Various Fracturing Fluids Used in the Codell Formation in the Denver-Julesburg Basin of Northeastern Colorado
Authors: DeBlasio C.J. , Bailey D.E. , Bergthold J.L. 
Published: December 2007

Abstract
This paper presents an overview of the stimulation treatments performed on the Codell Sandstone Formation in the Denver-Julesburg (D-J) Basin northeast of Denver, Colorado. The Codell Formation, a recently-targeted oil and gas zone, provides a basis for much speculation and a variety provides a basis for much speculation and a variety of treatment designs.
Due to a high percentage of clays and an intense natural fractured network, considerable formation damage is encountered through conventional drilling operations. The Codell Sandstone will not economically produce without a stimulation treatment to bypass the damaged zone. Hydraulic fracturing is the best method to date to stimulate this formation.
Treatment fluids used on the Codell Sandstone are classified into two categories: aqueous and non-aqueous based fluids. The majority of field treatments producing the most satisfactory results thus far are the aqueous based fluids consisting of a gelling agent mixed in a potassium chloride (KCl) base. Sand concentrations as high as eight pounds of proppant per gallon of fluid have been hydraulically pumped into this low permeability formation with mixed results.
Recently, non-aqueous based fluids utilizing hydrocarbons or methanol as a treatment fluid base in either a gelled or foamed state have experienced some degree of popularity with some operators in the D-J Basin. This particular type of treatment is considered to be experimental with results currently indicating research is still needed before more extensive application is widely advocated.
Case histories on producing offset wells are presented. presented.

GEOLOGY
The Codell Formation is a bioturbated, fine-grained, thin-bedded argillaceous sandstone underlying more than 2,000 square miles in the Denver-Julesburg (D-J) Basin northeast of Denver, Colorado (Fig. 1). This formation is the uppermost member of the Carlile Shale group of mid-Cretaceous age, unconformably overlain by the Timpas Limestone (also known as the Fort Hays Limestone) with a gradational basal contact with the Blue Hill Shale (Fig. 2). The Codell Sandstone is a marine coastal deposit laid down in a near-shore environment. Reserve estimates of 6 billion barrels of oil and/or condensate plus 10 trillion cubic feet of gas may be present in the Codell Sandstone of the D-J Basin.
Detailed seismic and subsurface work indicate the presence of a major fault near the Loveland, Colorado area resulting in the absence of the Codell Sandstone east of Larimer County, Colorado. It has been suggested that the associated flexure and stress faults underlying the Greeley-LaSalle-Johnstown area provide some degree of stratigraphic trapping mechanism for the Codell reservoir fluids. Many geological characteristics such as clay content, porosity, and permeability appear to be of a localized nature and are best defined as drilling in the respective area dictates.

FORMATION CHARACTERISTICS
Laboratory studies of core samples indicate that the Codell Sandstone contains a high percentage of clay minerals intermixed within a percentage of clay minerals intermixed within a framework of very fine moderately sorted grains of quartz, chert, feldspars, and other rock fragments. The intergranular pore spaces are heavily lined with illite, smectite and mixed layer (illite/smectite and chlorite) clay particles with traces of calcite occupying the remaining void spaces. Iron compounds such as chlorite, siderite, and pyrite also may exist in small amounts throughout the rock pore spaces (Fig. 3).
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