Study On Prediction Method And Drilling Method Of Gas Drilling Formation

Gas drilling has many advantages, such as increasing the rate of penetration and single well production greatly, and preventing the vicious lost circulation effectively. So it has been widely used at home and abroad. However, gas drilling use gas instead of drilling fluid as circulation medium, the bottom hole is in a state of extreme negative pressure. So it will not balance the formation fluid pressure and easy to cause formation water production. Numerous gas drilling practices have shown that the formation water production is a huge hidden trouble, which will affect the drilling safety significantly. Once the formation water production occur, it will bring huge difficulties, such as sticking, wellbore wall collapse and other complex ac-cidents. Therefore, it is important to understand which formation has the possibility of water production and predict the water yield accurately.Based on the geology, seepage flow mechanics, rock mechanics and log interpretation method, monitoring while drilling technique, wellbore multiphase flow theory, etc., through theoretical analysis, modeling and visualization simulation, combined with real gas drilling data, a method to predict the formation water production and recognition formation while drilling for gas drilling has been put forward. The main results are as follows:(1) On the basis of understanding formation water, summarize and analyze comprehen-sively the factors that affect formation water production in gas drilling. The results showed that the factors that affecting formation water production in gas drilling include hydrological geology, climate, formation structure, lithology combination, reservoir space, formation fluid parameters, formation temperature and pressure characteristics.(2) In view of the shallow and deep formation, respectively, influence factors for for-mation water production have been put forward. Moreover, explain how to combine the fac-tors to make prediction. On the basis of comprehensive analysis in combination with the en-gineering and geological data for the all-round situation, make water prediction method sys-tematically, while a forecasting process of the formation water production for gas drilling has been established.(3) Considering formation stress sensitivity, coupling formation pore fluid transient flow and well circumferential stress changes, matrix formation and fracture-matrix formation coupling flow water yield prediction model has been established. Numerical simulation by using MathCAD software has been performed, which establish water yield computing meth-ods that consider the strata stress sensitivity of the coupling flow, not consider strata stress sensitivity of the unstable seepage and stable seepage. Then analyze these results by com-bined with the water yield of real drilling data. Results showed that the coupling flow model considering strata stress sensitivity is more conform to the actual water yield.(4) The capacitive temperature and humidity sensor of UBD gas monitoring system has been taken to identify whether the formation water production occur. Based on the judgment of wellbore gas-liquid two phase flow type for formation water production in gas drilling, a calculation method for water yield by stand pipe pressure parameters increasing has been proposed. The results demonstrated that using capacitive temperature and humidity sensor while drilling to identify water formation is more advanced and accurate; it is considerable to calculate water yield rely on the well stand pipe pressure parameters.This study can provide theoretical basis for the well location and formation optimization, and subsequent media transformation. It has important guiding significance for the designing scheme optimizing and well construction safety.