Abstract

Refracturing wells, completed in the Codell formation, has caused a resurrection in activity in the Wattenberg Field of the Denver-Juleseburg (D-J) Basin. HS Resources (HSR) has increased average oil and gas rates by greater than 500% by restimulating over 750 Codell wells. The evolution of applied fracturing fluid technology has played a major role in the success of the Codell refrac program.

This paper will identify and evaluate the benefits to well performance and economics gained from the evolution of fluids used in the Codell refracture program. In this effect, the fluid systems will be compared using treating pressure characteristics, production analysis, fluid properties, and rheological property evaluation via specialized testing apparatus and economic results.

Reservoir Geology

The Codell Sandstone is Upper Cretaceous in age and produces condensate and gas with little water at true vertical depths of 7,000 to 8,000 feet. The Codell is a highly bioturbated marine bar-margin sandstone deposit which was initially over pressured (0.6 psi/ft pore pressure gradient, in the central portion of the field). Bottomhole static temperature ranges from 240 to 260°F across the field with 250°F as the expected average. The Codell is 10 to 20 feet thick and is rich in clay (15 to 25% by volume). Sediment sorting is poor with mixed layer illite/smectite clay occupying pore filling and pore-lining habitats. Trapping is enhanced by regional basement fault trends to the west and north, and an erosional pinchout to the south, southeast and northeast.

Pore volume (porosity x thickness) patterns parallel the erosional edge of the Codell to the southeast of the Wattenberg field. Typical pore volume phi-h values range from 1.5 to greater than 2.0. Pore volume is only important if the value falls below the 1.5 value as exists in the unproductive southeast area of the Wattenberg Field.

The permeability in the Codell interval is very low due to the small and tortuous pore network1. Typically, mercury injection into the core shows that 94% of the rock pores has a radius of one micron or less. Numerous interpretations of post frac permeability have showed the effective permeability ranging from 0.01 to 0.09 md.

Evolution Towards Refracturing

Early activity in the basin consisted mainly of drilling to, and exploiting, the J-Sand formation. The Codell was only completed sporadically until the early 1990's. At that time, the Codell and Niobrara zones were typically completed together in a limited entry treatment2. The primary reason for commingled zone treatment was the favorable economics. Two other treatment styles were also investigated and performed by a variety of operators in the basin: Codell only completions and dual completions in the Codell and Niobrara.

From the extensive analysis of the three types of completions pumped on the Codell and Niobrara it was determined that:

  1. Dual completions in the Codell and Niobrara correlated with higher well productivity.

  2. Production from the limited entry treatments was 5 to 6% lower than the dual completion treatments.

  3. Production from the Codell only stimulations was 20 to 21% lower than the dual completions.

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