Development of Daqing Oil Field by Waterflooding

Abstract

Summary Combining geology and engineering to exploit the Daqing oil field effectively is the topic of this paper, which also describes the principal procedures of oil development. After the reservoir characteristics of the field were established, waterflooding was initiated at an early stage of development, which allowed pressure maintenance by the separate-layer technique and achieved an improved result. The Daqing oil field is located in the Songliao basin. There are seven structures in the field, but only the northernmost Lamadian structure has an original gas cap. This oil field has three commercial reservoir formations--Saertu, Putaohua, and Gaotaizi. They are between 700 and 1200 m [2,295 and 3,934 ft] deep. The reservoir rocks are lacustrine and fluvial-deltaic sandstone, with siltstone deposits in the middle of the upper Cretaceous layer. The physical properties of reservoir crude oil are described in Table 1. The Daqing oil field was discovered in 1959. In June 1960, a 20-km2 [8-sq mile] pilot test unit was established in the central part of the field. On the basis of pilot results, the field has been developed sequentially since 1962. By 1976 the annual output had reached 58 × 106 m 3 [366 million bbl], as established by the development program. Oil output per well was 41 m3/d [256 B/D] at the beginning of waterflooding, and is currently 43 m3/d [271 B/D]. The average increase in water cut is controlled at 2 to 3 %. By the end of June 1981, 23.5 % of the original oil in place (OOIP) had been produced, with a water cut of 61.7%. Currently, the field is in a high-watercut stage of development (see Table 2). Waterflooding Development of the Reservoir In the initial development stage, reservoirs were divided into 30 to 50 layers and tens of thousands of oil-bearing sand bodies by means of correlating depositional rhythmics and marker beds; maps of these areas were drawn to show their thickness, permeability distribution, and degree of intercommunication. Subsequently, the distribution of these sand bodies, their depositional environments, and heterogeneities were studied in detail by sedimentary-phase identification. Eight types of sand bodies were identified. Each type has different sedimentary origins, characteristics of oil/water movement, and development characteristics. For example, the main reservoirs, which occur in fluvial facies of a flood-plain region, are composed of sand bodies originally deposited in different river systems. Permeabilities of these sand bodies show heterogeneities between sand bodies and within each sand body itself. Obviously, the movement and distribution of injected water in a reservoir becomes very complicated. In a river system, the injected water channels rapidly through highly permeable sections (i.e, the main streamline of the old river), then spreads to other parts (Fig. 1). This channel becomes a "time-unit" of the sand body. Thus, in a sedimentary unit that has a highly permeable channel section as its central feature the injected water displaces oil. This effect should be a primary consideration when adjusting the field-development program to obtain higher production. Some thicker sand bodies were not continuously deposited, but underwent several deposition-washing-deposition cycles. Therefore, there are thin, low-permeability argillaceous barriers between different time-units. Although the barriers may be only 10 to 40 cm [4 to 15 in.] thick, within a group of wells or in a small area they are often relatively stable, which permits the division of a thick sand body into two or more pay zones (Fig. 2). Field experience has shown that the thick-sand-body time-unit should be the basic area for water injection to displace oil. Inspection wells were drilled in the waterflooded areas and the undisturbed core samples were analyzed and compared with logging data. On the basis of this analysis, three layer types have been identified in thick sand bodies: bottom-, uniform-, and multizone-flooding. Reservoirs deposited in fluvial facies of a flood-plain area are bottom-flooding. They are very heterogeneous and have a small flood thickness and a positive rhythm--i.e., the injected water advances rapidly along the bottom. In a vertical section, flooding behavior varies from point to point and the water cut rises rapidly after breakthrough. For example, in Zhong Jian Well 4–24, which is 300 m [984 ft] from the injection-well array, undisturbed cores were taken and analyzed after 6 years of water injection. The net pay thickness of Layer PuI-2 is 8 m [26.9 ft]. Core analysis showed that only 1.5 m [4.59 ft.] at the bottom of the highly permeable section were flushed out; 1 m [3.28 ft] at the bottom was strongly flushed out. At this level the displacement efficiency is 75 %. JPT P. 269^