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
This paper describes the initial planning and pre-job laboratory testing for a gravel pack operation using coiled tubing in the Statfjord Field. The paper also describes the execution of the operations and methods involved. Potential problems related to the placement of the gravel were identified in pre-job meetings and were verified through a comprehensive test programme. A well specific cased hole completion was modelled using an acrylic gravel pack simulator, and a number of tests were performed to determine the optimum solution for gravel concentration, brine viscosity, viscosifier and slurry placement rate. The tests were utilised as an aid in designing the actual gravel placement operation. The gravel packing operation was conducted using small batches of slurry consisting of a non-viscosified brine carrier fluid along with a low gravel concentration. Two separate radioactive tracers were injected into the slurry to give better resolution of the gravel pack logs. The operation and completion technique is reviewed and verified by extensive post completion analysis. This includes calculated sand height verified by gravel pack placement logs. Initial gravel pack productivity is comparable to conventionally gravelpacked wells. Introduction The Statfjord Field is one of the largest offshore fields in the world. It is located on the boundary line between the Norwegian and British continental shelves, 200 km North West of Bergen. Production started in 1979 from one concrete gravity based production platform. Field development was completed in 1985 with three fully integrated platforms based on the Condeep design. Each platform has two drilling shafts and a total of 42 well slots. To date 124wells have been drilled including 10 redrills.
A laboratory scale flow loop for drilling applications has been used for evaluating the effect of lubricants on skin friction during drilling and completion with oil based or low solids oil based fluids. The flow loop included a 10 meter long test section with 2″ OD free whirling rotating drill string inside a 4″ ID wellbore made of concrete elements positioned inside a steel tubing. A transparent part of the housing was located in the middle of the test section, separating two steel sections of equal length. The entire test section was mounted on a steel frame which can be tilted from horizontal to 30° inclination. The drilling fluids and additives in these experiments were similar to those used in specific fields in NCS. Friction coefficient was calculated from the measured torque for different flow velocities and rotational velocities and the force perpendicular to the surface caused by the buoyed weight of the string. The main objective of the article has been to quantify the effect on mechanical friction when applying different concentrations of an oil-based lubricant into an ordinary oil based drilling fluid and a low solids oil based drilling fluid used in a North Sea drilling and completion operation.
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