Geological Factors Influencing Reservoir Performance of the Hartzog Draw Field, Wyoming
Authors: Hearn C.L. , Ebanks W.J. , Tye R.S. , Ranganathan V. 
Published: December 2007

Summary
The Hartzog Draw field, a major sandstone oil reservoir in the Powder River basin of Wyoming, was discovered in 1975 and unitized in 1980. The field is under waterflood and is being considered for CO2 miscible flooding. To aid in evaluating enhanced recovery, the working interest owners approved a comprehensive geological-engineering study using data from existing wells and new infill wells. Geologic facies identified in the study provided the stratigraphic basis for mapping reservoir flow units. These flow units more precisely describe variations in rock properties that control fluid flow. Within the flow units, bedding characteristics of the reservoir sandstones cause permeability anisotropy. Mineralogy and diagenesis affect permeability, porosity, water saturation, and sensitivity to injection fluids. The reservoir description resulting from this study provides insight into reservoir performance and forms the provides insight into reservoir performance and forms the framework for reservoir engineering studies to predict water flood and CO2-flood recovery.

Introduction
An accurate reservoir description-the internal, three-dimensional variation of reservoir rock properties-is essential to effective reservoir management. During primary production, areal variation of properties such as primary production, areal variation of properties such as permeability, porosity, thickness, and sand continuity permeability, porosity, thickness, and sand continuity influence both oil recovery and its distribution in the field. During secondary and enhanced production, vertical reservoir heterogeneity is a predominant factor affecting oil recovery. Knowledge of reservoir geology is particularly important when reservoir simulation models particularly important when reservoir simulation models are used to evaluate and predict reservoir performance. This paper presents the results of a study conducted to improve the reservoir description of the Hartzog Draw field, a large sandstone reservoir in the Powder River basin of northeastern Wyoming (Fig. 1). The development, reservoir properties, production history, and unitization of the field were described by Hunt and Hearn, and are summarized here. The field is approximately 22 miles [35.4 km] long and from 1 to 4 miles [ 1.6 to 6.4 km] wide. Production is from the Shannon sandstone at 9,400 ft [2865 m]. Maximum sand thickness is 60 ft [18.3 m]; average thickness is 20 ft [6.1 m]. Average porosity is about 12%, and average air permeability is about 12 md. The reservoir contains a black oil with an original bubble point pressure of 1,550 psig [10.7 MPa] and solution GOR of 292 scf/STB [52.6 std m /stock-tank m ]. The volumetric initial oil in place (IOIP) was 350 million STB 155.7 × 10 stock-tank m ]. Early engineering studies recognized that calculated IOIP and predicted oil recovery were subject to uncertainties in reservoir properties and geology. properties and geology. Field development began in 1975. Drilling of the original wells, on 160-acre [64.8-ha] spacing, was completed in 1979. During primary production, reservoir pressure dropped from the initial value of 5,000 psig pressure dropped from the initial value of 5,000 psig [34.5 MPa] to about 1.000 psig [6.9 MPa] average pressure. Cumulative production through 1981 was pressure. Cumulative production through 1981 was 32X 10 STB [5.1 × 10 stock-tank m ].
The effects of areal variation of reservoir properties became evident during the primary production period in Hartzog Draw. There was a significant difference in reservoir performance between the east and west sides of the field. Fig. 2 is a contour map of cumulative oil production through 1980 per wellbore porosity-foot. Such production through 1980 per wellbore porosity-foot. Such production maps must be interpreted with caution, since production maps must be interpreted with caution, since the performance of an individual well is related to production practices (such as well completion and artificial production practices (such as well completion and artificial lift methods) in addition to reservoir properties. However, a consistent trend is evident in Fig. 2. On the average, wells east of the long axis of the field were much better performers than wells west of the axis. It will be shown that this trend is related to the reservoir geology. The field was unitized in 1980 for secondary recovery, and development of a water flood system was begun in 1981. Water flooding started on a 640-acre [2.6× 10 -M ] inverted nine-spot pattern (Fig. 3) with plans to convert later to a 320-acre [1.3×10 M ] five-spot. There was concern that water flooding on such wide spacing would not be optimum for this reservoir.
JPT
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