Cost-Effective Data Acquisition for the Odin Field

Abstract

Summary This paper describes a cost-effective method for obtaining reservoir data to define the depletion mechanism in the small Odin gas field. The addition of a simple work program to the drilling plan for the first development well provided basic reservoir and aquifer data, and the well completion scheme provided for continuing reservoir surveillance. Cost-effective methods for reservoir data acquisition are particularly relevant for marginal developments. This discussion should be of interest to operators of gas fields where the depletion mechanism is not well defined and where early acquisition of additional data can indicate the mechanism. A brief development history of the Odin field illustrates the need for good reservoir data. Initially, some uncertainty surrounded the amount of bottomwater influx into the Odin gas sand and the resultant reservoir depletion mechanism. Conclusions about the expected type of reservoir depletion mechanism were drawn from data that included conventional core analysis, measured pressure gradients, fluid saturation logs, and production well tests. Production history that confirms the conclusions made from the initial data acquisition is shown. Field History Location and Correlation to Other Fields. Odin, North East Frigg, and Frigg gas reservoirs share a common aquifer in the Frigg formation sandstone. At discovery, reservoir pressures in the three fields were the same when corrected to a common datum. Odin lies north and North East Frigg lies northeast of the larger Frigg accumulation. The three fields lie at different subsea depths with the gas/oil contact (GOC) at 6,629 ft [2021 m] subsea for the Odin field. No other fields lie within sufficient proximity to influence the Odin field development. Discovery. The Odin field was discovered in early 1974 when Well 30/10-2 was drilled on the crest of the field, encountering 174 ft [53 m] of gas column. On a production test, the section flowed 15.5 MMscf/D [0.44x 10(6) std m3/d] of gas through a 4 3/64-in. [1.7-cm] choke. A second exploratory well, Well 30/10-3, drilled later the same year on the southern flank of the field, confirmed both the gas column and a thin (13- to 16-ft [4- to 5-m]) heavy-oil column below the gas. Development-Intrafield Interactions. The sands forming the reservoir at the Odin field are an extension of the submarine fan sands of the Eocene Age that form the Frigg field reservoir. Seismic data indicate that the Frigg sand in the Odin and Frigg fields is connected through a narrow channel. The fan lobes are limited in extent and are essentially flat at their bases and concave on the tops. This mounded nature of the sand deposition forms the hydrocarbon trap. The resultant structure is filled approximately to the spill point. Production from the Frigg field, beginning in late 1977, reduced the pressure in the entire reservoir system and caused a pressure differential to develop from the Odin field toward the Frigg field, resulting in a physical loss of gas from the Odin and other Frigg satellite fields. The pressure drop experienced at the Odin field before any significant production was 150 psi [1034 kPa]. The massive Heimdal formation aquifer underlies the Frigg clastic tongue, as shown in the geological cross section in Fig. 1. The Frigg sand, however, is separated from the Heimdal sand by 325 to 650 ft [100 to 200 m] of alternating Eocene sand and shales. The Heimdal sand is continuous over a large area and would provide significant pressure support if in direct communication with the Frigg sand. In the southern portion of the Frigg field, geological correlations and data from an aquifer observation well indicate that a restricted pressure communication exists. The Eocene sand/shale sequence between the Frigg and Heimdal sands, however, becomes very shaly in a northwest direction (as seen in Wells 30/10-2, 30/10-3, and 30/10-A4), providing an effective barrier under the Odin field.