Field Development Options for a Waterflooded Heavy-Oil Reservoir

Abstract

Summary Battrum Unit 4 is a moderately heavy-oil reservoir in Saskatchewan producing under waterflood from a thin sand. This paper describes a producing under waterflood from a thin sand. This paper describes a history match of previous field behavior and systematically analyzes through the use of numerical simulation the potential benefits to production of further waterflooding (with and without infill production of further waterflooding (with and without infilldrilling), steamflooding, and horizontal drilling. It is found that the remaining oil recovery potential of a steamflood with horizontal wells is significantly higher than that of any of the waterflood options. Introduction This reservoir study concerns oil recovery options for a waterflooded, moderately heavy-oil reservoir. the Battrum Northeast pool, located 45 km northwest of Swift Current, Sask. Various pool, located 45 km northwest of Swift Current, Sask. Various field development plans were examined through numerical simulation and screening for a suitable EOR method. Steamflooding was the EOR technique selected, although the reservoir conditions were somewhat borderline for application of this process. Use of horizontal wells also was studied. Reservoir Characteristics and Geology The pool, discovered in 1955, started to produce under natural depletion in Aug. 1963. A waterflood was initiated in 1966 to arrest declining pressures. The field was later unitized and divided into four units. Mobil Oil Canada Ltd. operates Units 1 through 3, which are being fireflooded; Sceptre Resources Ltd. operates Unit 4, the subject of this study. BP Resources Canada Ltd. (now Talisman Energy Inc.) has a 25 % interest in Unit 4. The original pattern used for the waterflood was an inverted nine-spot drifted on a 32-ha spacing. In1979, when water cuts became excessive, infill wells were drilled on a 16-haspacing to improve sweep efficiency. Further infill drilling was carried out in1983. Fluid migration across the boundary between Unit 3 and the more easterly Unit 4 was thought to have been controlled by water injection at the boundary. Oil production peaked initially in 1970 and subsequently in 1985, with an apparent rapid decline underway at the time the study began. The pay zone in Unit 4 consists of the Roseray (main producing zone) and Success sands. Fig. 1shows the isopachs for the Roseray, which communicates with the Success over a good portion of the reservoir. The well pattern can also be seen. Unit 4 has no gas cap. Table 1 lists the main characteristics of the reservoir rock and its fluids. The reservoir varies considerably in quality, especially in the more productive Roseray sand. Table 2 gives some of the variations in Roseray properties. These variations are included automatically in the history-matching process through the usual specification of reservoir data for the field. They also play a direct role in steamflood modeling when decisions must be made on where to locate a pattern element and what local reservoir properties to use for the pattern model. properties to use for the pattern model. Note that field wide fracturing was carried out at Unit 4 for two reasons: to improve productivity from the tighter sand and to prevent plugging of the near-well formation and sand screens by prevent plugging of the near-well formation and sand screens by fines. While the fracturing program only marginally improved productivity from the tighter sand, its effect on preventing sand productivity from the tighter sand, its effect on preventing sand plugging was much more beneficial. plugging was much more beneficial. Preliminary Reservoir Screening Preliminary Reservoir Screening Battrum Unit 4 was screened carefully for applicability of various EOR methods. Miscible processes (hydrocarbon or CO2)were rejected because the required pressures would have exceeded fracture pressures. Immiscible CO2 might have been successful, but there pressures. Immiscible CO2 might have been successful, but there was no economical CO2source in the area. Alkaline and surfactant processes were discarded because of their complexity and their processes were discarded because of their complexity and their generally mixed field performance. Polymer flooding was rejected because the foreseeable benefits were few; sweep has been good in this reservoir. Polymer diversion similarly was judged to be of little benefit because of existing crossflow between the two formations. In-situ combustion had been in operation in the neighboring Unit 3 and had not yet proved its effectiveness. Laboratory and reservoir studies indicated that Unit 4 may be a candidate for steamflooding, considering the advanced state of technology available for operating steamfloods and completing thermal wells. This investigation was intended partly to provide additional information on the viability of steamflooding Unit 4. Study Objectives The main objective of this study was to determine the best way to operate Battrum Unit 4. A history match of previous production was carried out first to determine present conditions and reservoir parameters. parameters. P. 888