Development of Gas Cap Reservoir in Pingfangwang Field and Studying on the Communication Between Formation Members
Authors: Wang Hua-fen 
Published: December 2007

Abstract
The Pingfangwang field contains reservoirs at the burial hill with a large gas cap and edge/bottom water. The main objects of development are the bioherm limestone of Upper S4 Member and the sandstone of Middle S4 Member in Tertiary system. However, the formation members communicated each other through faults, "skylight" and individual well-bores during development process, which seriously affects the development efficiency. In order to enhance the oil and gas recovery, many pilot projects had been advanced. For Upper S4 limestone, we firstly produced oil with high rate in oil ring, and then commingled oil and gas production, at last transferred to recover gas extensively. Now, 30.8% of the estimated oil reserve and 82% of estimated gas cap reserve from the Upper S4 Member have been produced, respectively. For Middle S4 Member, in the light of studying on the reservoir and the communication, we adopted the noncommingled development projects in which efforts are focused mainly on the oil recovery and non-swept oil zones. Furthermore, in accordance with the residual oil distribution, we optimized continuously the development projects, which increased not only the oil production, but also the expected ultimate recovery by 9%.

Introduction
The major reservoirs of the Pingfangwang oil field have been producing Upper S4 and Middle S4 Members with a large CO2 gas cap. There were great differences of reservoir property and the significant vertical communication between the two members. For this reason, the development optimization of the oil field was difficult. Thus, our reservoir development planning, which begins during pre-development phases, had been emphasized to adapt to the reservoirs of different properties and to minimize the unfavorable effect of communication throughout the productive lives of the oil field. There were three main challenges in achieving higher oil and gas recovery factors from Pingfangwang:determining and controlling communication,improving the balance between oil-water and oil-gas interfaces,improving well patterns of waterflooding in more heterogeneous zones.

Reservoir Geology and Development
Pingfangwang field as a buried hill structure with an area closure of about 31 sq km, is situated in the north-western part of Dongying depression which is enclosed partially by Lijun (east), Boxing (south), and Lizhezhen (west) depression, and connected with Dudian and Shangdian (north) anticlines. These parts belong to the same secondary structural strip. The major reservoirs in the Pingfangwang consist of the Upper S4 Member and Middle S4 Member, which have edge -bottom water support and natural gas caps (Fig. 1). The flat structure dips 1 7. The reservoirs lain at a shallower depth have an unified oil-gas surface of about 1510 1520m and an unified oil-water surface of about 1560 1570m. Gas contains higher CO2 concentration (30 90%). Three major faults divide the structure into four fault blocks: West Block, East Block, North Block and Center Block, which are complicated by other smaller faults with throw of 10 20 m. The Upper S4 Member consists of bioherm limestone, the Middle S4 Member consists of sandstone. Main properties of the two reservoirs are given in Table 1.
The field was discovered in 1968. The development started in 1971. Sustained injection was initiated in 1975. The Middle S4 Member has been producing continuously crude oil throughout the reservoir's life, but in 1994 gas cap in individual wells were perforated. Before 1977, the Upper S4 Member mainly had been recovering crude oil. P. 303^