The Ultrashort-Radius Radial System

Abstract

Summary. A group of interrelated horizontal drilling and completion technologies, collectively called the ultrashort-radius radial system (URRS), was developed and is being progressively applied in the field. Multiple radials can be placed at the same level and on multiple levels. Three-dimensional (3D) surveying is supplied. Horizontal completions can be provided, including 100%-fill gravel packing, in-situ electrolytic perforation and cutting, and flexible sand barriers (FSB's). Initial field applications were in unconsolidated formations. Introduction The URRS was developed, tested, and applied over the past 10 years. This paper reviews the drilling and completion technologies, presents results from initial field applications, and outlines ongoing research. With this technology, more than 27,000 ft [8230 m] was drilled and more than 500 horizontal radials were placed by use of various embodiments of the system. System Concepts The objective for the URRS is to provide an extended wellbore radius by means of multiple radials from a vertical wellbore (i.e, to effect an extended completion or extended piped perforations). These radials may be placed in one layer or multiple layers, depending on reservoir thickness and vertical communication. Figs. 1 and 2 show two arrangements of multiple radials in multiply layers. The choice of radial length, number of radials, and radial array is a function of the reservoir properties. A study to optimize these radial parameters for various reservoir conditions is currently under way. The specific variables included in this study are reservoir thickness, vertical and horizontal permeabilities, oil properties, well spacing, outer-boundary reservoir pressure, gravity drainage, thermal and nonthermal processes, and presence of impermeable partings within the reservoir. The choice of radial length and arrangement generally is unique to each reservoir. System Processes and Equipment The basic URRS uses an erectable whipstock lowered downhole by a 4 1/2 -in. [114-mm] workstring into an underreamed cavity or hydraulically slotted opening of 22-in. [56-cm] diameter. The whipstock (Fig. 3) is designed for use in a 7-in. [178-mm] casing. The drillstring is made of 1 1/4-in. [32-mm] electric-resistance welded tubing (A-606). The drillstring may be provided from a coiled-tubing rig or it may be fabricated on site from 30- to 40-ft [9- to 12-m] tubing joints. A hydraulic drill head is welded to the nose of the first joint of the drillstring (radial tube). If the drillstring is fabricated on site, subsequent 30- to 40-ft [9- to 12-m] joints of drillstring are welded by automatic computer-controlled welding on the rig floor to form the drillstring. A hydraulic motion controller that regulates rate of penetration (ROP) is welded to its tail. As the drillstring is fabricated, it is lowered inside the vertical 4 1/2-in. [114-mm] workstring. The nose (drill head) of the drill-string enters a high-pressure removable seal at the top of the whipstock. The seal provides the bottom closure of the workstring. Hence, the 1 1/4-in. [32-mm] drillstring is fully contained within the 4 1/2-in. [114-mm] workstring at the outset of drilling (Fig. 3). A wireline cable attached to the tail of the drillstring runs to the surface within the workstring and passes through the top closure of the workstring. Thus, a long sealed chamber containing the 1 1/4-in. [32-mm] drillstring and its connecting cable is created by the 4 1/2 -in. [114-mm] vertical workstring. Water drilling fluid at 8,000 to 10,000 psi [55 to 69 MPa] is pumped into the long vertical workstring at the surface with a conventional fracture pump. The drilling fluid is then pumped down the workstring where it enters the drillstring. The internal water pressure of the drilling system propels the drillstring through the high-pressure bottom seal and through the bending and confining slides and rollers of the whipstock. Traversing the 12-in. [30-cm] radius and 90 degrees bend of the whipstock, the drill head enters the formation horizontally. The drillstring is not rotated. These separate components of the URRS-the drillstring, the motion controller, and the whipstock-combine to propel and to control the motion of the drillstring into, through, and out of the whipstock, resulting in three load conditions of the drillstring. The first URRS component related to propulsion and control is the drillstring (radial tube), which is propelled out of the vertical workstring by the fluid pressure within the workstring. The second component is the motion controller (Fig. 4) on the tail of the drillstring, which acts as a hydraulic restraint. In essence, it is a piston with external seals that slide within a special smooth borehole portion of the vertical workstring. The high-pressure water pushes on the top of the motion controller, and water is trapped between it and the high-pressure seal at the bottom of the workstring. Water can escape only through a central orifice within the controller (Fig. 4). The result is a hydraulic restraint, or brake, on the forward motion of the 1 1/4 -in. [32-mm] drillstring. The third URRS component of the propulsion and control system is the whipstock, which bends the drillstring from vertical to horizontal. Fig. 5 shows the loads on the drillstring that result from propulsion and restraint forces. In its passage into, through, and out of the whipstock, the drillstring is subjected to axial, internal-pressure, and bending loads. In Section A of the drillstring (above the high-pressure seal), the drillstring stresses are below the elastic limit. In Section B, where the drillstring is below the high-pressure seal and within the whipstock, the drillstring stresses exceed the elastic limit and the drill-string deforms plastically. Because the drillstring is internall pressurized and is constrained by rollers and slides within the whipstock, it does not buckle while it is being bent. In Section C, the 1 1/4-in. [32-mm] drillstring exits the whipstock horizontally. There it is under only axial and internal-pressure loads. Again, the stresses are below the elastic limit. The pressure on the water drilling fluid in the system not only propels the drillstring, but also drills the horizontal borehole in the formation. To drill the formation, the water drilling fluid is acceler-ated through the conical-jet drill-head nozzle, creating a conic shell of water particles traveling at 800 to 900 ft/sec [244 to 274 m/s]. Fig. 6a shows a schematic of the conical jet. At the top of the figure is a standard collimated jet nozzle. The addition of fixed vanes within the nozzle causes a conical shell of high-velocity water particles to form a conical jet (Fig. 6b). The size of the horizontal bore-hole is established by the twist of the vanes, which in turn controls the angle of divergence of the cone of water particles. Figs. 6c and 6d show vanes for two different conical angles. Fig. 7 shows water jets resulting from various degrees of vane twist in 1-microsecond flash photographs of a collimated jet and two different conical jets. The conical angle is not affected by drilling-fluid pressure. These conical jets function at both ambient and elevated backpressures. At higher backpressures, cavitation does not appear to be an important cutting mechanism. Fig. 8 shows test results of submerged conical jets at ambient and elevated backpressures (2,000 psi [13.8 MPa]). The conical jets cut through unconsolidated and consolidated formations and produce a radial borehole with a diameter of about 4 in. [10 cm] or more in unconsolidated formations; a smaller-diameter hole is produced in hard rocks.