Compaction Monitoring in the Ekofisk Area Chalk Fields
Authors: Menghini M.L. 
Published: December 2007

Summary
In late Nov. 1984, the subsidence phenomenon was recognized in the Ekofisk field. To determine the magnitude and areal extent of the formation compaction, a program for measuring compaction with electric logging tools was initiated. Initial time-lapse surveys performed with cased-hole neutron tools indicated that reservoir compaction was occurring, but the accuracy of the determination of compaction rate was low. In addition to the cased-hole neutron survey, radioactive markers and a gamma ray (GR) detection tool were used to determine compaction rate in the reservoir more accurately and to determine whether compaction was occurring in the overburden. A program for implanting radioactive-marker bullets and subsequent monitoring with a four-detector GR tool was implemented. There are currently 13 wells equipped with radioactive markers in the compaction monitoring program. Compaction monitoring accuracy using the four-detector GR tool was found to depend on wellbore geometry, completion design, and radioactive-marker placement. This paper gives the results of the program to date and describes the operational procedures and analysis techniques used for compaction monitoring in the greater Ekofisk area chalk fields.

Introduction
The greater Ekofisk area fields are located in the central graben in the southern sector of the North Sea. The Ekofisk field in Block 2/4 was discovered in 1969. Since then, seven major oil and gas fields have been developed by Phillips Petroleum Co. Norway.
In late Nov. 1984, the subsidence phenomenon was recognized in the Ekofisk field through measurements from fixed platform references to mean sea level. Subsidence in the Ekofisk field is caused by compaction of the reservoir resulting from the pressure decline caused by hydrocarbon production. The pressure in the reservoir normally depends on accumulation depth. The weight of the overlying sediments is transmitted to the fluid and grain structure in the reservoir. As the hydrocarbons are produced, the fluid pressure declines and the load on the formation rock increases. The pressure declines and the load on the formation rock increases. The increased load on the formation rock causes the reservoir to be compacted. The amount and rate of compaction depend on the formation matrix strength. Total reservoir compaction is therefore related to the decrease in formation fluid pressure, the physical properties of the rock, and formation thickness. The amount of subsidence properties of the rock, and formation thickness. The amount of subsidence at the surface depends on the areal extent of the reservoir, the magnitude of compaction, reservoir depth, and the mechanical properties of the overburden.
The primary producing horizons in the Ekofisk area are the Ekofisk and Tor formations. These formations consist of Danian- and Maastrichtian-Age fractured chalk layers with porosities as high as 50%. The gross reservoir thickness (Ekofisk and Tor) is approximately 305 m [ 1,000 ft]. The top of the reservoir is located at 2957 m [9,700 ft] subsea. The initial formation pressure was approximately 48.3 MPa [7,000 psi] and the present reservoir pressure is approximately 24.1 to 27.6 MPa [3,500 to 4,000 psi].

Compaction Monitoring Experience
Before 1984, Phillips Petroleum Co. had minimal experience in subsidence and compaction monitoring. Literature indicated that several operators in various parts of the world had experience with compaction monitoring. In the Groningen field (onshore Holland), Nederlandse Aardolie Mij. has monitored formation compaction since 1964. In 1967, the use of radioactive markers was initiated for compaction monitoring in the Groningen field. In 1982, the four-detector formation subsidence monitoring tool (FSMT) was introduced. Compaction measurements were obtained in the Groningen field with a reported accuracy of 1.5 mm [0–06 in.] per 10.5-m [34.4-ft] monitoring interval. According to this experience, the four-detector FSMT seemed to be the most promising technique to monitor formation compaction in the Ekofisk field.

Compaction Monitoring at Ekofisk
The primary objective of the compaction-monitoring program is to determine the amount and areal extent of formation compaction. The data obtained from time-lapse compaction surveys can then be used for verification of theoretical studies, subsidence simulation models, and laboratory research.
The compaction-monitoring program at Ekofisk began with the review and analysis of open- and cased-hole neutron-logging surveys. Because of the less accurate mechanical depth-measurement system before the introduction of computerized logging units and nondistinct formation signatures in some wells, the compaction results from these surveys were rough estimates. The accuracy of the conventional time-lapse neutron-logging techniques was of the same order of magnitude as the yearly compaction rate, about 61 cm/a [24 in./yr]. Currently, neutron-log time-lapse surveys are used to supplement FSMT data.
The second phase of the compaction-monitoring program began in 1985 with the installation of radioactive markers. The radioactive marker consists of a weak radioactive source (cesium 137) of 100 to 150 phiCi [370 × 10 to 555 × 10 Bq] strength. The source is encapsulated in a stainless-steel sphere 2.3 mm [0.09 in.] in diameter. The sphere is fitted inside a stainless-steel bullet that is fired from a modified 12.7-cm [5-in.] core-sample-taker gun. The gun carries 12 marker bullets that can be fired selectively and a GR detector that can be used to verify the placement of the bullets. Currently, marker bullets are implanted only in openhole sections. Bullets can be fired through casing, although there is some uncertainty about optimum penetration through the cement sheath and into the formation. Additionally, firing marker bullets through the casing is limited to the production interval below the packer to maintain well integrity in the Ekofisk completion configuration.
Because the detector spacings on the FSMT are 10.5 m [34.4 ft] apart, the bullets are placed at 9.5-to-11.5-m [31.2-to-37.7-ft] in-tervals. Bullet intervals of 10.5 m [34.4 ft] result in the optimum measurement accuracy. To place the markers in the best position, knowledge of openhole conditions is necessary. To ensure proper placement, the markers should not be fired in washed-out hole placement, the markers should not be fired in washed-out hole sections. Caliper logs are run before marker placement to verify borehole conditions. Formation porosity and mechanical properties are required for evaluation of the proper explosive charge size to use when the markers are implanted.
Effective placement of the radioactive markers in the first few wells was difficult because of the limited data available on explosive charge strength and penetration depth vs Ekofisk formation porosity and hardness. The first well to have markers implanted was Well 2/4 B-19A. Markers were placed in the 31.1 -cm [12 1/4 -in.] hole section in the overburden shales. Results from this first well showed that 18- and 12-g charges were too large because the resultant GR response was weak and often not measurable; the bullets had penetrated too far into the formation. It was obvious that much penetrated too far into the formation. It was obvious that much smaller charge loads were required in the soft overburden intervals for optimum marker placement. The second well selected for installation of radioactive markers was Well 2/4 K-13B. In this well, bullets were also placed in the overburden formations. The charge sizes used were 13, 10, and 8 g.
JPT
P. 725