Multiple-Fracture Horizontal Wells: Performance and Numerical Simulation

Abstract

Abstract Six Multiple Fracture Horizontal (MFH) Wells have been drilled in the Dan Field, located in the Danish North Sea. The field is a low permeability chalk reservoir producing under a combination of solution gas and gas cap drives. Single well numerical models were built to determine the primary factors controlling the performance of these complex primary factors controlling the performance of these complex wells. In the two wells which have been successfully history matched, dynamic hydraulic/natural fracture behavior under drawdown and gas coning were key processes. The models are being used for the management by existing MFH well concept. Introduction The Dan Field, located in the Danish sector of the North Sea (Fig 1) was discovered in 1971 and put on production in 1972. The field is a domal structure covering 5900 acres (2390 ha) and is separated into two fault blocks by a northeast-southwest trending fault (Fig. 2). The reservoir rocks are Danian and Maastrichtian chalks with average thicknesses of 150 and 800 ft. (45 and 244 m) respectively. There is a long oil-water transition zone with an average oil-water contact (Sw=100%) at 6500 ft (1980 m) subsea. The total productive hydrocarbon column consists of approximately 100 ft (30 m) of gas 400 ft (120m) of oil and 200 ft (60 m) of oil-water transition zone. The chalks are characterized by high porosities (19-40%) and low permeabilities, commonly around 1 mD. The lower part of the Danian (designated D2) has considerably lower permeabilities (in the range of 1.1-0.5 mD) than the upper Danian (designated D1). In parts of the field a relatively impermeable "hardground: layer of chart and highly cemented chalk exists between the Danian and Maastrichtian chalks. This layer, where present, has a maximum thickness of 10 ft (3m). The reservoir oil is 30deg. API gravity (0.88 g/cm3) with an initial solution GOR of 630 scf/STB (110 m3/m3). The original reservoir pressure is 3820 psia (26.3 MPa) at the gas-oil contact of 6060 ft. (1850m) subsea. The field originally contained some two billion STB (320 million m3) of oil, 1.2. trillion scf (34 billion m3) of solution gas and 0.5 trillion scf (14 billion m3) of free gas in the gas cap. In the northwest fault block (A-block) the crest of the structure is relatively flat and the gas cap lies almost entirely above t he low permeability D2 and the hardground. The southeast fault block (b-block) dips to the southeast from the main fault at approximately six degrees. Here, the gas cap exists in both the Danian and Maastrichtian reservoirs. A total of 42 conventional deviated wells were drilled between 1972 and 1986. Six MFH wells have recently been drilled to assess their potential for increasing offtake from the field. Currently, the six MFH wells contribute 45% of the total field's oil production rate. A further 19 MFH wells are planned to complete primary development. The feasibility of secondary recovery, u sing patterns comprising MFH production wells and conventional deviated water injection wells, is currently being investigated. PROBLEM DEFINITION PROBLEM DEFINITION The primary purpose of drilline MFH wells in the Dan Field is to increase productivity and hence economic ultimate recovery of oil. Production enters the wellbore of MFH wells only via the hydraulic fractures. Thus a MFH well can be thought of as several hydraulically fractured conventional wells connected underground by a low resistance conduit (at flow rates typical in these wells). P. 167