The Study and Application of In-situ stress Distribution in Ultra-deep Fractured-caved Carbonate Reservoirs

Abstract

There are abundant oil and gas resources in carbonate reservoirs of Tarim Basin, and more than 90% come from fractured-caved carbonate. These fractured-caved reservoirs are formed by the superposition of large fracture zone and karstification. The fractured-caved bodies generally appear in strips along the fracture zone and are distributed irregularly in vertical and horizontal directions. In addition, the ultra-deep burial (6500-7500m) leads to strong heterogeneity of the reservoirs. This paper presents using horizontal wells to cross the fractured-caved bodies is an effective means to improve the economic profit of oil and gas field development. Based on geomechanical study, the development mechanism of high-quality fractured-caved bodies can be determined, the contradiction between multiple fractured-caved bodies and safe drilling can be solved, and the reservoir stimulation plan can be optimized. The study can provide quantitative data basis for well trajectory optimization, prediction of drilling safety window, determination of maximum build-up slope in deviated section, how to use one well to cross multiple fractured-caved bodies and optimization of reservoir stimulation program. The results show that the ultra-deep carbonate reservoirs are affected by four factors: faults, fractures, irregular karst and in-situ stress, resulting in strong heterogeneity and anisotropy of reservoir. Therefore, the probability of high-quality fractured-caved bodies encountered by well trajectory drilling in different directions and depths is different, and the stability of wellbore in different directions and deviation is also different. When the drilling trajectory crossed the fault zone, it experienced a large range of in-situ stress, so the stimulation effect varied greatly in different well sections. The geomechanical study of ultra-deep carbonate reservoirs shows that the strike of the fault is similar to the present in-situ stress, and it is easier to develop high-quality fractured-caved bodies in relatively weak stress environment. Through the study of the relationship between fracture system and in-situ stress, it provides a method to optimize the well trajectory in consideration of both high-quality reservoir and drilling engineering safety.On the basis of optimizing the best well trajectory, it can predict the safe drilling window more accurately, guide drilling through multiple fractured-caved bodies as much as possible, and implement reservoir stimulation effectively, laid a foundation for high-quality and high-yield drilling. So far, three carbonate reservoir oilfields’ 3D stress modelling have been completed and applied to well placement, drilling and development. The average oil production per well exceeds 300 tons under 6mm nozzle conditions.