Research On Gas-liquid Displacement In Vertical Crack And Drilling Fluid Plugging Technology To Prevent Gas Cut

Overflow induced by gas invasion is often encountered during the processes of gas wells drilling, which brings serious harm to the safety of drilling. The mechanism of gas invasion at negative pressure differential has been known. However, sometimes gas invasion also occurs at positive pressure differential condition. That is to say that gas invasion occurs when the fluid pressure in the well bore is greater than the formation. Since the phenomenon is understood less currently and no experimental and theoretical studies on it, so it is urgent to be researched. This dissertation studies the mechanism of gas-liquid displacement which occurs within the fractures and its influenced factors. This thesis proposes a new kind of drilling fluid technology which could prevent gas invasion by plugging the fracture channels actively and let the narrow safe drilling fluid density window become wider.Theoretical researched find that mechanism of gas-liquid displacement type gas invasion occurs in deep vertical fractures formation in the slight condition firstly. The drilling fluid drops into fracture channels at positive pressure difference, occupies the lower space of the crack to compress the gas in the crack and results in gas pressure increased gradually until it exceeds the pressure in the well bore at a certain point, the gas-liquid displacement’s gas invasion occurs. This kind of gas invasion is pulsate, which has closely relationship with the crack’s geometry, drilling fluid performance, original formation pressure and pressure difference between fluid pressure in the well bore and formation pressure.In order to verify the result of the theoretical research, based on the concept of visualization, the gas-liquid displacement and sealing anti-gas invasion experimental test device was developed which has automatic data acquisition and automatic recovery feature, removable cracks panel and crack width (height) adjustable has been developed in this dissertation. Based on this, gas invasion caused by the gas-liquid displacement is studied in depth. The occurrence and development of gas-liquid displacement at slight conditions are researched, which clarifies the causes and mechanism of gas invasion because of gas-liquid displacement, analyzes affecting factors of the gas invasion, proves that the wider crack’s width, the lower the apparent viscosity of drilling fluid, and the gas invasion is more likely to happen with higher drilling fluid density and original formation pressure. FLUENT, the numerical simulation software, is utilized to simulate the vertical crack gas-liquid displacement phenomenon and the mechanism, the influenced factors of gas-liquid displacement are verified with experimental studies.Based on the analysis of mechanism and influenced factors of the gas invasion of gas-liquid displacement, the technique is proposed to seal the fracture channels preventing gas invasion while drilling. Accordingly, a series of blocking experiments that block the cracks with the width less than lmm are carried out with the vertical cracks gas-liquid displacement and sealing seam to prevent gas invasion experimental device. The cracks wedge degree, blocking pressure difference, cracks surface roughness, rigidity calcium concentration and particle size distribution and synergy with other sealing material effect on blocking are studied and evaluation parameters:the blocking position, the amount of leakage and the sealing layer resistance capability on the gas reverse pressure when preventing gas invasion are given. Results show that the cracks with the same entrance width, the greater the degree of its wedge, the sealing material is closer to inlet portion with sealing position of same formation; the larger block pressure difference, the sealing position closer to the outlet portion; the less the leakage amount and the higher the capability to resistant gas reverse pressure; the more rough fracture surface, the sealing position closer to inlet portion; blocking effect is not affected by blocking material characteristic values D(50) or D(90) separately, yetaffected by the synergy; the higher the concentration of the blocking material is, the faster the blocking layer is formed and the more closer position blocking the entrance slit. But if the concentration is too high, it’s not conductive to seal cracks; the sealing effect is the best while rigid calcium carbonate and paper fibers synergy.Based on the theory of bridge plugging and dense blocking, using high temperature and high pressure filtration apparatus, the micro-pore is simulated by filters. Several deformable materials at a certain temperature and pressure are tested experimentally. Results show that the blocking effect to micro pores of a thermoplastic resin FTJ-1is the best. The rigid calcium carbonate particles of a certain size grading, staple fiber and resin are composed by a certain percentage to constitute sealing material for preventing gas invasion(Code TYFC-1). The drilling fluid technology preventing gas invasion brought by vertical fracture gas-liquid displacement is formed.The sealing materials and drilling fluid technology preventing gas invasion are studied in this dissertation. Field tests are complished in Tarim Oilfield with two horizontal wells. The results show the good preventing the gas invasion performance are achieved. In summary, a new drilling fluid technology for gas well drilling which has narrow safety drilling fluid density window is formed.