Rock Mechanics of the Ekofisk Reservoir in the Evaluation of Subsidence

Abstract

Summary Mechanical properties have been determined for Ekofisk reservoir rock for use in subsidence simulation. Tests were to describe the mechanical properties of the Ekofisk chalk for reservoir conditions. The mechanisms and time dependence of chalk compaction and the effect of waterflooding on chalk strength were also examined. Introduction The origin of the seafloor subsidence observed in the Ekofisk field is the compaction of the reservoir rock caused by production, as observed in certain other fields. The effective stress production, as observed in certain other fields. The effective stress on the rock (the difference between the overburden load on the rock and the pore pressure within the rock) increases as hydrocarbons are withdrawn and reservoir pressure declines. Reservoir rock will compact under these circumstances by an amount determined by its mechanical properties. Accurate prediction of subsidence requires that the mechanical properties of the reservoir rock be determined. These can then be properties of the reservoir rock be determined. These can then be combined in simulators with such information as reservoir pressure, overburden properties, and field structure to predict the amount of compaction and the resulting surface subsidence. The objective of this study-to determine the mechanical properties of the Ekofisk reservoir rock for use in such simulators-requires determination of the mechanical properties of all types of rock in the reservoir for all conditions encountered during its production life. Field Geology and Nature of the Reservoir Rock The Ekofisk reservoir consists largely of chalk of Maastrichtian (Late Cretaceous) through Upper Danian (Early Paleocene) Age. The chalk section in this region of the North Sea is subdivided into five formations: in ascending order, the Hidra and Plenus Marl (Cenomanian Age), the Hod (Turonian through Middle Campanian Age), the Tor (Upper Campanian through Maastrichtian Age), and the Ekofisk (Danian Age). The reservoir chalks in the Ekofisk field occur in the upper Tor and Ekofisk formations. The fairly similar patterns of chalk sedimentation within the Tor and Ekofisk formations patterns of chalk sedimentation within the Tor and Ekofisk formations constitute a rather continuous background of chalk deposition interrupted in places by masses of chalk material deposited elsewhere and introduced in the form of turbidites, slumps, and debris flows. Excluding the Tight Zone, the reservoir is composed almost exclusively of chalk having generally similar compositional and mechanical characteristics. The chalk consists almost entirely of calcite derived from the tests of planktonic, shallow-water calcareous algae (Coccolithophorids) set in a structureless matrix of very fine crystalline calcite. Coc-colithophorlds are composed of low-magnesium calcite platelets arranged into disks and rosettes (called coccoliths) that overlap to form a coccosphere. During deposition and subsequent sediment reworking, the coccospheres are decomposed and abraded to provide the coccolith-rich material that constitutes the bulk of the chalk sediment. Other calcite bioclasts occur in the chalk, including foraminifers, calcispheres, bryozoans, brachiopods, molluscs, echinoderms, and ostracods. The noncarbonate fraction of the chalks consists of detrita feldspar, quartz, clay minerals, phosphate, pelletal glauconite, and silica. pelletal glauconite, and silica. The Ekofisk chalk is a soft, finely textured, somewhat friable, highly porous limestone. It is a high-porosity (up to 50%, much of it above 35 %), low-permeability (1 - to 10-md) rock that is pervasively fractured. The fractures typically are steeply dipping and pervasively fractured. The fractures typically are steeply dipping and show a variety of trends across the reservoir. Production data indicate that the fractures more than the matrix permeability are responsible for the high fluid production rates. Reservoir Conditions and Test Procedures The Ekofisk reservoir is large and shaped like a shallow, elliptical dome about 22,000 ft [6700 m] wide and 30,800 ft [9390 m] long. The crest of the reservoir is approximately 9,500 ft [2900 m] below sea level, and the pay zone is nearly 1,000 ft [300 m] thick. The overburden consists largely of very-fine-grained, clay-rich shales and mud rocks. These mechanically weak rocks exert a vertical stress of about 9,000 psi [62 MPa] on the reservoir. At discovery, the Ekofisk reservoir was strongly overpressured, with initial formation pressures of about 7,000 psi [48.3 MPa]. After 17 years of production, the formation pressure has been reduced to about 4,000 psi [27.6 MPa], increasing the effective overburden stress on the chalk from 2,000 to 5,000 psi [13.8 to 34.5 MPa]. The objective of this test program is to define the mechanical behavior of the reservoir rock under test conditions that simulate the stress, temperature, and fluid saturations that occur in the field. The tests must subject the rock samples to the stress levels encountered during the producing life of the field. In addition, the manner in which the stress is applied must take into account the nature of the stress environment in the reservoir. All rock in the reservoir is surrounded by adjacent rock; there are no free faces, except immediately surrounding the wellbores. A large ratio of lateral extent to vertical thickness, as in the Ekofisk reservoir, means that compaction will occur largely in the vertical direction with minimal horizontal displacements. Tests to simulate chalk compaction in the subsurface should be confined tests that prevent the rock from undergoing any lateral deformation. The type of laboratory compaction test that most closely simulates compaction of the reservoir rock is the uniaxial-strain test (Fig. 1A). The axial stress on the cylindrical sample is increased and the radial stress is adjusted so that the sample radius remains constant throughout the test. The sample compacts axially with zero radial deformation. During a test, core deformation is measured with linear displacement gauges called linear variable differential transformers (LVDT's). Three axial LVDT's measure changes in sample length; three radial LVDT's measure changes in sample radius. To begin our uniaxial-strain tests, the core is brought to reservoir conditions at discovery of axial stress (9,000 psi [62 MPa]), lateral stress (8,000 psi [55.2 MPa]), and pore fluid pressure (7,000 psi [48.3 MPa]). The 8,000-psi [55.2-MPa] lateral stress was calculated from material properties of the chalk for the overburden and initial pore pressures. The temperature is raised to reservoir temperature (268 degrees F [ 131 degrees C]), and the sample is allowed to equilibrate at these "initial reservoir conditions" for 24 hours. The pore fluid pressure is then reduced, thereby increasing the effective stress on the sample and causing it to compact. This test procedure closely simulates the increase in effective stress on the reservoir chalk as the formation pressure decreases during production. As the pore fluid pressure is reduced, the sample will deform laterally unless the confining pressure is adjusted. The confining pressure is controlled through the radial LVDT'S, which constantly measure the core's radial dimension. Signals from the radial LVDT's are evaluated continually by a computer, which activates a confining pressure pump when the radial dimension of the core exceeds a pressure pump when the radial dimension of the core exceeds a narrowly defined limit. A second type of laboratory compaction test is the hydrostatic test (Fig. 1B). JPT P. 717