An Evaluation Of Nitrogen Injection As A Method Of Increasing Gas Cap Reserves And Accelerating Depletion - Ryckman Creek Field, Uinta County, Wyoming
Authors: Calvin James W. , Vogel John L. 
Published: December 2007

The Ryckman Creek Field is an asymetrical anticline containing a thick gas cap, an oil zone and a water zone. It is important to prevent oil migration into the gas cap during depletion, or oil will be trapped and reserves reduced. Two common practices have been to either leave the gas cap shut in until the oil zone was depleted or to produce the gas cap for NGL recovery while injecting plant residue and makeup hydrocarbon gas to hold the gas-oil contact steady. Both techniques result in substantial delays in gas cap sales. If a strong water drive is present, as is thought to be the case at Ryckman Creek, considerable gas cap gas will be trapped during blowdown as a result of the water influx.

At Ryckman Creek high purity nitrogen will be manufactured in the field at a rate of 12 MMcfd (340 km3/d) and injected into the top of the gas cap. The gas cap will then be produced simultaneously with the oil column. Predictions indicate that a 27 year improvement will be made in the timing of gas cap sales and that a minimum 15 Bcf (425 Mm3) gas reserve increase over the blowdown case should be realized by the project. Today's higher natural gas prices make this process economically attractive. process economically attractive.

Introduction

The Ryckman Creek Nugget field is located in Uinta County, Wyoming, on the complex Overthurst Belt (Figure 1). The discovery well, the Champlin 224 Amoco A No. 1, was completed in September 1976. This well tested gas at rates in excess of 3000 Mcfd (85 km3/d) from an overlaying gas cap, and oil at 500 BOPD (79.5 m3/d) along with 700 Mcfd (20 km3/d) of associated gas from the oil zone. Initial field development was begun on 80 acre spacing.

Ryckman Creek is an asymetrical anticline approximately three miles (4.82 km) long and one and one-half miles (2.4 km) wide. A structure map is presented on Figure 2. As shown on the typical well log of Figure No. 3, the overall Nugget sand thickness is 815 feet (248 m). The maximum gas cap thickness is approximately 400 feet (122 m) and the oil column is about 200 feet (61 m) thick. The field is completely underlain by water. A strong water drive is thus expected. The original oil in place is estimated at 78 MMstb (12.4 Mm3), with 110 Bcf (3115 Mm3) of solution gas and 90 Bcf (2549 Mm3 of gas cap gas. The gas reserve therefore represents a significant portion of the field's total value. The timing of production from both the oil zone and the gas zone is thus a key factor in an optimum plan of depletion, in addition to the normal goal of maximizing recovery.

Rock and Fluid Properties

The Jurassic Nugget sandstone at Ryckman Creek is located at a depth of approximately 7600 feet (2316 m). The sand is very clean, containing practically no clays or other impurities. Pertinent practically no clays or other impurities. Pertinent data for the more important rock and fluid properties are presented in Table 1. properties are presented in Table 1.The Nugget, although very cross-bedded, is a continuous sand at Ryckman Creek. An indication of this can be seen by the almost complete lack of shale breaks on the log shown on Figure No. 3. This factor is very important in the type of ultimate depletion technique chosen.

A native state core through the entire gas cap, oil zone, transition zone and partially through the water zone was taken early in the drilling program. This core provided two very important pieces of information. First, the Nugget sand was shown to be very consistent in porosity and permeability properties. When the native state core was compared to the porosity and permeability cutoffs which were established for the field, only 16 feet (5 m) of the 336 feet (102 m) cored could not be considered "net pay". Secondly, the efficiency of water displacement was found to be quite high.