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.
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.
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.