Damage Evaluation And Mechanism Of Drilling Mud In Reservoirs

This paper aims at depth investgating the damage characteristics and mechanism of drilling mud in reservoirs.XRD,scanning electron microscopy and energy spectrum etc analytical methods or technology are used to study the physical and chemical characteristics of Lower Paleozoic reservoirs rocks and the properties of the flowback fluid.The results show that the minerals of G22-5,G22-3,Shan 225 and Shan 25 these four wells rocks are mainly carbonates whose value are of 93.4 to 99%,and no or only a very small amount of clay.Shan44 well rock contains about 61.7% quartz and 32.7% clay.The dolomite crystal is microcrystalline structure in which intergranular microporosity can be seen,it is in close contact between the mosaic-like crystals.The rocks' permeability are of(3.21×10-4~3.69)×10-3?m2 and the porosity are of 0.86%~6.54%.In a word,the reservoirs are low porosity and low permeability formations.Mechanics test of rocks shows that vertical geostress of G22-5 well is 80.87 MPa,meanwhile the maximum horizontal geostress is 66.33 MPa and minimal horizontal geostress is 51.81 MPa.Tectonic fractures direction of the core is axial.The uniaxial antitensile strength shows a relatively higher value except G22-3 well.The rocks' Poisson's ratio are of 0.22 to0.33,angle of internal friction are of 32.17°to 53.91°and simultaneously brittleness index are of 27.94 to 63.93%.The above figures illustrate these five wells rocks are more rigid and brittle and not easily deformed.They have a good formation mechanical stability,but prone to breaking into pieces at a high pressure.Drilling muds have higher fluid loss.Colloid particle size distribution are of 0.01~4?m.Mud of Jing Ping 22-4-2 and Jing Ping 22-4-3(4850m)well have bigger viscosity because higher levels of organics.Inorganic of mud is mainly composed of sodium salts,silicates,calcium carbonate and barium sulfate.Drilling mud will inject into the rocks when the extra pressure exists and causes a damage of 1.3 to 8.7 times to permeability decrease that is of mild or serious degree.The damage mainly happened in the first two hours on test.The smaller viscosity and solid content of drilling mud,the greater degree of damage to the rock.The maximum penetration depth of the drilling fluid is less than 25 mm except for 4# rock.Factors that lowered reservoir permeability are solid particles what directly plug the flow channe and low permeability rock where organic components into the gap that lead to water lock and adsorption effects.Overall organic components of drilling mud inject core results in relative smaller permeability damage,and the longer the soaking time,the greater the damage.