Rock Compressibility, Compaction, and Subsidence in a High-Porosity Chalk Reservoir: A Case Study of Valhall Field

Abstract

Summary A case study of the North Sea Valhall chalk reservoir demonstrates the significant impact that rock compressibility can have on field performance. Porosity reduction, reservoir interval compaction, and seabed subsidence have been observed in conjunction with reservoir pressure depletion. Full-diameter samples from a recently cut core of the unconsolidated high-porosity chalk were subjected to a series of uniaxial-strain experiments to determine compaction and PV compressibility. The laboratory measurements were corrected to field stress rates and pressure, and porosity-dependent rock-compressibility curves were developed. The uniaxial compaction data were used both in a reservoir model to recognize the significant additional reservoir energy resulting from the lithic drive of large-scale rock compaction and in a subsidence model to predict the impact of reservoir depletion on seabed displacements. Introduction The Valhall field is an overpressured, undersaturated Upper Cretaceous chalk reservoir located about 290 km [180 miles] offshore in the central graben of the North Sea at the southern tip of the Norwegian sector. The field was discovered in 1975, and field development began in 1981 with the location of a three-platform complex in 69 m [226 ft] of water in the field's central area. Oil and gas production from the field began in Oct. 1982 and cumulative oil production to the end of March 1989 was about 19x(10)6 stock-tank m3 [120 MMSTB]. The reservoir is at a depth of about 2400 m [7,875 ft] subsea and consists of two oil-bearing formations: the Tor and Hod. About two-thirds of the oil and the majority of the productivity are in the Tor, which is a soft chalk formation characterized by very high purity (95 to 98% calcite), very high porosity (up to 50%), and purity (95 to 98% calcite), very high porosity (up to 50%), and very high oil saturations (90% and greater). Fig. 1 shows a well log of a typical crestal well. At discovery, the Tor pressure was only 3.4 MPa [500 psi] less than the 48.3-MPa [7,000-psi] overburden weight, implying only minor formation compaction during burial. As the field is developed, depletion of the reservoir pressure causes large changes in net stresses on the reservoir rock that result in compaction. The effects of rock compaction manifest themselves in two important ways: a significant contribution to the reservoir energy in the form of a lithic drive and a partial transfer of this compaction through the overburden, resulting in mudline subsidence. This paper reviews the manner by which laboratory compaction tests of Valhall core were used to improve prediction of both field performance and surface subsidence accompanying depletion of the performance and surface subsidence accompanying depletion of the reservoir. Compaction Tests of Valhall Cores Historically, many strength-related core compaction tests have been performed on Valhall chalk, but before 1987 these tests tended to performed on Valhall chalk, but before 1987 these tests tended to concentrate on hydrostatic loading. The majority of these tests were addressed primarily at issues related to well completions to prevent production of the soft chalk into the wellbore. production of the soft chalk into the wellbore. In 1986, after only 3 1/2 years of Valhall production, it became apparent that measured reservoir pressures in the crestal, high-porosity portion of the field were deviating significantly from reservoir-model predictions, suggesting the presence of unaccounted-for reservoir energy. Attempts were made to history match the repeat-formation-tester (RFT) pressure measurements in the field by use of alternative sources of supplemental energy, such as greater oil in place, greater water influx on the flanks, greater transmissibility between the Tor and Hod formations; and higher-bubblepoint-pressure fluids in the crestal area. All these efforts failed to match adequately both observed field performance and the high pressures measured in the crestal wells. Improved history matching pressures measured in the crestal wells. Improved history matching was realized, however, by a rock compressibility that gradually increased beyond the constant value of 2.5x10–6 kPa-1 [17x10–6 psi-1] assumed in previous simulations. psi-1] assumed in previous simulations. Further support for significant rock compaction in the crestal area was obtained in mid-1986 when surface subsidence was first observed through satellite surveys. Continuing surface subsidence was monitored by both satellite surveys and infrared-wave-height measurements, while downhole compaction was indicated by cased-hole logs showing reductions in both porosity and pay-interval thickness. Direct measurements of downhole compaction were made by monitoring the distance between radioactive bullets shot into the chalk in the vertical well located immediately under the platform complex. Unfortunately. the well log estimates of porosity and pay-interval-thickness reductions were, at best, qualitative. Complexities pay-interval-thickness reductions were, at best, qualitative. Complexities were introduced in the interpretation of compaction associated with a particular porosity and reservoir pressure because of heterogeneities particular porosity and reservoir pressure because of heterogeneities within the reservoir, variations in the pressure transient away from the wellbore, uncertainties in the logging measurement, and the influence of casing and wellbore deviation. To obtain quantitative predictions of compaction, laboratory rock sample tests were predictions of compaction, laboratory rock sample tests were required. A core from the high-porosity Tor formation was cut. An oilbased mud system designed to include a minimum of surfactants with minimal fluid loss was used in an attempt to retain a condition as close to native state as practical. Earlier studies of high-porosity Valhall chalk suggest that the Tor formation may be weakened by contact with water, so efforts were made to recover and to preserve the core without altering its mechanical strength. Full-size preserve the core without altering its mechanical strength. Full-size core was used for the compaction tests to avoid damaging the fragile core by cutting plug samples. The core diameter was 66 mm [2.6 in.] and samples were 120 to 130 mm [4.7 to 5.1 in.] long. To avoid further weakening of the core material by cleaning. the samples were used in their native state and were flushed with 1 PV of filtered Valhall crude to resaturate the core before the compaction tests. Samples were tested in a triaxial cell under both hydrostatic (equal vertical and horizontal stresses) and uniaxial-strain (no lateral expansion) conditions. The hydrostatic tests were performed to establish a relation between hydrostatic and uniaxial tests on Valhall core tested under the same closely controlled conditions. Sample response during uniaxial-strain testing more closely represents the response of the reservoir rock during compaction and was the primary input in the numerical simulations. This latter test type is primary input in the numerical simulations. This latter test type is difficult to perform on any sample but particularly on soft chalks. Long-term creep tests were performed to correct the data to the compaction that takes place at the low rate of stress increase in the reservoir. Test Equipment. The triaxial test apparatus was based on a Hoek cell with a pressure capability up to 69 MPa [10,000 psi] for the large-diameter Valhall samples (Fig. 2). The ends of the core samples were trimmed flat, and the core was held between two movable end pistons and inserted in a rubber membrane to prevent fluid communication between the sample and the pressure chamber. JPT July 1989 P. 741