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The Codell formation is a low permeability, clay rich, late Cretaceous agesandstone within the Wattenberg field of the DJ Basin. Since 1997 over 1500Codell wells have been restimulated. Results on the past 200 refractured Codellwells using a reduced CMG polymer fluid system have yielded incremental monthlyproduction results in excess of 1900 BOE/well or approximately 80% of theoriginal initial production. Success of this program is believed to be thecombination of stringent well selection criteria, high fluid quality controlguidelines and effective operational field practices. It is believed that dueto this recent success in restimulating the Codell, over 4000 additional wellswithin the DJ Basin may be restimulated with economic benefits.
This paper will discuss completion history of the Codell formation and howcriteria from candidate selection to fluid quality may impact the success ofsuch a program.
Records indicate that the first Codell completion in the Wattenberg field ofthe Denver-Julesburg basin occurred in 1955. It was not until the early 1980'sthat the Codell became a major gas play. Since that time thousands of Codellwells have been developed within an area of approximately 100,000,000acres.1 The DJ basin shown in Figure 1 is an asymmetricalbasin just north of Denver, Colorado with the axis of the basin runningparallel to the Front Range uplift.
The Codell, described as Type 2 sandstone by Weimer andSonnenberg2, is a member of the Upper Cretaceous Carlile shale. Itis a bioturbated, reworked fine-grained marine shelf sandstone without acentral bar facies and is laterally continuous across the field area. Figure2 is a stratigraphic sequence of formations bounding the Codell within theDJ Basin. Over the years, many wells have included the Niobrara formation bymeans of separate completions on both the Niobrara and Codell or simultaneouslystimulating both formations with limited entry techniques.3Production from the Niobrara often times has proved to be limited due tonanodarcy matrix permeability. Due to the Niobrara inconsistency, a greatnumber of wells were completed solely in the Codell interval.
These reservoirs were initially over pressured with a pore pressure gradientof ±0.60 psi/ft.4 Pay thickness in the Codell can range from 14 to20 ft. within the central portion of the basin at typical depths of 7000 - 7200feet. Bottom hole temperatures are generally between 230-250° F BHST. It is aclay rich sandstone (15-25% by volume) with pore filling and pore lining mixedlayer illite/smecite clays. Permeability is low (i.e., <0.1 millidarcies),with Density log measured porosity ranging from 8 to 20%.
Aggressive exploration and development of the Codell led to severalexperimental completion techniques throughout the years. Wells were stimulatedwithout regard to geological and lithologic variations. A wide range of fluidtypes and treatment designs were implemented in order to achieve an economicalrate of return.
Original Codell Completions
An obvious contributor to the potential for restimulations is the originalcompletion. The Codell formation has a history that begins in the middle 1950'sand continues today with the continued drilling of acreage within the DJ Basin.Original completion techniques and stimulation fluids utilized have affectedthe results of restimulation programs.