Summary
Recently acquired geological, geophysical, and engineering data at the Nash Draw Brushy Canyon Pool revealed that the initial reservoir characterization was too simplistic to capture the critical features of this complex Delaware formation. A new reservoir description provides sufficient detail to indicate that compartmentalization exists in the Brushy Canyon interval. This new reservoir description is being used to identify "sweet spots" for a development drilling program as well as to optimize reservoir management strategies. This paper presents recent results of an integrated reservoir characterization effort that is being used at Nash Draw as a risk reduction tool.
Introduction
A producing property operated by Strata Production Company (Strata) in the Nash Draw Brushy Canyon Pool, Eddy County, New Mexico is a cost-shared field demonstration project in the U.S. Department of Energy Class III program. A major goal of the Class III Program is to stimulate the use of advanced technologies to increase ultimate recovery from slope-basin clastic reservoirs.
The basic problem at the Nash Draw Pool (NDP) is the low oil recovery that is typically observed in similar Delaware reservoirs. By comparing a control area using standard infill drilling techniques to a similar area developed using advanced reservoir characterization methods, the goal of the project is to demonstrate that a development program based on advanced methodology can significantly improve oil recovery.
During Phase I of the project, six new wells were drilled as data acquisition wells, several hundred feet of whole core was obtained from one of the new wells, and vertical seismic profiles and a 3D seismic survey were acquired. The advanced characterization effort is integrating geological, geophysical, petrophysical, geostatistical, production, and reservoir engineering data. The stratigraphic framework is being quantified in petrophysical terms using innovative rock-fabric/petrophysical relationships calibrated to wireline logs, and 3D seismic attributes are being used to extrapolate petrophysical properties into the interwell area. Using the geological model developed in the first year of the project, a detailed reservoir description of the pilot area has been made, and current efforts are concentrating on defining the next generation geological model that will include 3D seismic input and greater use of statistical methods. Reservoir characterization and simulation studies are being used to predict the distribution of remaining oil saturation and to optimize development drilling programs.
Production and Recovery Challenges
Production at the NDP is from the Brushy Canyon formation, a low-permeability turbidite reservoir of marginal quality. A challenge in developing the reservoir is to distinguish oil-productive pay intervals from water-saturated, nonpay intervals. Additionally, because initial reservoir pressure is only slightly above bubble-point pressure, rapid oil decline rates and high gas/oil ratios are typically observed in the first year of primary production. Further, limited surface access, caused by underground Potash mining and surface Playa Lakes in the area (see Fig. 1), prohibits development with conventional drilling in some parts of the reservoir.
Various combinations of vertical and horizontal wells combined with selective completions are being considered for optimizing production performance. Based on the production constraints due to high gas-oil ratios observed in similar Delaware fields, pressure maintenance is a likely requirement at the NDP.
Project Management Concept
The project involved the demonstration of a virtual company concept involving a small independent oil producer and geographically diverse experts. This concept is described in a companion paper.1
Initial Reservoir Description
Reservoir and fluid data are listed in Table 1.2,3
The sandstone units of the basal Brushy Canyon sequence of the Delaware Mountain Group in this study represent the initial phase of detrital basin fill in the Delaware Basin during Guadalupian time. The Delaware sands are deep-water marine turbidite deposits. Depositional models4,5 suggest that the sands were eolian derived and were transported across an exposed carbonate platform to the basin margin. Interpretations of the associated transport mechanisms6,7 suggest that the clastic materials were deposited episodically, and were transported into the basin through shelf by-pass systems along an emergent shelf-edge margin.
The Brushy Canyon sequence lies above the Bone Spring Formation. The top of the Bone Spring is marked by a regionally persistent limestone varying from 50 to 100 ft in thickness. This surface provides an excellent regional mapping horizon. Regional dip is to the east-southeast at about 100 ft per mile in the area of the NDP. The structural dip resulted from an overprint of post-depositional tilting, and this overprint is reflected in the reservoir rocks of the Delaware formation and impacts the trapping mechanism in the sands.