Study On Creep Mechanics And Safe Drilling Techniques For Large Salt-gypsum Layers In Amudarya Right Bank Block

The salt and gypsum layer is an excellent reservoir cap formation,especially a thick salt and gypsum layer.Usually,large oil and gas reservoirs are bred under the thick salt and gypsum layer.Different from the conventional sedimentary rock properties,the salt and gypsum layer has strong plasticity,and the thick salt and gypsum layer also has certain fluidity.It is easy to cause complex due to salt rock reduction and dissolution during the exploration and development of subsalt reservoirs.The resources of the subsalt reservoirs in the right bank of Amu Darya River in Turkmenistan are huge.By the end of June 2015,the Project of the right bank of Amu Darya River has supplied 130×108m3/year gas to our country.Although the exploration and development of the sub-salt oil and gas reservoirs in the right bank of Amu Darya River has achieved initial success,the drilling problems in the thick salt and gypsum layer are still prominent,such as the reduction of well diameter caused by thick salt and gypsum layer creep,inaccurate formation pressure prediction and salt rock dissolution which may cause lost circulation and overflow.In order to reach safe,economical and efficient development of the sub-salt oil and gas reservoirs in the right bank of Amu Darya River,this paper firstly analyzes the geological characteristics and actual drilling conditions of right bank of Amu Darya River area to identify the difficult points of drilling;secondly,establish Then,based on the experimental results,obtain the creep law of the thick salt and gypsum layer,and use the finite element software to find out the minimum safe drilling fluid density and carry out simulation of stress loading on casing.Optimize drilling fluid system for drilling through thick salt and gypsum layer,and finally,conclude a safety drilling technology about thick salt and gypsum for on-site application.The main conclusions of the paper are as following:(1)The thick salt and gypsum of the right bank of Amu Darya River is about 800 m?1300 m,and the depth is about 1800 m?3300 m.The gradients of three principal stress are:?H=2.58 MPa/m,?v=2.36 MPa/m and ?H=2.04 MPa/m;The modulus is 5050 MPa,the average Poisson ratio is 0.454,and the collapse pressure coefficient of the sand-shale formation up the salt and gypsum layer is 1.0-1.1,the pore pressure coefficient of which is 1.2-1.3,and the fracture pressure coefficient of which is 2.0-2.25.(2)The creep rule of thick salt and gypsum layer in the right bank of Amu Darya River is match with the Heard model to maximum extent.The model constants are:A=40.238,B=0.568 and Q=20027;the density of the drilling fluid for safe drilling through the thick salt and gypsum layer should be higher than 1.80g/cm3.The steel grade 125 TT,15.88 mm casing is applicable for the thick salt and gypsum layer.(3)The optimized drilling fluid additives are:FA-367,XY-27 and JT-888(or SMP-?).The drilling system using for upper part of the salt and gypsum layer is 4%slurry+0.3%FA-367+0.3%XY-27+0.5%JT-888+1.5%QS-2+1.5%EP-1+ barite.The drilling system using for the salt and gypsum layer is:6%slurry+0,5%FA-367+0.5%XY-27+0.5%JT-888+ 3%ZD-1+2%SMP-?+25%NaCl+2%EP-1+barite.(4)The safe drilling technique for thick salt and gypsum layer in the right bank of Amu Darya River has a significant effect.The drilling rate of the thick salt and gypsum layer is shortened by more than 3 times,and the drilling accident rate and drilling complexity rate are greatly reduced.The average drilling period of the vertical well in Zone A of the right bank of Amu Darya River is reducing from 82 days to 50 days;the average drilling period of the derection well in Zone A is reducing from 120 days to 90 days;the average drilling period in the vertical well in Zone B of the right bank of Amu Darya River is decreasing from 130 days to 110 days;the average drilling period of the derection well in Zone A is reducing from 173 days to 156 days.The drilling engineering technical measures described in this paper are based on the current drilling practice,which can solve the technical problems of drilling engineering and provide technical support and guidance for the later complex salt and gypsum layers.