To find out the inherent causes related to the borehole wall collapse in drilling the Shiqianfeng formation-Shihezi formation in the east of the Yan'an gas field,samples were taken from the rig-sites and were studied for their mineral composition,physical-chemical features and mechanical characteristics.Based on the study a measure dealing with the borehole wall collapse,which is to inhibit the self-imbibition of liquid into the formation rocks,is presented.From the study it was found that the Shiqianfeng formation-Shihezi formation in the east of the Yan'an gas field contain 15.44%-47.52%clays,and are a formation with weak expandability and moderate dispersibility.The rock samples are developed with micro-fractures and micro-fissures which are the main causes for the invasion of liquids and hence the collapse of borehole walls.The percent cuttings recovery of the samples tested on hot roller tester in distilled water is less than 67.2%,and the linear percent expansion of the cores made of the samples in distilled water is less than 8.14%.The Shihezi formation shows stronger dispersibility and stronger hydrophilicity than those of the Shiqianfeng formation.After soaking in a drilling fluid,the triaxial compressive strength of the Shiqianfeng sample is reduced from 186.04 MPa to 98.13 MPa,and the triaxial compressive strength of the Shihezi sample is reduced from 90.09 MPa to 49.21 MPa,indicating that the invasion of the drilling fluid into the rocks along the micro-fractures and micro-fissures causes the strengths of the formations to reduce.Using 0.3%self-imbibition inhibitor ZXS-1,the degrees of saturation of water phase and oil phase in the rock samples can be reduced from 72.6%and 86.6%to 4.7%and 33.5%,respectively,and the ZXS-1 additive can form a dense layer of adsorption around the surfaces of the rocks,thereby turning the wettability of the rock surfaces from hydrophilicity to hydrophobicity.By inhibiting the water imbibition into the rocks,the micro-fractures and micro-fissures are plugged and the liquid can no longer invade into the rocks,and the borehole walls are thus stabilized.
Borehole wall destabilizationPhysical-chemical characteristicsTriaxial compressive strengthInhibit rock from water imbibitionYan'an gas field
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