Restimulation of wells completed in the Codell formation, a tight gas sand, has proven to be successful in the Wattenberg Field in Colorado. Beginning in 1997, Codell refracturing evolved into a massive program involving hundreds of wells per year. To date, HS Resources has restimulated over 750 Codell wells, increasing reserves and resulting in a project ROR of 100% with finding costs below $4.23 per barrel oil equivalent. This is a case study of the program conducted by HS Resources. This paper summarizes the refrac program, and its evolution, since its inception. The candidate selection process is examined, as well as geological and operational considerations when restimulating old wells. The evolution of stimulation treatments and fluids are also investigated. The use of 3D fracture simulations, run in real-time during the refracture treatments, and their results are discussed. Finally, both economic and production results are presented. Introduction Fracture treatment fluids and designs have varied greatly since the full-scale development of the Codell zone in the Wattenberg Field of northeast Colorado (Figure 1) began in 1981. Initially, small sand volumes and a variety of fluids were used to complete the Codell zone. With the improvement of frac fluids and placement techniques in the 90's, frac treatment designs changed dramatically towards larger sand concentrations and higher pump rates. Furthermore, with the trend towards completing the Niobrara in addition to the Codell, cost cutting techniques such as limited entry completions were employed. This technique controls the anticipated placement of the frac treatment by the number of perforations shot across each interval. In many of these wells, there are only 4 to 6 perforations in the Codell zone, which has been shown to be the most prolific reservoir of the two in most areas of the field. Varying degrees of effectiveness have been demonstrated by the historical evolution of frac design and placement. Current fracture modeling has shown that the techniques discussed above can limit the induced fracture lengths in the Codell, and thereby, negatively affect the production performance of the well. Operators in the Denver-Julesburg (D-J) Basin discovered, as early as 1989, that restimulating wells with small or ineffective original treatments yielded impressive and sometimes dramatic results. In June 1997, HS Resources began their refrac program. General Geology In the Wattenberg Field, the Codell Sandstone (Figure 2) is a highly bioturbated marine bar-margin deposit, flanking a central bar facies to the south. Moderately low depositional energies and considerable authigenic alteration of feldspar and rock fragment grains have sourced clay contents as high as 30% in some portions of the field. These interstitial clays exhibit grain-coating, pore-lining and pore-occluding habitats, often reducing permeabilities below 0.1 md, although porosities range from 10 to 25%. As a result, hydraulic stimulation is used to establish high permeability fairways (hydraulic fractures) that connect larger cross sections of low permeability matrix and micro-fracture networks to the wellbore. Regionally, the Codell Sandstone in the central portions of Wattenberg Field contains unique reservoir characteristics and is bounded by multiple trapping factors. Reservoir pinchouts to the south, southeast and northeast of Wattenberg, regional basement faulting to the west and north and corresponding porosity and permeability reductions toward these features help create an effective regional oil and gas trap within the Codell in the central field area. Correspondingly, the central field area contains the greatest maximum porosity and permeability reservoir in the Codell play, with the highest GOR's, exceeding 15,000 scf/bbl. All of these factors mutually overlap to create an overpressured cell in the Codell reservoir in the central field area (Twps. 3 to 5N, Rgs. 65 to 66W). Pressure gradients range from about 0.445 psi/ft, on the flanks of the field, to a maximum of 0.669 psi/ft in the center of the overpressured cell.

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:Dual completions in the Codell and Niobrara correlated with higher well productivity.Production from the limited entry treatments was 5 to 6% lower than the dual completion treatments.Production from the Codell only stimulations was 20 to 21% lower than the dual completions.

The Altamont-Bluebell field produces oil and gas from fractured, tight, lacustrine deposits. Initial field discovery is dated back to 1950. The main producing intervals are the Wasatch and the lower portion of the Green River formation. The lower Green River intervals are found at a depth of 8,000-10,500 feet and are the focus of this study. Previous stimulation in the lower Green River formation mainly consisted of acid breakdown treatments. With the advent of better fracturing fluid additives and proppants, hydraulic fracturing has shown to be more effective when compared to acid stimulation treatments.This work compares production data from over 20 hydraulically fractured recompletions done in the lower Green River formation. Fracture modeling with a fully functional 3D simulator is also presented to gain better insight into the fracture design. Finally, we present pitfalls and techniques to better stimulate the tight formations encountered.