Study On Mechanism Of High Pressure Gas - Tight Sandstone Gas Drilling

Due to reducing formation damage and detecting reservoir effectively, Air drilling has been used to develop tight sand reservoirs. However, during the development of the tight sand reservoir drilled by air drilling, the phenomenon of borehole instability is different from the common air drilling. The well log showed that the tight sand collapsed seriously and leaded the accidents of drill bit being buried, which limited the development of the tight sand gas reservoir. In order to explain this phenomenon and supply the basis for the risk estimation in tight sand reservoir when used air drilling, the paper is based on the in-situ condition when the tight sand formation drilled by the air drilling, designs the mechanical test and stress sensitivity tests of tight sand, and builds the elastic and elastic-plastic stress distribution models around the wellbore during air drilling. The main contents of this paper are followed as below:(1) Study the in-situ conditions of air drilling and geological characteristics of the tight sand reservoir. The research shows that the borehole instability is easy to occur in the tight sand formations. When using air drilling to drill through the tight sand formation, there are shear band and compaction band, which lead the stress field around borehole to become complex, and the wellbore instability easily occurs.(2) Carry out the rock mechanical test to study the effect of the pore pressure, confined pressure and axial pressure on the mechanical parameters. The research finds:the pore pressure has an important effect on the triaxial compressive strength. The bigger the pore pressure is, the smaller the compressive strength is. As the confined pressure increases, the elastic modulus increases as the power function. The rate of axial load has an effect on the residual procees of axial compressive strength of rock. The bigger the rate is, the easier the rock breaks out completely.(3) Use the changing confined pressure and changing pore pressure stress sensitivity test to study the stress sensitivity of tight sand. The tests show that the changing confined pressure test is more sensitivity than the one in the changing pore pressure test. And in the changing confined pressure test, when the confined pressure increases to some extent, the permeability of tight sand does not obviously decrease.(4) Consider the pore pressure, non-linear formation and the shape of the borehole, based on the elastic stress distribution around the hole in the common air drilling, analyse the stress distribution during air drilling. When using the air drilling to drill through the tight sand formation with high pressure, because of the change of pore pressure, the stress distribution around hole will change. This could lead the permeability to drop down, and form a compaction band, which prevents the tight sand gas from flowing into the hole. In the other hand, when the borehole collapses, the stress concentrates around the hole, which leads the collapsed zone extends into the formation. This tends will not stop until the stress gets into the new balance. (5) Use the model of elastic-plastic stress distribution around the wellbore in air drilling to study the elastic and plastic zone distribution when the tight sand gas flow into the hole or there is no gas flowing into the tight sand. The researches show that:the plastic zone will increase when the cohension and internal friction angle decreases, which leads the borehole instability. And the gas seepage will increase the plastic zone, which leads the borehole easily collapses.