Geomechanical Approach Lead to Successfully Improving Wellbore Stability and Drilling Rate in Kuqa Depression in Tarim Basin
Authors: Cai Zhenzhong , Zhang Hui , Yuan Fang , Yin Guoqing , Wang Haiying 
Published: November 2019

The Kuqa depression located in northern Tarim Basin is the second largest natural-gas field in China, however, drilling engineering is faced with extremely complex geological conditions, such as complex structural movement history, complex formation conditions (huge thick gypsum salt rock, and different thickness of alternating sand/shale sequences and conglomerate), abnormal high pore pressure systems and strong anisotropy in situ stress. These complex geologic conditions result in severe wellbore instability problems.

An integrated research was conducted combining geology, geomechanics and drilling engineering to solve drilling problems caused by complex geological conditions. Firstly, geomechanical models are established according to the geological characteristics of different formations to get orientation and magnitude of stress, pore pressure and rock mechanical parameters. Secondly, based on rock mechanics experiments and wellbore information, the geomechanical mechanism research of wellbore instability was carried out under complex geologic conditions. Finally, the geomechanical model, wellbore stability parameters and the mechanism of drilling problems are applied to the drilling engineering design optimizing mud parameters, wellbore structures and trajectory of high deviation wellbore.

It is shown that geomechanical approach can improve wellbore stability and drilling rate. (1) For the uppermost conglomerate formation, according to the experimental study of the failure mechanism of conglomerate, accurate mud density is the key avoiding causing extension fracture around conglomerate grains. (2) The mechanical stability of borehole in alternating sand/shale sequences is good, but it is easily affected by hydration. Therefore, high quality mud properties can be matched with low mud density to maintain wellbore stability and improve drilling rate. (3)The interior of gypsum-salt sequences is divided into six lithologic sections. Based on the detailed analysis of different lithology, an in-situ stress model is established optimizing the mud density to find a balance between creep resistance and preventing from lost circulation. (4)Pay zone belongs to fractured sandstone under strong stress background. It shows strong anisotropy in stress field and rock strength. The mud density window determined by this mechanism can not only maintain wellbore stability and prevent lost circulation, but also protect reservoir.(5)The feasibility of highly deviated well was demonstrated based on geomechanical approach. And the wellbore trajectory was optimized in four aspects: avoiding shallow fracture, maintaining wellbore stability, traversing more effective fractures, and easy fracturing after drilling.

Geomechanical research under complex geologic conditions promoted the recognition of the mechanism of wellbore instability, and optimized the program of drilling engineering. The drilling incidents of formation above salt were reduced by 50% and the non-productive time was reduced by more than 20%. At the same time, this research project also promotes the successful implementation of the first highly deviated wells which provided a new way to further improve the gas productivity in this area.