Drilling and Production Aspects of Horizontal Wells in the Austin Chalk
Authors: Shelkholeslami B.A. , Schlottman B.W. , Seidel F.A. , Button D.M. 
Published: December 2007

This paper discusses testing of horizontal technology for use in the highly fractured Giddings field. Three short- and seven medium-radius wells were drilled successfully in the Austin Chalk formation. The paper discusses well plans, bottomhole assemblies, trajectory control, telemetry, mud systems, hydraulics, hole cleaning, casing design, cementing, problems encountered, formation evaluation, completions, and reservoir response.

Introduction

The Austin Chalk formation of the Giddings field in central Texas (Fig. 1) is an Upper Cretaceous fractured limestone oil and gas reservoir that was developed in the late 1970's and early 1980's. Initial development was accelerated by the explosive oil prices of the time and by the earlier development of the Pearsall field.

Although the Giddings field was discovered in 1961, its potential was not evident until 1973, when re-entry into the Giddings No. 1 flowed 300 BOPD. Field development began in 1976 and accelerated rapidly. Because of the field's highly variable natural fracture system, initial performance of vertical wells ranged from highly prolific to exceptionally poor. Field development declined rapidly after 1982, partly because of failing oil prices.

By 1984, reservoir studies and stimulation analyses had revealed why recovery efficiency from existing vertical wells remained low: the vertical wellbores were linked to only one natural fracture system, leaving unpenetrated fracture systems and matrices poorly drained. Instead of drilling infill vertical wells to drain the reservoir, we tested horizontal drilling and completion technology to determine whether multiple natural fracture systems could be linked to a single wellbore.

Since 1985, a total horizontal displacement of 14,500 ft has been drilled within the pay. The longest horizontal displacement, pay. The longest horizontal displacement, 2,200 ft, was drilled within a 30-ft target.

Geology and Background

The Austin Chalk produces along a trend 10 to 25 miles wide and 250 miles long from Mexico to Brazos County, TX, with scattered production in east Texas.

The most prolific area to date is the Giddings field, covering 1,200 sq miles in Burleson, Lee, Fayette, and Washington counties. Structural strike in the Giddings field is northeast, with the productive portion of the field ranging from 6,500 to portion of the field ranging from 6,500 to 12,000 ft. Porosity ranges from 3 to 12%, with very low matrix permeability (less than 0.1 md). Net pay thickness ranges from 10 to 240 ft.

Reservoir development in the Austin Chalk is almost wholly the result of fracturing, creating a permeability system of near-vertical fractures that run parallel to structural strike (i.e., northeast/southwest). Because these linear fracture systems, which connect the tight matrix with a wellbore, are isolated flow channels, only limited areas can be drained by a vertical wellbore. Horizontal drilling, however, connects multiple vertical fracture systems with a single perpendicular wellbore to drain a larger area perpendicular wellbore to drain a larger area at higher producing rates.

All 10 horizontal wells targeted the bottom portion of the Austin Chalk because of its abundant fractures, greater oil saturation, and proximity to the Eagleford, which also may be a major source-rock contributor of hydrocarbons.

Definitions for Horizontal Wells

The popularity of horizontal drilling during the last few years has led to use of the term "horizontal drilling" for many conventional directional-drilling operations, river crossings, extended-reach drilling, and other directional applications. We think horizontal drilling should be defined as a drilling and completion technique in which the wellbore remains in a high-angle trajectory roughly parallel to the formation, thereby exposing significantly more pay to production than would be exposed by a vertical production than would be exposed by a vertical wellbore. Many of the more recently developed, aggressive curve-building techniques are not actually horizontal drilling unless the lateral wellbore remains in the target for a distance 10 or more times the thickness of the net pay. In many reservoirs, a lateral length 20 to 50 times the thickness is necessary to make horizontal drilling economically viable.

JPT

P. 773