Compaction Monitoring In The Ekofisk Area Chalk Fields
Authors: Menghini M.L. 
Published: October 2008

In late November 1984, the subsidence phenomenon was recognized in the Ekofisk field. In order to determine the magnitude and areal extent of the formation compaction, a program for measuring compaction utilizing electric logging tools was initiated. Initial time-lapse surveys performed with cased hole neutron tools indicated that reservoir compaction was occurring, although the accuracy for determination of compaction rate was low. In addition to the cased hole neutron survey, the use of radioactive markers and a gamma ray (GR) detection tool was initiated in an attempt to more accurately determine compaction rate in the reservoir, and to determine if compaction was occurring in the overburden.

A program for implanting radioactive marker bullets and subsequent monitoring utilizing a four detector GR tool was implemented. There are currently ten wells equipped with radioactive markers in the compaction monitoring program. Compaction monitoring accuracy using the four detector GR tool was found to be dependent on wellbore geometry, completion design and radioactive marker placement. This paper will give the results of the program to date and describe the operational procedures and analysis techniques utilized for compaction monitoring in the Greater Ekofisk Area chalk fields.

INTRODUCTION

The Greater Ekofisk Area fields, operated by Phillips Petroleum Company Norway on behalf of the Phillips Norway Group, are located in the Central Graben in the southern sector of the North Sea (Figure 1). The Ekofisk field in block 2/4 was discovered in 1969. Since then, seven major oil and gas fields have been developed by Phillips. In 1981, a peak production rate of 99,370 m3 (625,000 barrels) per day was achieved. The current rate rom the seven fields is approximately 23,850 m3 (150,000 barrels) per day.

In late November 1984, the subsidence phenomenon was recognized in the Ekofisk field through measurements from fixed platform references to mean sea level. It was later determined that the seabed had subsided approximately 3 m (9.8 ft) in the vicinity of the Ekofisk Complex. The Ekofisk Complex, which consists of nine steel platforms and one concrete storage tank, is located directly above the main Ekofisk producing structure (Figure 2). As of November 1987, the seabed at the center of the field has subsided approximately 4 m (13.1 ft).

Subsidence in the Ekofisk field is caused by compaction of the reservoir resulting from the pressure decline due to the production of hydrocarbons. The pressure in the reservoir is normally dependent on depth of accumulation. The weight of the overlaying sediments is transmitted to the fluid and grain structure in the reservoir. As the hydrocarbons are produced, the fluid pressure will decline and the load on the formation rock wll1 increase. The increased load on the 2 formation rock causes the reservoir to be compacted. The amount of and rate of compaction depends on the strength of the formation matrix. Total compaction of the reservoir is therefore related to the decrease in formation fluid pressure, the physical properties of the rock and the thickness of the formation.