Stimulation Of The Niobrara Formation Using Foamed Methanol-Water
Authors: Rohret Matthew T. , Jones T.C. 
Published: December 2007

The Smokey Hill Chalk section of the Niobrara formation has historically presented problems using conventional stimulation techniques.

The Niobrara formation is a very porous, poorly consolidated micrite. Its high porosity and fracture permeability contributes to its fairly low rock embedment strength and its tendency toward fluid imbibition. A moderately high clay content is also evident (Table 1).

A discussion of formation analysis, drilling program, and conventional stimulation treatments program, and conventional stimulation treatments is presented.

Stable foam fracture treatments using 6260 gallons of 2 percent potassium chloride water for a final volume of 31,300 gallons of .80 quality foam carrying 32,000 pounds of sand has proven to be superior to past treatments.

A more recent improvement has been the elimination of 50 percent of the water with the substitution of 3130 gallons of methanol. Thus, all of the advantages of an alcohol based fracturing fluid are obtained, while at the same time keeping the fluid content and cost increase down to a minimum.

Introduction

The earliest reported production of the Niobrara in this area was in 1919. One well, drilled in the 1920's has produced locally for a rancher for many years. The early completion was drilled with cable tools and completed open hole with no stimulation.

Four wells were drilled in 1972 by setting casing above the Niobrara and drilling the gas zone with air, resulting in natural completions from 50 to 120 MCFD. However, all of the open-hole sections eventually collapsed, resulting in recompletions with liners or abandonment of the wells. It should be noted that later DST's of mud-drilled holes flowed only about 5 MCFD, indicating that wellbore damage does occur as compared to the air-drilled holes.

DRILLING AND COMPLETION PROGRAMS

The wells are normally drilled to the top of the Niobrara with native mud, changing to a low fluid loss mud while drilling through the pay zone, which averages 30 feet in thickness. A conventional casing program calls for running 4 1/2" casing with a guide shoe and float collar, and with scratchers and centralizers located through the pay zone.

A 500 gallon mud flush spacer is pumped, followed by 125 sacks of 50-50 Poz Mix or Class G cement. Casing is reciprocated during cementing operations to effectively clean the formation walls and to facilitate an even cement cover.

At the time of completion, the casing is swabbed down to remove all of the water or mud. The formation is then perforated one shot per foot over a 20 foot interval using a casing gun. Natural unstimulated production ranges from 0 to 50 MCFD.

STIMULATION OF THE NIOBRARA CHALK

With an acid solubility in excess of 80 percent, the Niobrara would appear to be a likely percent, the Niobrara would appear to be a likely candidate for acid stimulation. However, the release of formation fines after acidizing has almost precluded the use of acid.

Laboratory tests were run on a core sample (83 percent soluble) and the results showed that 182 to 376 lbs. of fines would be released for each 1000 gallons of 15 percent HCL pumped.