Liu, Zhiyu (China University of Petroleum) | Zhou, Fujian (China University of Petroleum) | Feng, Xiaoxu (China University of Petroleum) | Yang, Zhao (China University of Petroleum) | Fan, Fan (China University of Petroleum)
ABSTRACT: Wellbore stability is an important issue for drilling a horizontal well or an extended reach well. In this paper, the distribution of the breakdown pressure and collapse pressure is calculated with varying well inclination and azimuth angles. Considering the formation seepage, the mud density window for wellbore stability is obtained for different cases of in situ stress. The open-hole extension limit of the horizontal section in an extended reach well can be determined using these results. The results show that when (equation) the mud density window will become narrower with increasing deviation angle. Meanwhile, the mud density window becomes narrower or disappears as the in situ stress disparity increases. In addition, when the horizontal well section is drilled along the direction of σh, the wellbore stability achieves the safest state. The extension limit length of the open-hole horizontal section in the extended reach well can also reach its maximum value. The open-hole extension limit will continue to increase as the annular wellbore friction is reduced.
The research presented in this paper provides guidance for the optimal design of wellbore stability during drilling operations.
With the development of modern exploitation of unconventional and offshore oil and gas reservoirs, horizontal well or extended reach well technology is commonly used. It is considered not only for economic reasons, but also for the requirement for reservoir exploitation. Extended reach wells are commonly used, particularly in the exploitation of offshore reservoirs. Owing to the large deviation angle of horizontal wells and extended reach wells, there are significant differences in the behavior of the wellbore stability compared to that of vertical wells. Wellbore stability in extended reach wells is not only related to the formation rock strength characteristics and underground in-site stress state, but is also significantly affected by the wellbore trajectory, which is described by parameters including the deviation angle, azimuth angle of the well, and depth.
During the drilling operation process, wellbore stability is one of the most important issues. Wellbore failure causes economic losses of hundreds of millions of dollars every year. Before a drilling operation, deep underground rock is in a state of equilibrium between the loading of original in situ stress and pore pressure. After the wellbore hole is created, the surrounding stress will be redistributed. Then, due to the effects of stress concentration around the wellbore, shear failure may occur if the stress exceeds the compressive strength of the rock itself. In addition, tensile damage may occur if the stress excesses the tensile strength in a local area.
Xuerui, Wang (China University of Petroleum) | Baojiang, Sun (China University of Petroleum) | Zhiyuan, Wang (China University of Petroleum) | Yang, Zhao (China University of Petroleum) | Jintang, Wang (China University of Petroleum) | Zhennan, Zhang (China University of Petroleum) | Gao, Yonghai (China University of Petroleum)
The arctic could hold about 30% of the world's undiscovered gas and 13% of the world's undiscovered oil according to an assessment by the United States Geological Survey (USGS). While, arctic oil and gas exploration is faced with various challenges such as the extremely cold polar environment, the ice scour, the permafrost. Permafrost is the perennially freezing soil (actually freezing pore water), and it can be an important geologic hazard and constraint. Permafrost is very common in arctic region, about one-half of the total land area of Canada and Russia and 85% of Alaska is underlain by permafrost. Besides, sub-bottom permafrost is also wide spread phenomenon of Arctic offshore areas.
At present, the traditional thermal models of wellbore use an algebraic approximation to describe the heat transfer between wellbore and formation. Thus a new model should be established considering the effect of permafrost thaw during drilling in arctic region. As a matter of fact, the phase change of ice in permafrost needs to absorb heat from wellbore which can have an effect on the wellbore temperature. At the same time, migration of water from warm to cold regions will happen during the process of the thaw of permafrost. The migration of water will have an effect on the temperature due to the heat transfer along with the water. In addition, temperature in wellbore can be negative after long shut-in time due to the extremely cold polar environment which could lead to the blocking of wellbore.
In this paper, a transient thermal model of wellbore during arctic drilling is established considering the effect of permafrost thaw in this paper. On the basis of the new model, a simulation of an arctic well is made and some conclusion are made from the case study: The temperature fields in wellbore and permafrost interact with each other. Besides, the temperature in wellbore drops to negative due to the extremely cold environment after long shut-in time, and the drilling fluid in wellbore could be frozen consequently. Long shut-in time should be avoided during drilling in arctic region.
The new model established in this paper can determine more reasonably the wellbore temperature in artic permafrost region than the traditional model. A method to determine the reasonable shut-in time is given in this paper which can provide safety guidance during drilling in arctic permafrost region.
Yang, Zhao (PetroChina) | Fenjin, Sun (PetroChina) | Bo, Wang (PetroChina) | Xianyue, Xiong (PetroChina) | Wuzhong, Li (PetroChina) | Lianzhu, Cong (PetroChina) | Jiaosheng, Yang (PetroChina) | Meizhu, Wang (PetroChina)
Compared with the conventional oil and gas reservoirs, hydrogeological gas controlling process linking CBM accumulation, enrichment and high yield is one of the important scientific problems for the development of a CBM field. Previous research results are mainly focused on the impact of hydrodynamics on CBM dissipation, preservation and enrichment, whereas relatively less work has been done on the quantitative evaluation of the hydrochemical field of CBM and establishing evaluation indicators of CBM enrichment. Therefore, taking BQ Well area of Hancheng block in east Ordos Basin as an example, this paper tried to initiate a systematic analysis of the controlling function of hydrogeological conditions on the enrichment and high yield of CBM in the study area. Hydrological evaluation indicators for hydrocarbon enrichment zones are established and two favorable hydrocarbon enrichment zones are optimized. It is of great significance for the established analytical method of hydrogeological rule on the studies of CBM enrichment characteristics and development in Hancheng CBM block, and subsequent exploration & development in the neighboring blocks.
Firstly, the relevant principle of hydrodynamics is applied to identify substantive parameters, such as measured in-situ reservoir pressure and CBM reservoir water level in the production wells to calculate the reduced water level and analyze groundwater level distribution characteristics; secondly, combined with the analysis of groundwater water types, the sources of the produced water from coal beds are identified, and the sealing property of the reservoir is demonstrated; on this basis, the study area is divided into the weak runoff zone and the stagnation zone. It is considered that the runoff intensity is relatively weak and the sealing capability is good in the study area, with no external water intrusion; finally, it is considered that, through integrated studies on the hydrochemical field, the desulfuration coefficient and sodium chloride coefficient can reflect the diversity of CBM reservoir conditions in a more elaborated way. Hydrological indicators based on hydrochemical characteristics are established, and two favorable enrichment zones are predicted.
This work proved that hydrogeological features of CBM reservoirs are able to characterize their accumulation conditions elaborately. In particular, the establishment of hydrological indicators can classify favorable enrichment zones and hereafter guide following CBM exploration & development. This methodology has been satisfactorily applied in BQ well area of Hancheng block where the data of gas bearing capacity is limited. High single well production rates have been obtained in the two predicted favorable enrichment zones. The hydrological indicators established in this paper are expected to be popularized and applied in other well areas of Hancheng block, which may accelerate the overall exploration & development progress in this block.