This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 143066, ’Optimization of Completions in Unconventional Reservoirs for Ultimate Recovery - Case Studies,’ by Daniel J. Snyder, SPE, and Rocky Seale, SPE, Packers Plus Energy Services, originally prepared for the 2011 SPE EUROPEC/EAGE Annual Conference and Exhibition, Vienna, Austria, 23-26 May. The paper has not been peer reviewed.
Over the last decade, an industry-wide shift to unconventional plays has been made possible by advances in technology, allowing the recovery of previously uneconomic reserves. The primary objective of completions in these unconventional reservoirs is to increase the effective surface area of the well to maximize reservoir contact. The full-length paper provides an introduction to unconventional reservoirs, describes the main methods of horizontal multistage completions, and discusses how the choice of method can affect good fracturing practices as well as long-term production.
Unconventional Reservoirs. Over the last decade, an industrywide shift to unconventional plays has occurred as a result of the depletion of mature conventional reservoirs, increased demand, and advances in technology. Unconventional reservoirs have been defined as formations that cannot be produced at economic flow rates or that do not produce economic volumes of oil and gas without stimulation treatments or special recovery processes and technologies (Fig. 1). Types of unconventional reservoirs include those with poor fluid-flow characteristics because of small inter-pore connections and/or with stacked pay units.
The primary objective of completions in these unconventional reservoirs is to increase the effective surface area of the well to maximize reservoir contact. Horizontal drilling and multistage fracturing are two technologies that accomplish this. The two main methods of horizontal multistage completions currently used in unconventional reservoirs are cemented liner “plug and perf” and openhole multistage-fracturing systems. Although both methods have the same goal of increasing access to the reservoir by the induction of fractures along the entire length of the horizontal wellbore, they differ significantly from an operational perspective.
Cemented-Liner Multistage-Fracturing Method. This type of completion involves cementing production casing in the horizontal wellbore and plug-and-perf stimulation. Mechanical isolation in the liner is accomplished by setting bridge plugs using pump-down wireline or coiled tubing (CT), followed by perforating and then fracturing the well to provide access to the reservoir. The cement provides the mechanical diversion in the annulus, while the bridge plug provides the mechanical diversion inside the liner. This process then is repeated for the number of stimulations desired for the horizontal wellbore. After all stages have been completed, CT is used to drill out the composite plugs, thus re-establishing access to the toe of the horizontal wellbore. Although an effective method of creating diversion along the horizontal section for discrete-stage stimulation, the inherent cost of multiple interventions with CT, perforating guns, and deployment of fracturing equipment needed for each stage is extremely high, not to mention being very inefficient and time consuming. Production using this method also can be limiting because cementing the wellbore closes many of the natural fractures and fissures that would otherwise contribute to overall production.