Influence of Production on Chalk Failure in the Valhall Field

Abstract

Chalk failure in six production wells without gravel pack in the Valbail field are observed to occur in two main periods, the early and the late. Early period failures are related to pre-existing natural damage of the formation and wellbore operations such as drilling and well completion/perforation. Late-period failures are related to depletion in reservoir pressure. As pore pressure fails the effective stress on the formation chalk increases until its yield stress is reached. Early and late-period failures are both triggered by stresses related to pressure gradients in the formation near a wellbore. Shut-in and re-opening of production wells, for example, give rise to high pressure gradients that cause chalk failure. Fluid production below the bubble point also triggers chalk failure. The analyses of 19 chalk failure incidents are expressed in a conceptual damage-release model. Introduction Solids production occurs in many oilfields. When the amount of solids is unacceptable, gravel packs are often installed in the production wells. Gravel packs are designed to reduce or eliminate the solids production. But the installation of gravel packs can be difficult and expensive, especially on offshore platforms in fields with deviated wells. An understanding of solids production and the technology for its control have been developed primarily for wells producing from sand formations. Much less is known about wells producing from chalk formations, for example in the greater Ekofisk area of the North Sea. The several oilfields in this area contribute about one-quarter of the total oil production in Norwegian waters. improved understanding of solids production in chalk reservoir, therefore, can be of considerable technical and economic significance. The Valhall field is in the southern part of the greater Ekofisk area, in Norwegian waters. Wells without gravel pack in the Valhall field have suffered from influx of chalk solids at a constant low-level and in major bursts. The solids are associated with oil production from the highorosity, low-permeability Tor and Hod chalk reservoir formations. When the field came on production in 1982, the production wells were hydraulically fractured. Because of persistent solids production, however, it was later decided to gravel pack the production wells. By the end of 1989, 19 of the 24 production wells had been re-completed with a gravel pack. pack. Chalk is the main reservoir formation in the greater Ekofisk area, including the Valhall field. Rock mechanics has been the focus of most published studies on well stability in chalk reservoirs in the Ekofisk published studies on well stability in chalk reservoirs in the Ekofisk area. As field data are gathered, however, it has become possible to study chalk instability terms of well production data. possible to study chalk instability terms of well production data. Relationship between major chalk failures and well production in the Valhall field are reported in this paper. The cue history of one well is presented in detail and that of four other wells in less detail. Two other presented in detail and that of four other wells in less detail. Two other wells are also mentioned. The paper deals with major chalk influx incidents, characterized by bursts of solids production. VALHALL FIELD AND RESERVOIR The Valhall reservoir is at a depth of 2400-2600 m subsea and consists of two oil-bearing formations: the Tor and the Hod. About two-thirds of the oil and the main productivity is in the Tor, which is a soft chalk formaion consisting of high purity calcite (95 to 98 percent). The formation chalk in the Valhall reservoir is very soft (weak) because of its low silica content (the chalk lacks cementation). The formation is characterized by high porosity (40-50 percent) and low matrix permeability (1-15 mD). initial oil saturation in the Valhall reservoir was 90 percent and above. Despite the low matrix permeability the effective permeability of the reservoir formation is higher due to natural fractures. P. 117

Static and dynamic effective stress coefficient of chalk

“…Compaction in the reservoir and subsidence at the surface may occur due to this deformation. This is a major challenge during production, as exemplified in the North Sea chalk fields (Hermansson and Gudmundsson, 1990;Kristiansen, 1998;Barkved and Kristiansen, 2005).…”

Homogenized constitutive laws for rocks with elastoplastic fractures

“…Numerous engineering studies of fractured reservoirs show that reservoir productivity may be altered by production [1][2][3]. This is attributed to changes in the hydraulic properties of fractures due to production-induced stress variations.…”