Short-Radius Horizontal Drilling System Optimization in Yates Field Unit Provides Excellent Directional Control and Highly Economical Completions

Abstract

Summary Yates field unit in Pecos County, Texas, has recently drilled its 100th short-radius horizontal drainhole. The program has been a technical and economic success for the field, providing efficient completions in this highly fractured reservoir. In the early stages of the program (1986 1990), short-radius drilling in Yates field unit was performed using strictly rotary drilling systems. In 1992, Odorisio and Curtis1 reported on the introduction of articulated downhole mud motors to the program and the impact of the initial modifications to the system. Since that time over 75 additional drainholes have been drilled. Two other drilling systems have been tested and many improvements have been made to the original system. This paper documents the optimization that has occurred in the drilling systems and the results obtained from this work. Introduction The Yates field unit is located at the southern tip of the Central Basin Platform, 90 miles south of Midland, Texas (Fig. 1). It consists of an asymmetric, horseshoe-shaped anticline with greater than 400 ft of closure that covers 26,400 acres.2 The main producing formation is the Permian age San Andres formation found at 1,200 to 1,700 ft. Being at a shelf edge, the formation was well worked from wave action depositing porous limestone grainstones. Tectonic events after deposition raised the field above sea level, causing fracturing and allowing solution enhancement (karsting) from fresh water.3 Subsequent burial led to nearly complete dolomitization, and the formation of an anhydrite seal from the Seven Rivers formation. The resulting reservoir today is a highly fractured dolomite with average matrix porosities of 18%, and initial well-productivity index measurements frequently exceeding 50 BOPD per psi of drawdown.2 Gravity drainage is the drive mechanism for the reservoir. A relatively thin oil column of 50 ft is bounded by an expanding gas cap and bottom aquifer. The density difference between the oil-filled matrix and the gas-filled portion of the fracture system above the gas/oil contact allows for a very efficient recovery mechanism. Since discovery in 1926, cumulative production has been in excess of 1.3 billion bbl, with current production averaging 54,300 BOPD. Since the introduction of the system in 1986, the objective of the short-radius horizontal program has been to enhance production in those areas of the field where localized areas of the reservoir are not as productive because of facies changes such as interbedded shales and secondary calcite cementation.1 Exposing more of the rock with a horizontal borehole minimizes gas coning and offers the opportunity to connect up with the major fracture trends in the field. Properly placed boreholes maximize the production efficiency in these areas of the field. The short radius system is the most cost-effective method of drilling horizontals in Yates field unit because it eliminates problems of drilling through a cavernous gas cap, can be used in the nearly 1,630 existing wells, and does not limit the lateral length necessary in this field with 10-acre spacing. General Discussion of the Short-Radius Technique The short-radius technique is almost exclusively used in existing vertical wellbores, and is typically drilled with workover rigs instead of drilling rigs. With less equipment being required to drill a short-radius well and most being re-entries, a large cost savings can be achieved over medium- and long-radius wells typically drilled as grassroots wells. This system builds a radius from vertical to horizontal in 20 to 100 ft and produces laterals ranging from 100 to 1,300 ft. If avoiding troublesome overlying zones are key to a successful well, short radius provides a method by which one can drill under these zones with casing isolation above. For example if a 45-ft radius is required to remain below difficult formations, and lateral lengths are limited to 1,000 ft due to well spacing, a short radius will provide nearly 50% more exposure to the zone of interest than a comparable medium radius well (Fig. 2). Even though some initial development of the system took place in the 1940's, short radius did not begin to be used until the 1980's.4 Initial short-radius horizontal wells were drilled using a rotary drilling system, in which drillpipe was articulated to negotiate the severe doglegs of the curve. Surveying was only possible after tripping for the steering tool. The flexible nature of the system, while allowing negotiation of very tight radii, also provided control problems. In the late 1980's downhole mud motors were developed that would negotiate the tight radii and allow continuous surveying with a downhole steering tool, or more recently measurement-while-drilling (MWD).5 For radii less than 70 ft, the mud motors were articulated in order to pass through the curve. The majority of short-radius horizontals are drilled in slide mode where the drill string is not rotated. For radii under 80 ft, rotation has to be limited to a rolling of the tool face, 3 to 5 revolutions every 5 to 10 ft.6 The limitation is the drillstring, not the motor systems. Above 80 ft, 20 to 30 rev/min rotation is possible with careful examination of drillstring integrity on a regular basis. Some work is being done with composite drillpipe, which is a combination of metal and fiberglass for added flexibility. Yates Field Unit Short-Radius Drilling Operations From 1986 through the end of 1995 a total of 105 short-radius horizontal boreholes have been drilled in the field (Fig. 3). This total includes 16 boreholes that were redrilled once gas/oil contact movement downward made the existing borehole unproductive because of excessive gas production. As previously discussed, the role of short-radius horizontal wells in Yates field unit is to maximize completion efficiency in those parts of the field where lower productivity exists by exposing additional reservoir and intersecting fractures. Minimizing gas coning is the ongoing production management objective of all wells in the field, and the short-radius horizontals have proven the most effective method to combat this problem where traditional vertical completions are insufficient.

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