Performance of Hawkins Field Unit Under Gas Drive-Pressure Maintenance Operations and Development of an Enhanced Oil Recovery Project

Abstract

Abstract In-depth studies of the Hawkins Field Unit revealed that gas drive-gravity drainage had a recovery efficiency of over 80 percent whereas the recovery efficiency from water drive was about 60 percent. These studies led to unitization of the Hawkins Field on January 1, 1975 and initiation of a gas injection project using produced gas and inert gas. This gas injection project has been very successful in converting the combination gas drive - water drive recovery mechanism to essentially all gas drive. Recent studies indicate that immiscible gas drive of the water invaded portion of the reservoir should recover additional oil. These studies show that the minimum residual oil saturation from gas displacement of water invaded oil column is essentially the same as that from gas displacement of original oil column. Thus, potential exists for reducing the average residue oil saturation in the water invaded oil column from 35 percent to about 12 percent by gas drive-gravity drainage. The process of gas displacement of a water invaded oil column has been termed Double Displacement Process (DDP). This paper reviews the performance of the Hawkins Field Unit under the existing gas drive-pressure maintenance project. It also discusses the development work recently completed supporting the Double Displacement Process and the initiation of an enhanced recovery project using DDP in the East Fault Block. Introduction The Hawkins Field was discovered in December 1940. The field is located in the central portion of the East Texas Woodbine Basin, about 100 miles (161 km) east of Dallas at the town of Hawkins. During the early 1940's, it was rapidly developed to 20-acre (8.1-ha) spacing. Production comes primarily from the Woodbine formation of Upper Cretaceous age. The Woodbine formation is divided into upper, Lewisville sands and lower, Dexter sands (Figure 1). The Dexter sands, thicker and more permeable than the Lewisville sands, contained about 70 percent of the field's original oil-in-place. Hawkins is extensively faulted into over 100 productive fault blocks as the result of a deep-seated salt dome. All of the fault blocks are in communication because of sand to sand juxtaposition except for those separated by a large NE-SW fault on the eastern side of the field. This fault divides the field into two productive areas, the East Fault Block and the West Fault Block. The fault pattern is shown on Figure 2, a structure map on top of the Dexter sands. The field originally contained over 1.3 billion barrels (207 Mm3) of oil and over 430 Bscf (12 Gm3) of gas cap gas. The East Fault Block contained about 20 percent of the original oil and about 7 percent of the original gas cap gas. The primary production performance of the field was influenced by a combination of sand quality and areal extent, the existence of an asphalt layer, and pressure draw down of the Woodbine Basin because of 10 years of production from the East Texas Field prior to the discovery of Hawkins. The Lewisville sands are commonly ashy, shaly and very fine grained. Most of the sand deposits are lenticular and discontinuous and, as a result, communication with the Woodbine aquifer is limited. The Dexter sands, on the other hand, are composed of clean, fine-to-medium grained, well-sorted sandstones. They are relatively massive, correlatable over the field, and extend into the Woodbine aquifer. Average rock properties for the Lewisville and Dexter are given in Table 1. P. 49^