Simulation Of In-situ Stress Near Wellbore And Research On Wellbore Stability

Great progress has been made in the exploration and exploitation of the Xujiahe Formation gas reservoir in recent years ,which is located in the west Sichuan depression. This reservoir is well known as typical tight pay zone with ultra high pressure and temperature, and the geological condition is quite complex. The distinct heterogeneous lithologic character ,such as the interbedding of sand and clay, along with nature fracture system, is the major reason to the frequent accidents ,which consists of leak, collapse ,kicking and sticking, and the drilling operation have been suffered much from this. To speed up the progress of gas reservoir exploitation, horizontal drilling operation have been introduced to put into pilot operation to improve the probability of disclosing nature fracture system as much as possible. At present, to get a better understanding of the safety complication and accident prevention, it is believed that the theoretical research on well bore stability could be the key factor, since few experience has been accumulated from actual horizontal drilling operation in this area. To take the Xujiahe gas reservoir as an example, based on the multidisciplinary knowledge from mathematics, geophysics, geology and civil engineering, by means of integrated approaches which consists of simulation and computation , the information from geophysics ,geology, drilling ,wire line logging and laboratory mechanics experiments are utilized to carry out the simulation on regional in-situ stress and the wellbore stability. The results can be listed as follows:1. Based on the primitive methods of rock mechanics ,the laboratory results and the wire-line logging data are utilized to summarize the static and dynamic mechanics parameters. The relationship deduced from laboratory mechanics parameters can be the basis to establish transformation model for this studied area, and can be the basic data for the regional stress field simulation and wellbore stability analysis. 2. The mechanical model has been built on the basis of joint application of various in-situ stress measurements, along with mechanism of earthquake focus and the distribution of fracture system. Taken into accounts of the different lithological belts , this model is solved by means of finite elements ,and the results of in-stitu stress are compared with that from on-site downhole measurements to show its accuracy . The general in-situ stress character of the target zone can be the basis for the further analysis of the stress distribution near-wellbore.3. Compared with the traditional analytic method for in-situ stress near wellbore, the feasibility of finite elements is proved. Herein , different models have been established to specify the unique character of various well bore axis, and the fractures cutting wellbore .4.Based on the global in-situ stress distribution of Xujiahe Formation, the calculation of in-situ stress near wellbore for vertical well is discussed ,the results of which could be adopted to interpret the different model of in-situ stress distribution .The results herein are also brought forward to justify the well bore stability for vertical well ,which could be proved by traditional analytic methods.5.Taken into account of elasticity theory, the mathematical models of in-situ stress near well bore for high inclination well , including horizontal well, are established, and the results from which could be utilized to deduce the in-situ stress distribution for different kinds of engineering conditions. The results herein are also brought forward to justify the well bore stability for high inclination well ,which could be proved by traditional analytic methods.6.Combined with deep well drilling operation in Xujiahe Formation of western Sichuan depression, well log interpretation, on-site drilling data , numerical simulation of mechanical modeling, the mechanical criteria for the borehole stability is systematically summarized. It is showed by this research listed herein that the regional mechanical characteristics, in-situ stress distribution in the pay zone or near wellbore can be useful for the research on borehole stability effectively and systematically, and can be strong technical support for the drilling and fracturing operation. All of the ideas and methods could be applicable to the similar gas reservoir home and abroad .