Controls on Water Cresting in High-Productivity Horizontal Gas Wells

Abstract

It is widely agreed that gas reservoirs with a component of water drive should be produced at high rates to minimize the volume of gas which is trapped at high pressure by the advancing water (often termed 'outrunning the aquifer'). Yet high production rates are also associated with coning (in vertical wells) or cresting (in horizontal wells) of the encroaching water, leading to early water breakthrough. In vertical wells, the formation of an inverse gas cone means that high gas rates can be maintained post-breakthrough until almost the whole perforated interval is flowing water. However, in horizontal wells, water breakthrough is a serious threat to gas deliverability, because the inverse coning mechanism does not apply and the well rapidly loads with water. Consequently, it is not clear whether producing at high rates is the best strategy to maximize recovery in gas reservoirs developed using horizontal wells. We investigate the risk associated with producing horizontal wells at high rates by simulating gas recovery and aquifer response over a broad range of reservoir properties and production scenarios. We find that high rates always result in lower gas recovery unless the ratio of vertical to horizontal permeability is very low, in which case water cresting is suppressed. However, there are many instances where accelerating production recovers only slightly less gas over much shorter timescales, so may be economically favorable. Rate sensitivity increases in low permeability reservoirs with thin gas columns, because these conditions increase the tendency for water cresting, and decreases in reservoirs with strong aquifer support, since water breakthrough occurs regardless of the rate at which the well is produced. Our results can be used as a reference framework to rapidly assess gas production behavior and aquifer response within a wide range of field development scenarios. Introduction Horizontal and highly deviated wells are increasingly being used in gas field developments worldwide.1–5 Large-bore horizontal wells can deliver significantly higher gas production rates than conventional completions,2 reducing field development costs by allowing reserves to be targeted with fewer wells.4–6 However, realizing the potential of high-productivity gas wells requires an understanding of the subsurface risks to deliverability, to ensure sustained gas production, maximize profitability, and establish large-bore completions as an economically viable development option. A key subsurface risk in gas reservoirs with a component of water drive is early water breakthrough.3–5,7,8 In large-bore horizontal wells, early water breakthrough is a particularly serious threat to deliverability, because of the significant reduction in gas flow capacity associated with flowing entrained water to the surface.4,9–11 At best, substantial water production will require expensive processing facilities; at worst, it will effectively 'kill' the well.3–5,12 Based on material balance considerations, it is widely agreed that gas reservoirs with a component of water drive should be produced at high rates. This approach (often described as 'outrunning the aquifer') maximizes gas recovery by reducing the volume of gas which is trapped at high pressure by the advancing water.13–18 In this context, high productivity horizontal wells might be expected to make a positive contribution to gas recovery, because they can generally produce at much higher rates than vertical wells. However, material balance approaches assume that the gas-water contact (GWC) remains flat during production.13,14,16,18,20–22 Yet bottom-water drive gas reservoirs are associated with coning (in vertical wells) or cresting (in horizontal wells) of the GWC towards the well.7,23–28 Cresting occurs when viscous forces associated with pressure drawdown overcome gravity forces resulting from the density contrast between gas and water, causing a crest or cone of water to be drawn upwards towards the producing well 29 (Fig. 1). Water crest behavior has been described using analytical approaches to predict a 'critical rate' above which water breakthrough is expected,30,31 and time to water breakthrough.32,33 These approaches imply a sensitivity of gas recovery to production rate that conflicts with material balance techniques, as the severity of water cresting is increased with accelerated production, and therefore water breakthrough is expected earlier at higher rates. Water crest development also becomes more significant as the separation between the well and GWC is reduced, the horizontal reservoir permeability decreases, and the vertical permeability increases.29

Water Production Problem in Gas Reservoirs: Concepts, Challenges, and Practical Solutions

“…Cresting is more probable at high production rates from horizontal wells and especially near gas/water contact; thus, it is suggested the wells are drilled away from the waterfront [25]. e cresting is intensified by the increase in aquifer size and production rates, and subsequently the recovery decreases [29].…”

“…Sech et al [29] studied the impact of the gas production rate on the recovery of a reservoir being produced with horizontal and highly deviated gas wells. Large horizontal wells can provide remarkably higher production rates compared with conventional wells.…”

Design and Numerical Simulation Study of a Novel AICD for Water Control and Gas Production in Gas Wells

“…5 In addition, the water in the gas reservoir will quickly coning along the fractures in the reservoir while separating the gas area of the reservoir, it will also migrate to the area that is not invaded by water to generate “water-sealed gas”, thus affecting the recoverable reserves and recovery degree of gas fields. 6 Sech et al 7 and Qi et al 8 found in the experimental research of water flooding gas reservoirs that the elastic production water of the gas well advances suddenly along the high-permeability formation, and the water in the low-permeability formation rises slowly; it shows that in the high-permeability zone, water is more likely to invade. From this, it can be judged that in the process of gas reservoir exploitation, strong reservoir heterogeneity, large interlayer differences, edge-bottom water coning, etc.…”

Field Validation of a Non-logging Alternative Method for the Prediction of the Location of Water-Cresting in Horizontal Wells for Water-Drive Reservoirs

“…They reported that if the wellbore pressure drop is significant, then coning will take place at the heel and increasing the length of the horizontal wells will not delay the breakthrough. In 2007, Sech et al conducted parametric analysis and reported that large well diameter and production rate for horizontal wells will result in severe water coning with the only exception being when the vertical permeability relative to the horizontal permeability is poor . These theoretical studies listed above laid important foundation for the experimental studies that came after.…”