Geological Factors Study On A Large Scale Of The N₂ Bottom Layer Casing Damage In Daqing Oil Field

The casing damage has become one of the most important factors that affect the oil production in Daqing Oil Field. The statistics shows that almost 50% casing damage point occurr at the bottom of layer N₂ in the oil field. So the studies on the N₂ bottom layer casing damage have been emphasized greatly. In the previous studies, it was accepted that the reason for the casing damage was due to the standard layer drowning, the mudstone absorbed the water, swelled, changed slowly, untill the cases were squeezed, compressed and finally crushed. But after being accurately measured, 90% of the casing damage distribute among 2-5 meters which is corresponding to the third peak in the 2.5m relative resistance rate curve. The casing damage points are so concentrated that there must have a relation between the casing damage and the corresponding stratum. And this stratum is characterized by a dense fossil distribution, in which there are 3-4 fossil layers per centimeter. The existence of the fossil layers resulted in the rock broken easily along with the interface of the fossil layers,and the fossil layers became the weak interface. So it is nessesary to study the relationship between the casing damage point and the fossil layers to find out the real reasons for the N₂ layers casing damage. This research will have a great significance to recognize the reasons of the casing damage.This paper firstly contrasts and studies the difference of the rock nature, fossil distribution, fossil category between the casing damage stratum and undamaged stratum by observing the typical wells' core, and summarize the characters of the casing damage in these respccts. Secondly, logging response characters will be analysed and summarized by many logging information, such as electricity quality, well parameter, natural electricity potential, etc. Thirdly, some samples will be tested and analysed to determine the dynamic characters of the casing damage stratum. Forthly, the main strata and the points or the frequency of the casing damage will be determined by the statistic data, some distribution regularities of the casing damage will be analysed and summarized by carefully comparing and analysing the logging curves' characters of the casing damage points. Fifthly, the casing damage shape and stratum will be accurately described and relocated in terms of both the depth data of the printed lead mold and the multi-arm well diameter testing. Lastly, the main factors that effect the casing damage, a valid geological model and dynamical model that lead to casing damage will be established will be determined by comprehensive study.The geological characteristics in the casing damage strata show that there are many ancient bio-organism fossil in the N₂ bottom layers. The core observation show that the dense slices of the ancient bio-organism fossil distributed along with the mudstone layer, and it becomes a typical weak mudstone interface. In addition, due to the mudstone containing abundant fossils and the calcium, the N₂ bottom layers shows entirely some characters of a hard rock naturew and a weak interface. The existence of the weak interface must result in the stratum stress relief, and...