Application of Recently Developed Medium-Curvature Horizontal-Drilling Technology in the Spraberry Trend Area
Authors: Edlund P.A. 
Published: December 2007

Summary.
Recently developed horizontal-drilling techniques have been applied in the Spraberry trend area in Midland County, TX. The desired application includes drilling horizontally at a right angle to the naturally occurring vertical fractures in one of three Spraberry zones. The result was 1,180 ft [360 m] of vertical section within 10 [0.17 rad] of the planned azimuth of 115 [2.01 rad].
The W.M. Schrock 38 No. 7 was drilled vertically to 8,620 ft [2627 m] through the Upper Spraberry, Lower Spraberry, and Dean formations. After openhole-log evaluation, the decision was made to attempt horizontal- drilling operations in the Lower Spraberry. Seven-inch [17.8-cm] casing was set 342 ft [104 m] above the horizontal target of 7,889-ft [2405-m] true vertical depth (TVD), and a cement plug was set and dressed to 7,567-ft [2306-m] TVD. Using a 20/100-ft [11.45 × 10–3-rad/ml bent- housing motor and compressive-service drillpipe, the well was kicked off and angle was built at about 18/100 ft [10.3 × 10–3 rad/m] until a 90 [1.6-rad] inclination was achieved at 7,871-ft [2399-m] TVD. After the curved portion of the hole had been drilled, 3- and 1/100-ft [1.7 X 10–3 and 0.57 X 10–3-rad/ml motors were used in both rotational and orienting modes to maintain an inclination near 90 [1.6 rad] and an azimuth of about 115[2.01 rad]. During drilling at 8,598-ft [2621-m] measured depth (MD), the motor parted and a fishing job in the near-horizontal portion of the wellbore resulted. The motor and bit were successfully retrieved on the first attempt. From 8,400- to 8,835-ft [2560- to 2693-m] MD, inclination decreased from 90 to 82 [1.6 to 1.4 rad], resulting in the penetration of the base of the Lower Spraberry. Drilling operations ceased at 7,929-ft [2417-m] TVD and 8,930-ft [2722-m] MD with 891 ft [272 m] of formation exposed vs. a 70-ft [21 -m] exposure in a standard vertical-well completion in the Spraberry trend area.

Introduction
The W.M. Schrock 38 No. 7 is located in Section 38, Block 37, T-4-S, Midland County, TX. See Figs. 1 through 3. The three Spraberry formations-Upper Spraberry, Lower Spraberry, and Dean-are all siltstones that are fractured because of minor tectonic movements. The fractures generally are believed to be oriented north 25 east. In a standard Spraberry completion, these zones must be hydraulically fractured before they will produce hydrocarbons. It was theorized that a horizontal wellbore drilled at a right angle to the naturally occurring fractures would increase production more than a hydraulic fracture. It was also believed that increased formation exposure would add to production.
The drilling of a horizontal wellbore in the Spraberry trend area was first evaluated in late 1984 with what has become known as the minimum-radius method. This method uses flexible drill collars and has a radius of curvature of about 30 ft [9 m]. The minimum-curvature method has several negative aspects, including poor control of direction and inclination. Also, the method is undesirable for use in areas of lost circulation because of severe wall-sticking problems that would be inherent with drill collars lying on the bottom of a horizontal wellbore. Lost circulation is a common occurrence in the Spraberry trend area. The commonly used maximum-radius method (3,000-ft [914-m] radius) was not applicable because of well-spacing rules and also because of hole problems encountered in drilling similar wells (45 [0.79-rad] in-clination) in this area it was believed that these same problems would arise if a maximum-radius well were drilled. Therefore, if a horizontal well were to be drilled in this area, a new method was required that would eliminate some of the associated problems caused by lost circulation while the use of standard oilfield tools continued.
During this same time, researchers were developing the medium-radius method, which uses compressive-service drillpipe that incorporates wear knots along each joint, thus reducing the wall- sticking problem by decreasing the wall-contact area. The compressive-service drillpipe is installed above the measurement- while-drilling (MWD) equipment but below the drill collars. The drill collars are essentially inflexible and must remain in the vertical portion of the wellbore. This arrangement allows for much more efficient use of drill-collar weight than the previously used minimum-radius method. Because the compressive-service drillpipe will always be rotated in compression, it will ride on the low side of the hole, therefore reducing keysest formation. The new method increased the radius of curvature to about 300 ft [91.4 m] while allowing for an extended reach that was previously available only in methods with a radius of about 3,000 ft [914 m]. To reduce the risk of lost circulation and associated problems, an inverted-oil-emulsion drilling fluid weighing 7.4 lbm/gal [890 kg/m3 was used. Fresh water is normally used as the drilling fluid in this area. It is believed that the oil-based drilling fluid also reduced drag. The differences between pickup, stack-off, and rotating weights were very minimal, even during drilling at a vertical section of 1, 180 ft [360 m]. This indicates a very slick hole.

Planning
Four wells were to the drilled in the Spraberry trend area, one of which would be horizontal. All four were evaluated as candidates for application of the medium-curvature technology. Major aspects that were considered were (1) timing of permits, (2) equipment availability, and (3) sufficient area to accommodate horizontal-drilling operations while Texas Railroad Commission spacing rules were followed. With these major considerations, it was decided that horizontal-drilling techniques would be used on the W.M. Schrock 38 No. 7.
A larger-than-normal location was built to accommodate the larger rig that was required, the technical personnel who were to live on location, and the room needed for the specialized equipment that was required for this operation-e.g., additional strings of drill- pipe and horizontal-drilling tools (shown in Fig. 4). A rig was selected that was equipped with pumps that could deliver 2,500 psi [ 1 7.24 MPa] and a range of flow rates from 150 to 300 gal/min [34.1 to 68.1 m3/h]. The rate requirements were dictated by the downhole mud motors used.
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