Completion Length Optimization in Gas Wells

Abstract Water-drive gas systems are economically important but gas recovery may be limited by water loading of the production string or incomplete sweep in the reservoir. Efforts to improve recovery from water-drive gas wells have focused on limiting the completion length. However, limiting the completion interval decreases the gas production rate and may have a substantial negative effect on project economics. The effects of limited completion length on gas rate, cumulative gas production, and discounted revenue stream are considered for a range of reservoir and economic factors in this study. A multilevel factorial design in 8 factors (576 simulations) was analyzed using response models and Monte Carlo simulation. The gas price, discount rate and initial reservoir pressure are the most important factors affecting present value, followed by horizontal and vertical permeability and completion interval length. Because decreased completion interval length has a strong negative effect on gas rates and economics, methods that control water completion while maintaining longer completions are desirable. The completion length can be optimized using response surfaces and Monte Carlo methods. The optimum completion length depends on the distribution of the uncertain factors; computing the optimum at factor mean or median values is not equivalent and is less reasonable. Methods to address this uncertainty are presented. For the models considered in this study, productivity is often the most important factor and the expected project value is often maximized by using the maximum completion interval length. PROBLEM STATEMENT The behavior of gas wells under water drive has been investigated from many perspectives, from material balances(1,2) to detailed studies of completions.(3) These studies recognize a tradeoff between the additional drive provided by water influx versus the potential for gas trapping behind the advancing gas-water contact. In general, this has led to the conclusion that higher gas production rates are preferred to minimize trapping; the occurrence of a water-drive is widely perceived to lead to decreased ultimate recovery. Minimization of gas trapping is the motivation for schemes that produce water from the aquifer concurrent with gas production.(4) On the other hand, some studies have suggested that reservoirs with low gas productivity may benefit from water drives and indeed water injection may be advisable.(5) Because gas wells may not be able to produce with high water cuts, operators sometimes use partial completions to reduce coning and delay water loading. However, because the gas mobility is high, this strategy may be unnecessary and may decrease gas rates with little impact on ultimate recovery.(3) Partial penetration decreases productivity via a pseudoskin effect;(6) the attendant increase in near-well velocity exacerbates non-Darcy flow effects if the Reynolds number is greater than one.(7,8) In summary, the dependence of recovery on aquifer strength, reservoir properties and completion properties is complex. Although water influx traps gas, it sustains reservoir pressure; although limited completion lengths suppress coning, they lower well productivity. These tradeoffs can be assessed using numerical simulation and discounting to account for the desirability of higher gas rates. This study examines the factors that control the value of water-drive gas wells and proposes methods to analyze and optimize completion strategy.