Reservoir Management of the Hartzog Draw Field
Authors: Hunt Robert D. , Hearn Charles L. 
Published: December 2007

Summary
The Hartzog Draw field is a recently discovered major oil reservoir in northeastern Wyoming. Initial reservoir performance indicated that the field soon would be depleted below the bubble-point pressure. To evaluate secondary recovery possibilities, the working interest owners, cooperated in an extensive reservoir study. Although there were some unique problems. both in evaluating reservoir data and in obtaining a unitization agreement, this effort led to the formation of the Hartzog Draw Unit within 5 years of field discovery. Secondary recovery by waterflooding has begun, and enhanced recovery possibilities are being evaluated.

Introduction
The Hartzog Draw field, discovered in 1975, is in the Powder River basin of northeastern Wyoming, (Fig. 1). There are 177 wells in the field, producing on 160-acre (647 x 10 3-M2) spacing from the Shannon sand at 9,400 ft (2865 m). Initial oil in place (IOIP) was 350 MMSTB (55.6x 10 6 stock-tank M 3).
As the field was developed, pressure measurements revealed that reservoir pressure was dropping rapidly from the initial value of 5,000 psig (34.5 MPa). Reservoir fluid analysis indicated that the oil was an undersaturated black oil with a bubble point of 1,550 psig (10.7 MPa). Pressure reduction significantly below the bubble point would lower ultimate recovery if the field eventually were waterflooded. Prudent reservoir management required that the field be unitized. An operators committee was formed, and under the direction of various subcommittees an extensive reservoir analysis was conducted. This included geological correlation and mapping, reservoir fluid and rock analysis, pressure transient testing, and relative permeability tests. Reservoir simulation studies were conducted to predict remaining primary recovery and secondary recovery for various proposed methods. These efforts resulted in a recommendation to the working interest owners to unitize and begin secondary recovery immediately by waterflooding. The field was unitized in mid-1980, and waterflood facilities currently are being installed. A study of enhanced recovery possibilities by CO, injection is being conducted also.
The objectives of this paper are (1) to present the geology. performance history, and reservoir properties of the Hartzog Draw field, (2) to describe the unitization process, and (3) to discuss the current status of field development and plans for enhanced recovery.

Field Geology
The Hartzog Draw field is a northwest-to-southeast trending, stratigraphically trapped hydrocarbon reservoir. The field is approximately 22 miles (35.4 km) long and from 1 to 4 miles (1.6 to 6.4 km) wide.
Oil and gas production is from the Shannon sandstone member of the Cody shale of Upper Cretaceous age. The reservoir sands range from less than 5 ft (1.5 m) to more than 60 ft (18.3 m) thick (Fig. 2), and lie within a 20- to 90-ft (6- to 27-m) sequence of inter-bedded sand, silt, and shale. The Hartzog Draw discovery well is shown in Fig. 3 as a type log for the field.
It is believed that the Shannon sand of Hartzog Draw was deposited in an offshore marine shelf setting. The sand originally was spread over the shelf as a blanket deposit. The reservoir was formed by southward-flowing marine currents, which reworked and redistributed the sand in an elongate bar-type deposit. These marine currents varied in strength and energy, as indicated by the sedimentary structures, which range from ripples to large scale trough cross-beds within the reservoir.
The grain size of the sand ranges from fine to medium, with an average clay content of about 20%.
JPT
P. 1575^