A Study On Predicting Formation Water Production And Wellbore Stability For Underbalanced Gas/Foam Drilling In Daqing Oilfield
Authors: Jiang Yufang , Liu He , Liu Zhihuan , Liu Yonggui , Li Zengle 
Published: May 2016

Abstract:
The practices have shown that the premise for successful implementation of gas drilling is stable borehole wall, formation containing less water or no water, not high pressure formation and clear reservoir bed relation, etc. Therefore, accurately evaluating borehole wall stability, predicting formation water production and selecting reasonable well intervals for gas drilling to effectively prevent occurrence of complex conditions such as blocking, sticking and fracture of drilling tools, etc. caused by formation water production and borehole wall instability are the keys for success of gas drilling. The paper, aiming at the problems of formation water production, long interval reaming, bit freezing and drilling tool failure occurring in deep zone gas drilling of Daqing oilfield, has analyzed difference of formation water production between gas drilling and conventional drilling, sifted logging response parameters of water zone under the deep zone gas drilling conditions of Daqing oilfield by borrowing the reservoir fluid identification methods under conventional drilling conditions and combining the practical gas drilling data, and established qualitative water zone identification method for gas drilling; in addition, based on Darcy's law, the paper has optimized and formulated quantitative calculation method for unsteady flow formation water production under dynamic condition and established borehole wall stability prediction model under gas drilling conditions in Daqing deep zone volcanic rock formation.
The formation water production prediction method and borehole wall stability model established in the paper, as important basis for feasibility analysis, engineering design and operation of gas drilling, have been 100% popularized in engineering design of deep zone gas drilling in Daqing, China. The coincidence rate of water-producing horizon predicted by using this method, comparing with the practical water-producing horizon, is up to above 80%, and that of borehole wall stability evaluation result, comparing with practically measured borehole diameter data, is up to above 85%.

Engineering Background
In the volcanic rock gas reservoir in Xujiaweizi fault depression structural belt of Daqing oilfield, China, Xushen gas field[1], with the scale over 1000*108 m3, was formed since the beginning of 2005. To continue to expand the exploration result and prove the reserves scale earlier, it is necessary to accelerate natural gas exploration, increase amount of drilled exploration wells, enhance drilling speed and shorten drilling period on the premise of protecting reservoir beds. However, the rocks below Daqing gas field deep zone Q2 Member are of bad drillability, high hardness and low drilling rate. To enhance drilling speed, several speed increasing tests for conventional drilling, such as bit optimization and parameter optimization, etc., were carried out in Daqing oilfield in 2004. Although certain effect has been achieved, the requirement for accelerating exploration still can not be satisfied. It is urgently necessary to apply new drilling technologies to make a breakthrough in enhancement of ROP (rate of penetration).For this reason, studies and field tests of gas drilling supporting technologies have been performed in Daqing oilfield since 2005 and the drilling operation of 14 gas wells has been finished, in which the ROP, 3 to 5 times of that of conventional drilling, has been increased significantly. While in gas/foam drilling practice, the complex conditions such as borehole wall instability and blocking, sticking and fracture of drilling tools, etc., caused by formation water production occurred frequently, and the drilling operations of several wells were completed in advance since the designed well depths were failed to be reached due to formation collapse in crushed zone of volcanic rock. The problems of borehole wall instability and formation water production seriously influence the efficiency of gas/foam drilling, thus restraining the advantages of gas/foam drilling technology.