An Effective Hydrajet-Fracturing Implementation Using Coiled Tubing and Annular Stimulation Fluid Delivery

Abstract

Abstract The use of coiled tubing (CT) in fracturing was uncommon until a few years ago. Its applications have usually been limited to small jobs, and in reality, CT stimulation treatments have often been limited to acid placement treatments. With the inception of hydrajet fracturing technology, the use of CT has become more popular. Historically, CT fracturing methods were limited to placement of small fractures in the formation, and in some acidizing situations, near-wellbore damage-bypass methods. CT technology has progressed until now it is used to place large fractures, some reaching over 600-ft half-lengths. Obviously, in most formations, such fracture sizes would not be possible with fluid delivery originating from the CT alone, so the use of the annulus to deliver the primary fracturing fluid becomes vital. This paper discusses the knowledge gained from recent field applications that have used this new annular path hydrajet-fracturing technology. Unique features of this technology are presented. Added benefits, such as the ability to deliver large proppant volumes, will be a focal point of this paper. Case studies involving placement of multi-fracture treatments in three wells treated with this procedure will be discussed. Production improvements experienced in these wells will also be presented. Introduction Since its first implementation in September 1963, the use of CT has proliferated into many sectors of the oilfield services industry. These uses include pipe cleaning, perforating, placement of various tools, logging, fishing, chemical placement, matrix treatment, and drilling, just to name a few.[1–5] In most cases, CT is considered for an application because of its ability (1) to rapidly deploy without the use of drilling rigs, and (2) to go through small restrictions or production tubing. Additionally, the inherent safety backup feature of annular blowout preventers (BOPs) incorporated into many CT systems has made this technology increasingly popular. The latest implementation of CT is its use in fracture stimulation. In fracture stimulation, the BOPs work well with the high-pressure requirements of stimulation applications. However, fracture stimulation generally requires a high injection rate for good fracture extension, and small-diameter coils do not have the needed flow capacity. Therefore, when CT fracturing began, only small fracture placements were addressed.[6] With the advancement of hydrajet-fracturing technologies[7–12] however, the inherent multiple flow path feature is opening new possibilities for larger fracture sizes using CT systems. The development of larger CT systems makes ever larger fractures possible. In the following sections, hydrajet-fracturing stimulation technology will be discussed as well as jetting equipment reliability in large fracture placement. A new stimulation solution is provided, and three case studies are reviewed. Hydrajet Fracturing Revisited Although invented in 1998,8 hydrajet fracturing is a relatively new technology in the area of hydraulic stimulation of a well. Until 2001, few hydrajet-fracturing operations had been implemented, and, although these experiences were highly successful, most operators were still reluctant to try such a new technology. Since that time, however, acceptance has been increasing rapidly, although some skepticism still exists. The hydrajet-fracturing method uses tubing to deliver high-velocity fluids to the formation or casing wall through jets at up to 700 ft/sec. Because the fluid usually contains sand or other abrasive proppants, this jetted erosive fluid will cut a cavity in the casing and/or wellbore wall. Note that no separate perforating stage is needed when using this process. When a deep perforation is first created (this usually takes a minute or two), the fluid does not have a path to follow except backwards, and this abrupt u-turn causes pressure in the cavity to increase. Another aspect of this process is that pressure in the wellbore around the jet is reduced, based upon the Bernoulli equation.[7–12] This makes the pressure within the cavity much higher than the pressure within the annulus (Fig. 1).