| The drilling data of the oilfield in east part of south shows that HZ42block’s borehole stability is poor so that the drill string is easy to be sticked, and without leakage, the overflow problem during drilling time,only have a small amount of leakage in MDT logging process after drilling to the totle depth.These restrict the drilling speed in this region. In order to development of the oil and gas resources in the region better, formulate reasonable drilling design, improve the quality of drilling hole, lower exploration drilling and development costs, shorten the drilling period and improving economic efficiency of oil and gas development,we are badly in need of predicting and testing the formation drillability of this region.Based on the comprehensive analyses of the HZ42-7structure’s drilling geological data, engineering data, and logging data, and through statistic two well’s complex problems,and analyses the trend of pore pressure, in-situ stress, as well as the formation collapse pressure and fracture pressure on the changing with the vertical depth by using mechanics model of formation, project and analysis the pre drilling borehole wall stability,we proposed a drilling hole structure design based on the safe mud density window. From the angle of sand production we study the feasibility of open hole completion as well as calculation and analysis the try oil extreme pressure difference. The main research conclusions are as follows:1The conclusion of pore pressure researchYue Hai to Wen Chang group in HZ42-7structure have a normal pressure gradient, generally without abnormal high pressure, mainly in En Ping group to Wen Chang formation pore pressure is a bit high, pore pressure is about1.10g/cm3.The mainly target layer, the sandstone formation of Wen Chang group’s pore pressure coefficient is between1.01-1.09g/cm3.2In-situ stress research conclusionIn HZ42-7structure, about700-4100m formation upper cover basic principal stress equivalent density are between1.88to2.30g/cm3, the horizontal maximum principal stress equivalent density are between1.78to2.18g/cm3density, the horizontal minimum principal stress equivalent density are between1.41to1.62 g/cm3. The relative size of three principal in-situ stress:"Vertical principal stress> Horizontal maximum principal stress> Minimum horizontal principal stress".According to the fault strike, combined with the result of crustal stress obtained by borehole caving ellipse method in south China sea.we determined the horizontal maximum principal stress direction azimuth of this block is roughly Nl10°E～N110°E.3Formation strength research conclusionCombined with laboratory experiments, using the logging data analyzed the HZ42-7oilfield formation rock mechanics parameters, the calculation result shows that, with well depth increasing, HZ42-7oil field formation strength showed a trend of increase, generaly belongs to medium strength formation. Yue Hai to Han Jiang group formation strength are generaly between5to19MPa, the Pearl River group formation strength are between8-45MPa, Zhu Hai group and En Ping group formation strength are between14-60MPa, Wen Chang group of formation strength are between15to50MPa,the strength of granite basement is extremely high, for more than80MPa.4Safe drilling mud density window conclusionIn the HZ42-7structure, the collapse pressure increased with the depth slightly, the highest in1.20-1.42, the formation fracture pressure below Yue Hai Group is above1.80g/cm3, so the safe drilling density window between" collapse pressure and fracture pressure "is wide. The minimum leakage pressure below Yue hai group formation in the well at around1.40g/cm3, as a result,we should pay attention to prevent leakage when drilling through fault, but on the premise of drilling fluid sealing performance is good, the risk of leakage is lower. According to the seismic interval velocity analysis,during200-4000m in the pre drilling borehole has a normal pressure gradient. The recommended the drilling fluid density of HZ42-7-3Wells wenchang formation is1.42g/cm3. The original drilling fluid is recommended.5The drilling hole structure designBy considering the pressure constraint conditions of open hole section, drilling engineering construction condition and the risk of borehole instability problems,we designed a three open scheme to well HZ42-7-3, the first spud is to around805m, cementing the upper loose set; Second spud in upper stratum of Zhu Hai group, to around3005m, cementing the upper strata of Yue Hai to Zhu Hai group; Three spud to finish drilling.Because of the safe drilling mud density window is wide in well HZ42-7-3, so we mainly considered to meet the requirements of geological reservoir in the design of well structure, adjust depth of intermediate casing.6the feasibility of open hole completion and the limited well test pressure conclusionFrom the angle of sand production we study the feasibility of open hole completion. The early development of HZ42-7blocks are less likely to producing sands,open hole well completion is available, but consider the risks related,so we recommended "open hole+perforation" completion.In the Huizhou42-7oilfield, the critical reservoir sand producing pressure difference are above9MPa, it is recommended that the early oil test pressure should not more than9Mpa. As the reservoir pressure decrease and water influence on strength during the process of development, critical pressure differential will decrease slightly. |
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