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Abstract The paper has discussed some questions in unbalanced drilling, and state that the most important question is determining the information pressure gradient before drilling in the design. The paper put out some principles and formulae for determining the information pressure gradient, which are different from that were put forward by some scholars in past to some extent. The author introduced a case history about unbalance drilling - BS7 well, which was drilled in Dagang oil field, Tianjin city, People's Republic of China, and explain the application of the principles and formula in the well. There are two points in this paper. First, in term of new principle and formula of design formation pressure gradient, the time that is from starting show of some omen to having blow out in well, is longer than predicting in other method in past, and this predicting had been testified by the practise in the well. So the operator has more time to deal with well kick and protect the blow out of well, in other words, designer can take a risk to determine the formation pressure gradient more than in past. Second, when determining the formation pressure gradient, the designer would not consider abnormal layers, which are much smaller in thick than the length of naked section in the hole. Because not only the operator have enough time to deal with well blow out if the blow out really happen, but also some situations have not happened, in speciality to the block faulted oil field, for example, Dagang oil field. The mainly reason is the level of forecast the formation pressure gradient is not very accurate, and it is not worthy of raising the value of it in the whole section in well only according to a predicting value of the formation pressure gradient in shorter section, which predicting value of it mainly depend on the data from neighbor wells. Since beginning of 1999, the most of oil fields in main land of China have accepted these principle in the drilling of explored well so that decease the mud density, and get obvious effect, which have found several huge oil fields so far.
1 introduce In main land of China, there are about 19 oil fields in land and 3 oil fields in offshore. CNPC (China National petroleum Corporation) manage the land oil field, and CNOOC (China National Offshore Oil field) manage the offshore oil field. The production of oil and natural gas in the oil fields on land has been decreasing for years recently. How maintain the production of oil and gas in the old oil fields and developing the potential of its capability? By research and practice for long time, it has been proved that the important way to raise the production of well is using the quasi balance and balance and unbalanced drilling.
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