Review of Current Fracture Stimulation Techniques for Best Economics in Multi-layer, Lower Permeability Reservoirs

Abstract

Abstract Currently, thousands of wells are being drilled every year in low- to moderate- permeability sandstone and carbonate reservoirs that have many distinct zones or layers that can contribute to production if adequately stimulated. In lower-permeability reservoirs, and especially for CBM completions, well economics will limit how much can be spent on the stimulation portion of a completion program. Operators may have to execute a "balancing act" with the need to maximize return on investment (ROI) while trying to complete as many contributing zones as possible. This usually drives a well operator toward methods that can minimize the number of separate well interventions while maximizing the number of zones that can be effectively stimulated. This paper provides a State of the Art review on both old and newer completion/stimulation techniques that can help an operator accomplish production goals. When the fracturing treatments are pumped down the casing, different methods applied can vary from simple, single-stage fracturing treatments using "limited entry" perforating to multi-stage fracture treatments with bridge plugs (BP) or ball/baffle stage isolation. Additionally, there are various other techniques incorporating conventional tubing strings with or without packers or BP's, and even various methods for coiled-tubing (CT)-deployed treatments. A "scorecard" approach presented here may offer operators an improved understanding of when a certain completion method is appropriate or flawed, based on the operators' current understanding of specific reservoir characteristics of their field or well. Too often, a multilayer completion technique has been applied to several wells and later found to be a poor choice. The wrong choice may have been made because of a limited understanding of the completion method, or even of the reservoir itself. Often, an operator understands the variances of the reservoir, but does not realize the limitations that these variances should bring to the process of choosing the optimal simulation/completion method. In some cases, the choice has simply been made to take the low-cost completion/stimulation approach, and the resulting well(s) did not produce at (or sustain) economic levels. Introduction When an oil or gas well has penetrated multiple pay zones that are known to have the potential to contribute high production rates if adequately fracture stimulated, it is easy to justify the expense and effort of using mechanical isolation to help ensure effective stimulation of each zone or groups of closely spaced zones. What most operators consider the "preferred" method is as follows:Perforate initial zone only, then run a hydraulic-fracture stimulation (HFS).Flow-back/cleanup stimulated zone (10's of hours to 10's of days).Mechanically isolate the stimulated zone(s) and repeat the entire multi-day process again on the next zone uphole, and possibly a third or fourth time if needed.Remove any mechanical isolation hardware and complete well. The above is the "done right" approach---effective, predictable, but for many multi-zone wells in low-permeability or low-pressure reservoirs, it is just too costly! When the zones have low or an unknown potential to produce, this level of expense can seldom be economically justified. Operators drilling in marginally commercial reservoirs will usually be forced to use a low-cost completion technique. The basic purpose of this paper is to review completion options that operators might consider to improve on the economics, while attempting to maintain adequate stimulation effectiveness.