The Azimuth of Deep, Penetrating Fractures in The Wattenberg Field

Abstract

An experimental program was conducted in the Wattenberg gas field that included borehole measurements, surface measurements, and laboratory tests on oriented cores. Results indicated an apparent variation in azimuth over the field, good correlation between strength anisotropy and fracture azimuth, and that fracture length may not be symmetric around the wellbore. Introduction With better wellhead gas prices and increased demand for gas, it has become economically feasible to produce the so-called "tight" gas reservoirs. Massive hydraulic fracturing (MHF) has proved successful in producing these formations. An MHF stimulation, which may involve thousands of barrels of fluid and 1 million lb or more of proppant material, creates a deep, penetrating fracture proppant material, creates a deep, penetrating fracture extending from the wellbore. These fractures generally proceed along a single azimuth line in both directions proceed along a single azimuth line in both directions from the wellbore and may extend 1 mile or more from tip to tip. The optimum draining of such a reservoir demands a knowledge of the azimuth of the fractures. A field experimental program was set up to measure fracture azimuth in the Wattenberg field. The Wattenberg gas field is located north of Denver, Colo. The Muddy "J" formation is the major gas producing zone. This blanket sandstone formation is found at producing zone. This blanket sandstone formation is found at about 8,000 ft and has permeabilities ranging from 5 to 50 md. Natural gas production from this zone ranges from a small show of gas to 100 Mcf/D. The field can be produced economically only through massive produced economically only through massive fracturing. A more complete description of the field is found in Refs. 1 and 2. The success of massive fracturing in this field indicates that a deep, penetrating fracture is created. History of Fracture Orientation Work This study of the fracture azimuth in Wattenberg field began in 1973, when an attempt was made to monitor the progress of an induced fracture with an arrangement of progress of an induced fracture with an arrangement of seismometers located on the surface. These devices were placed both radially and linearly from the well. However, placed both radially and linearly from the well. However, acquisition systems and processing techniques were unable to discriminate the low-level signals in a relatively high noise environment. A second attempt to determine the direction of fractures was tried in Jan. 1975 at Well G (see Fig. 1), which was completed open hole rather than through casing in Feb. 1971. Wells in Wattenberg normally are completed with 4 1/2-in. casing set through the pay zone, perforated, and fractured. However, Well G was completed open hole and provided an excellent opportunity to try orientation work. Well G, located in the southwest part of the field, was completed originally with an initial flow rate of 791 Mcf/D after stimulation by fracturing with 38,000 gal of gelled water. In Jan. 1975, before restimulation by fracturing with 310,000 gal of polymer emulsion, impression packers were run to determine fracture azimuth. Two attempts were made with 22 ft of packer on tubing. However, both attempts resulted in packer rupture. A third attempt after refracturing was scheduled, but was cancelled because of hole sloughing. Analysis of the first two attempts indicated that hole enlargement caused by the first fracture was the primary cause of failure. JPT P. 185