have been developed to study and compare performance of these completions.