Rotary Drilling System Optimizes BHA Performance in Horizontal Austin Chalk Wells
Authors: Wisenbaker B.M. , Moser T.L. , Edwards M.L. , Eggleston W.H. 
Published: December 2007

The placement of an adjustable stabilizer on the bottom of a positive displacement motor (PDM) is being used to enhance horizontal drilling. These findings are unique in that these are the first known cases of its kind where an adjustable stabilizer has been successfully run below a motor. Several case studies of Sonat Exploration's work in the Austin Chalk, particularly in Newton County, Texas, and Vernon Parish, Louisiana, will be supplied, with field operating results highlighted.

The initial process of finding a bottomhole assembly (BHA) that could control inclination while rotating without sacrificing motor rate of penetration will be presented. General benefits of running a conventional steerable system will be reviewed, and the additional benefits offered by the new BHA configuration that was developed will be discussed. These benefits include reduction of trips, extension of horizontal sections (accessing additional reserves), reduction of mud additives, and increased life of the measurement-while-drilling (MWD) assembly and motor while drilling in deep, high-temperature wells. In addition, software simulation prior to field testing provided an accurate BHA for implementation, further reducing the cost of trial field testing. In conclusion, a comparison of earlier field data will be given to help quantify benefits of running rotary steerable systems as well as prejob planning with software simulation programs.

Introduction

The Austin Chalk is a naturally fractured carbonate reservoir that is productive over a region spanning 650 miles, extending from south Texas to east Louisiana (Fig. 1). In addition to fracture systems, the Austin Chalk also has extensive faulting and formation dip changes. These faults, fractures, and dip changes can significantly affect the build and drop tendencies of the BHA. To keep the BHA in the target zone, mechanical course corrections are usually required. The target zone in the Austin Chalk is typically a 15- to 30-ft window that is believed to contain the highest degree of fracturing (Fig. 2).

Previous BHA configurations

A bent housing PDM has been used as the conventional means of controlling inclination in the Austin Chalk. The motors have been used (1) without an adjustable stabilizer and (2) with an adjustable stabilizer placed behind the motor. PDM Without Adjustable Stabilizer. The BHA configuration shown in Fig. 3 is typical of those used in the Austin Chalk. A polycrystalline diamond compact (PDC) bit is used, and the assembly incorporates a dual power section with two points of stabilization. The medium-speed motor utilizes a fluid-cooled bearing assembly and provides about 240 rpm at 250 gal/min throughput. With a typical bend setting of 1.83, the assembly is capable of producing buildup rates (BURs) of 5 /100 ft to 7 /100 ft in the horizontal section.

To build angle with this steerable BHA, the tool face is oriented toward the high side of the hole. Weight is applied to the bit without rotation of the drillstring The drillstring slides along the low side of the hole, allowing the BHA to build angle. To drop angle, the same procedure is followed, except that the tool face is oriented toward the low side of the hole.

Sliding a steerable BHA to control wellbore inclination in the 15- to 30-ft target window of the Austin Chalk is very time consuming. Sliding becomes even more difficult as mud weights approach 12 lb/gal and lateral displacements approach 3,000 ft. P. 483^