Selectively Placing Many Fractures in Openhole Horizontal Wells Improves Production

Abstract

Summary This paper presents the results of a study on an openhole well that was treated with a new fracturing process - hydrajet fracturing. In only a single pass, hydrajet fracturing allows operators to use high-pressure jets to place multiple fractures accurately in several locations in a well. To fracture the well most effectively, the process uses surface and downhole data recorders to monitor well conditions. Study results show that the hydrajet fracturing process is an effective, economical method for fracture-treating openhole horizontal wells. Introduction Hydraulic fracturing is a common practice in today's oil industry. Since its inception in 1948,1 the process has been improved by new fluid systems, resulting in increased volumes, proppant concentrations, flow rates, and pressures. Originally used in vertical wells, hydraulic fracturing processes now can be used in horizontal wells, primarily in cased wells. When fracturing was a relatively new process, unknown phenomena caused numerous problems. Today, an understanding of well properties (massive leakoffs, tortuosities, limited entry, coplanar entry, fracture direction, etc.) allows operators to maximize fracturing efficiency. In cased horizontal wells, fractures perpendicular to the wellbore can be effectively initiated if operators reduce the length of the perforation interval, increase the perforation density, and slug the well.2 With the hydrajetting method, the well can be perforated coplanar with the preferred fracture plane, then fractured.3 In openhole horizontal wells, large, exposed wall surfaces cause high fluid loss, especially when natural fractures are present. Therefore, stimulations are limited to either uneconomical, random placement of fractures performed at ultrahigh rates, or well damage removal with movable pipe, such as coiled tubing.4 Hydrajet Fracturing Operators and service companies have searched continually for ways to perform effective fracturing treatments. One such solution, hydrajet fracturing, was recently introduced as an alternative to traditional fracturing processes.5 With this economical method, operators can quickly and accurately place multiple fractures in the same well without using sealing elements. The hydrajet fracturing method, based on a Bernoulli equation, maintains low wellbore pressures and effectively initiates strategically placed fractures. Fig. 1 shows a single horizontal well that consists of multiple large fractures, strategically placed where hydrocarbons may exist. Each fracture can be formulated with different fluids, such as sand slurries or acid, depending on the rock formation surrounding the fracture entry point. Moreover, numerous small fractures can be placed throughout the well, bypassing damaged areas. The process is economical because all the processes involved can usually be performed in one trip down the well. Hydrajet Fracturing Treatment Procedure. To perform a hydrajet fracturing job, operators must use a jetting tool with coplanar-adjusted jets. The jetting plane must coincide approximately (varying no more than 30°) with the reservoir's preferred fracture-extension plane. To place the jets on or near the fracture-extension plane, operators should first attempt to predict the expected fracture direction in the well. Predicting the fracture direction is generally done with existing techniques, such as extensiometers or other tools. When the fracture direction is known, a jetting tool can be designed and placed accordingly. Although it is possible to design a tool with a completely adjustable jet-positioning system, it is more economical to design one with fixed jet positions. Moreover, a fracturing process should be properly designed on the basis of conventional procedures, which help establish desirable pressures and flows for the well. The number of jets and jet sizes changes from well to well; therefore, a tool is designed specifically for a particular well. When the fracture direction is approximately perpendicular to the wellbore, positioning the tool requires only accurate depth placement. Although placement accuracy is generally not critical, some cases may require a high degree of accuracy. In those cases, sophisticated wireline equipment may be necessary for positioning the tool. However, in most horizontal well applications, this degree of accuracy is not necessary. When the fracture direction is not perpendicular to the wellbore, or if the fracture is to be initiated in a particular direction (such as only upward or sideways), downhole positioning of the tool is important because the pipe rotates, or uncoils, during insertion, and the tool's actual downhole position is unknown. For proper jet positioning, the tool is premanufactured with the jets fixed at an angular position, as shown in Figs. 2 and 3. Two scenarios are possible when the hydrajet tool is used for fracturing a well. In the first scenario, the jetting tool is rotationally positioned, by string rotation from the surface, in the downhole as soon as the correct depth is reached. To accurately determine the current position of the tool, the operator attaches a wireline tool to the jetting tool, with matching muleshoes, as shown in Fig. 2. If wirelines are not selected, a positioning sensor can be placed below the jetting tool and connected to a wireless data-transmission system similar to the one described in this paper (Fig. 4). For the second scenario, a swivel system uses gravity to correctly position the tool. This purely mechanical method is much simpler and more economical (Fig. 3) than the first scenario. The jetting tool also has a return bypass system, which is needed only if fluids must be reversed during the job. While pumping fracturing fluids through the jets, the operator uses the flow down the annulus to control the bottomhole pressure (BHP) and to supplement the proper fracture with fluid. Because the well becomes supercharged during fracturing, operators must use tools to maintain annulus pressure during pipe movement and must install a tubing valve in the tubing string downhole to allow new connections to be made.

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